Lewis pair polymerization by classical and frustrated Lewis pairs: Acid, base and monomer scope and polymerization mechanism

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Frustrated Lewis Pairs

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PAPER

Lewis pair polymerization by classical and frustrated Lewis pairs: acid, base

and monomer scope and polymerization mechanism†

Yuetao Zhang,^aGarret M. Miyake,^aMallory G. John,^aLaura Falivene,^bLucia Caporaso,^bLuigi Cavallo*^c

and Eugene Y.-X. Chen*^a

Received 22nd February 2012, Accepted 23rd April 2012

DOI: 10.1039/c2dt30427a

Classical and frustrated Lewis pairs (LPs) of the strong Lewis acid (LA) Al(C₆F₅)₃with several Lewis

base (LB) classes have been found to exhibit exceptional activity in the Lewis pair polymerization (LPP)

of conjugated polar alkenes such as methyl methacrylate (MMA) as well as renewable α-methylene-

γ-butyrolactone (MBL) and γ-methyl-α-methylene-γ-butyrolactone (γ-MMBL), leading to high molecular

weight polymers, often with narrow molecular weight distributions. This study has investigated a large

number of LPs, consisting of 11 LAs as well as 10 achiral and 4 chiral LBs, for LPP of 12 monomers of

several different types. Although some more common LAs can also be utilized for LPP, Al(C₆F₅)₃-based

LPs are far more active and effective than other LA-based LPs. On the other hand, several classes of LBs,

when paired with Al(C₆F₅)₃, can render highly active and effective LPP of MMA and γ-MMBL; such

LBs include phosphines (e.g., P^tBu₃), chiral chelating diphosphines, N-heterocyclic carbenes (NHCs),

and phosphazene superbases (e.g., P₄-^tBu). The P₄-^tBu/Al(C₆F₅)₃pair exhibits the highest activity of the

LP series, with a remarkably high turn-over frequency of 9.6 × 10⁴h⁻¹(0.125 mol% catalyst, 100%

MMA conversion in 30 s, M_n= 2.12 × 10⁵g mol⁻¹, PDI = 1.34). The polymers produced by LPs at RT

are typically atactic (P_γMMBL with ∼47% mr) or syndio-rich (PMMAwith ∼70–75% rr), but highly

syndiotactic PMMAwith rr ∼91% can be produced by chiral or achiral LPs at −78 °C. Mechanistic

studies have identiﬁed and structurally characterized zwitterionic phosphonium and imidazolium

enolaluminates as the active species of the current LPP system, which are formed by the reaction of the

monomer·Al(C₆F₅)₃adduct with P^tBu₃and NHC bases, respectively. Kinetic studies have revealed that

the MMA polymerization by the ^tBu₃P/Al(C₆F₅)₃pair is zero-order in monomer concentration after an

initial induction period, and the polymerization is signiﬁcantly catalyzed by the LA, thus pointing to a

bimetallic, activated monomer propagation mechanism. Computational study on the active species

formation as well as the chain initiation and propagation events involved in the LPP of MMAwith some

of the most representative LPs has added our understanding of fundamental steps of LPP. The main

difference between NHC and PR₃bases is in the energetics of zwitterion formation, with the NHC-based

zwitterions being remarkably more stable than the PR₃-based zwitterions. Comparison of the

monometallic and bimetallic mechanisms for MMA addition shows a clear preference for the bimetallic

mechanism.

Introduction

^aDepartment of Chemistry, Colorado State University, Fort Collins,

The “frustrated Lewis pair” (FLP) chemistry¹has caught che-

Colorado 80523-1872, USA. E-mail: luigi.cavallo@kaust.edu.sa,

mists’ imagination and thus attracted an explosive level of recent

interest since the seminal works²of Stephan and Erker, which

introduced the FLP concept to describe sterically encumbered

Lewis acid (LA, most commonly B(C₆F₅)₃) and Lewis base (LB,

e.g., P^tBu₃) pairs that are sterically precluded from forming clas-

sical donor–acceptor adducts. Instead, the unquenched, opposite

LA/LB reactivity of a FLP can promote unusual reactions, or

reactions that were previously known to be possible only by tran-

sition-metal complexes.¹In its relatively short history, FLP

eugene.chen@colostate.edu

^bDipartimento di Chimica, Università di Salerno, Via Ponte don Melillo,

I-84084 Fisciano, Italy

^cKing Abdullah University of Science and Technology (KAUST),

Chemical and Life Sciences and Engineering, Kaust Catalysis Center,

Thuwal 23955-6900, Saudi Arabia

†Electronic supplementary information (ESI) available: Cartesian co-

ordinates with BP86 and M06 solvent phase energies. CCDC 865899

(MMA-based zwitterion 1), 865898 (MBL-based zwitterion 2). For ESI

and crystallographic data in CIF or other electronic format see DOI:

10.1039/c2dt30427a

This journal is © The Royal Society of Chemistry 2012

Dalton Trans., 2012, 41, 9119–9134 | 9119

chemistry has achieved remarkable successes in many areas of

chemistry, chieﬂy activation of small molecules,³catalytic

hydrogenation,⁴and new reactivity/reaction development.⁵

Our interest in this area has been on the reactivity of the

of MMA by the most representative LPP systems has added our

understanding of fundamental steps involved in LPP.

6

Experimental

strongly acidic, sterically encumbered alane Al(C₆F₅)₃towards

LB substrates and subsequent application of the resulting active

species to polymerization catalysis. This interest was prompted

by several surprising ﬁndings during the period of 2000–2002 in

the LA/LB chemistry associated with Al(C₆F₅)₃, which is in

sharp contrast to the LA/LB chemistry with the borane congener

B(C₆F₅)₃. Notable examples include: (a) unique “double acti-

vation” of group 4 metallocene dimethyl bases with Al(C₆F₅)₃to

generate the corresponding highly active dicationic metallocene

catalysts for oleﬁn polymerization;⁷(b) formation of unusual

μ-alkyl dialuminate anions from the reaction of group 5 tantalo-

cene trimethyl with Al(C₆F₅)₃;⁸(c) high polymerization activity

of the enolaluminate/Al(C₆F₅)₃LB/LA pair versus inactivity of

the enolborate/B(C₆F₅)₃pair;⁹and (d) arene C–H bond acti-

vation taking place¹⁰when combining the strong LA C₇H₈·Al

Materials, reagents, and methods

All syntheses and manipulations of air- and moisture-sensitive

materials were carried out in ﬂamed Schlenk-type glassware on a

dual-manifold Schlenk line, on a high-vacuum line, or in an

inert gas (Ar or N₂)-ﬁlled glovebox. NMR-scale reactions were

conducted in Teﬂon-valve-sealed J. Young-type NMR tubes.

HPLC-grade organic solvents were ﬁrst sparged extensively with

N₂during ﬁlling 20 L solvent reservoirs and then dried by

passage through activated alumina (for Et₂O, THF, and CH₂Cl₂)

followed by passage through Q-5 supported copper catalyst (for

toluene and hexanes) stainless steel columns. Benzene-d₆and

toluene-d₈were dried over sodium/potassium alloy and vacuum-

distilled or ﬁltered, whereas CD₂Cl₂and CDCl₃were dried over

activated Davison 4 Å molecular sieves. NMR spectra were

11

(C₆F₅)₃with a bulky LB, 2,6-di-tert-butyl pyridine, which

1

does not form a classical acid–base adduct when unsolvated Al

(C₆F₅)₃is used. Preceeding the current term FLP,¹this result (d)

was an early example of the unusual reactivity of the FLP rela-

tive to what was generally recognized. Since then, our interest

has continued to focus on utilizing the high Lewis acidity of Al

(C₆F₅)₃and the unique catalytic feature of the active species

derived from this alane for the polymerization of functionalized

alkenes.¹²To this end, we recently communicated a signiﬁcant

development in this continuing effort: Al(C₆F₅)₃-based LPs

rapidly polymerize conjugated polar alkenes, including methyl

methacrylate (MMA) and naturally renewable methylene butyro-

lactones,¹³α-methylene-γ-butyrolactone (MBL)¹⁴and γ-methyl-

α-methylene-γ-butyrolactone (γ-MMBL),¹⁵at room temperature

(RT) to high molecular weight (MW) polymers.¹⁶The bases

examined therein included phosphines [P^tBu₃, PMes₃(Mes =

2,4,6-Me₃C₆H₂), and PPh₃] as well as N-heterocyclic carbenes

(NHCs), 1,3-di-tert-butylimidazolin-2-ylidene (I^tBu) and 1,3-

di-mesityl-butylimidazolin-2-ylidene (IMes). This Lewis pair

polymerization (LPP) was proposed to proceed via zwitterionic

phosphonium or imidazolium enolaluminate active species. Intri-

guingly, although the borane congener, B(C₆F₅)₃, can also form

analogous zwitterionic species, it is inactive for LPP. Most

recently, we found that reactive α-methylene-γ-butyrolactones

(e.g., MBL, γ-MMBL) can be directly and rapidly polymerized

by the NHC base I^tBu in DMF,¹⁷which represents the ﬁrst

conjugate-addition organopolymerization of polar alkenes by

organic catalysts such as NHCs.¹⁸

This contribution presents a full account of our combined

experimental and theoretical study on LPP, including experimen-

tal investigations into LA, LB and monomer scopes (Chart 1)

and computational study of active species formation and

polymerization mechanism. Notably, this study has examined a

large number of classical and frustrated LPs, consisting of 11

LAs and 10 achiral and 4 chiral LBs, for the polymerization of

12 monomers of several different types, including methacrylates,

an acrylate, acrylamides, α-methylene-γ-butyrolactones, a vinyl

phosphonate, and cyclic monomers (lactones). Computational

study of the active species formation as well as the chain

initiation and propagation events involved in the polymerization

recorded on a Varian Inova 300 (300 MHz, H; 75 MHz, ¹³C;

282 MHz, ¹⁹F), 400 MHz, or 500 MHz spectrometer. Chemical

1

shifts for H and ¹³C spectra were referenced to internal solvent

resonances and are reported as parts per million relative to

SiMe₄, whereas ¹⁹F NMR spectra were referenced to external

CFCl₃and ³¹P NMR spectra to H₃PO₄.

Methyl methacrylate (MMA), furfuryl methacrylate (FMA),

diethyl vinylphosphonate (DEVP), n-butyl acrylate (ⁿBA),

ε-caporalactone (ε-CL), γ-valerolactone (γ-VL), γ-butyrolactone

(γ-BL), and α-angelica lactone (α-AL) were purchased from

Sigma-Aldrich Chemical Co, while N,N-dimethylacrylamide

(DMAA), α-methylene-γ-butyrolactone (MBL), and γ-methyl-

α-methylene-γ-butyrolactone (γ-MMBL) were purchased from

TCI America. These monomers were ﬁrst degassed and dried

over CaH₂overnight, followed by vacuum distillation; MMA

was further puriﬁed by titration with neat tri(n-octyl)aluminum

to a yellow end point and distillation under reduced pressure. A

literature procedure was used to prepare N,N-diphenylacrylamide

(DPAA).¹⁹The puriﬁed monomers were stored in brown bottles

inside a glovebox freezer at −30 °C.

Phosphines, including PPh₃, P^tBu₃, and PMes₃were pur-

chased from Alfa Aesar Chemical Co. Chiral phosphines, includ-

ing (S)-(−)-(1,1′-binaphthalene-2,2′-diyl)bis(diphenylphosphine)

(CP-1), (−)-1,2-bis[(2S,5S)-2,5-diisopropylphospholano]benzene

(CP-2), (S,S)-1,2-bis[α-naphthyl(phenylphosphino)]ethane (CP-3),

and (2S,3S)-(−)-2,3-bis(diphenylphosphino)butane (CP-4) were

purchased from Sigma-Aldrich. Superbases, 1-tert-butyl-4,4,4-

tris(dimethylamino)-2,2-bis[tris(dimethylamino)-phosphoranylide-

namino]-2λ⁵,4λ⁵-catenadi(phosphazene) (P₄-^tBu, solution in

hexane) and 1-tert-butyl-2,2,4,4,4-pentakis(dimethylamino)-

2λ⁵,4λ⁵-catenadi(phosphazene) (P₂-^tBu, solution in THF) were

purchased from Sigma-Aldrich; the solvent was removed prior to

use. N-Heterocyclic carbenes (NHCs), including 1,3-bis(2,4,6-

trimethylphenyl)imidazol-2-ylidene (IMes) and 1,3-di-tert-butyl-

imidazol-2-ylidene (I^tBu), were purchased from Strem Chemical

Co. A literature procedure was used to prepare 1,3,4-triphenyl-

4,5-dihydro-1H-1,2,4-triazol-5-ylidene (TPT).²⁰Trimethylalumi-

num, triethylaluminum, and tri(n-octyl)aluminum were pur-

chased from Strem Chemical Co., while butylated

9120 | Dalton Trans., 2012, 41, 9119–9134

This journal is © The Royal Society of Chemistry 2012

Chart 1 Scopes of LAs, LBs, and monomers investigated in this study.

hydroxytoluene (BHT-H, 2,6-di-tert-butyl-4-methylphenol) was

two isomers A (major) and B (minor) in a 7 : 3 ratio. These

species undergo decomposition at RT after storing in solution for

2 h; a mixture of 1, free P^tBu₃, and other two unidentiﬁable

species, but clearly no free MMA·Al(C₆F₅)₃adduct or Al(C₆F₅)₃

present, was formed after 20 h at RT.

purchased from Alfa Aesar Chemical Co. Methylaluminoxane

solution (10 wt% in toluene, MAO), AlCl₃, triisopropylsilane,

and triethylsilane were purchased from Aldrich Chemical Co.

All chemicals were used as received unless otherwise speciﬁed

as follows. C₆₀(>99.95%) was purchased from Materials Tech-

nologies Research and dried in vacuo prior to use. BHT-H was

recrystallized from hexanes prior to use; ^tBu₃P was ﬁrst degassed

and dried over CaH₂overnight, followed by vacuum distillation.

¹H NMR (C₆D₆, 23 °C) for major isomer 1A: δ 3.63 (s, 3H,

OMe), 2.49 (d, J = 9.9 Hz, 2H, PCH₂), 1.70 (s, 3H,vCMe),

0.74 (d, J = 13.2 Hz, 27H, ^tBu); minor isomer 1B: δ 3.27 (s, 3H,

OMe), 2.80 (d, J = 10.8 Hz, 2H, PCH₂), 1.60 (d, J = 1.8 Hz,

3H,vCMe), 0.81 (d, J = 13.8 Hz, 27H, ^tBu). ¹H NMR (CD₂Cl₂,

23 °C) for 1A: δ 3.50 (s, 3H, OMe), 3.19 (d, J = 9.6 Hz, 2H,

21

Literature procedures were used to prepare MeAl(BHT)₂and

Al(C₆F₅)₂Cl.²²Tris(pentaﬂuorophenyl)borane, B(C₆F₅)₃, and

activator [Ph₃C][B(C₆F₅)₄]²³were obtained as research gifts

from Boulder Scientiﬁc Co., and B(C₆F₅)₃was further puriﬁed

by recrystallization from hexanes at −30 °C. Tris(pentaﬂuoro-

phenyl)alane, Al(C₆F₅)₃, as a (toluene)_0.5or (toluene)_1.0adduct,

or in its unsolvated form, was prepared by ligand exchange reac-

tions between B(C₆F₅)₃and AlMe₃or AlEt₃(for preparation of

the unsolvated form).²⁴(Extra caution should be exercised when

handling these materials, especially the unsolvated alane, due to

its thermal and shock sensitivity!) The MMA·Al(C₆F₅)₃adduct⁹

was prepared by addition of excess MMA to a toluene solution

of the alane, followed by removal of the volatiles and drying in

vacuo; its crystal structure was determined previously.²⁵Zwitter-

ionic species derived from the reaction of MMA·Al(C₆F₅)₃with

several phosphines and NHCs were previously described.¹⁶

t

PCH₂), 1.63 (s, br, 3H,vCMe), 1.52 (d, J = 13.2 Hz, 27H, Bu);

1B: δ 3.23 (s, 3H, OMe), 3.32 (d, J = 11.1 Hz, 2H, PCH₂), 1.82

(d, J = 1.8 Hz, 3H,vCMe), 1.55 (d, J = 13.5 Hz, 27H, ^tBu). ¹³

NMR (CD₂Cl₂, 23 °C) for 1A: δ 156.5 (d, J_C–P= 9.6 Hz, OC

C

3

1

(OMe)v), 150.3 (dm, J_C–F= 230 Hz), 141.0 (dm, J_C–F

=

246 Hz), 136.9 (dm, ¹J_C–F= 249 Hz), 67.78 (d, ²J_C–P= 7.3 Hz),

52.76, 39.94 (d, J_C–P= 25.9 Hz, CMe₃), 30.47 (CMe₃), 24.46

(d, J_C–P= 31.8 Hz, CH₂P), 18.76 (OMe); 1B: δ 160.6 (d, J_C–P

= 11.5 Hz, OC(OMe)v), 150.3 (dm, J_C–F= 230 Hz), 141.0

1

3

1

(dm, J_C–F= 246 Hz), 136.9 (dm, J_C–F= 249 Hz), 75.69 (d,

²J_C–P= 8.0 Hz), 57.49, 39.88 (d, ¹J_C–P= 26.5 Hz, CMe₃), 30.43

(CMe₃), 24.00 (d, J_C–P= 33.7 Hz, CH₂P), 17.93 (OMe). ¹⁹F

1

NMR (C₆D₆, 23 °C) for 1A: δ −122.6 (m, 6F, o-F), −156.8 (t,

J_F–F= 21.4 Hz, 3F, p-F), −163.3 (m, 6F, m-F); 1B: δ −122.3 (m,

6F, o-F), −156.9 (t, J_F–F= 21.4 Hz, 3F, p-F), −163.6 (m, 6F,

m-F). ¹⁹F NMR (CD₂Cl₂, 23 °C) for 1A: δ −123.4 (m, 6F, o-F),

−157.8 (t, J_F–F= 21.4 Hz, 3F, p-F), −164.3 (m, 6F, m-F); 1B: δ

−123.2 (m, 6F, o-F), −157.8 (t, J_F–F= 21.4 Hz, 3F, p-F), −164.4

(m, 6F, m-F). ³¹P NMR (C₆D₆, 23 °C) for 1A: δ 47.8 (s, P^tBu₃);

1B: δ 51.9 (s, P^tBu₃). ³¹P NMR (CD₂Cl₂, 23 °C) for 1A: δ 49.0

(s, P^tBu₃); 1B: δ 52.6 (s, P^tBu₃).

The molecular structure of 1 (E isomer) has been conﬁrmed

by single-crystal X-ray diffraction analysis. Single crystals suit-

able for X-ray diffraction analysis, obtained by layering a

Generation, NMR data, and crystal structure of MMA-based

phosphonium enolaluminate ^tBu₃P–CH₂C(Me)vC(OMe)O–

Al(C₆F₅)₃(1). A Teﬂon-valve-sealed J. Young-type NMR tube

was charged with P^tBu₃(10.1 mg, 0.05 mmol) and 0.3 mL of

C₆D₆or CD₂Cl₂. A solution of MMA·Al(C₆F₅)₃(31.4 mg,

0.05 mmol, 0.3 mL C₆D₆or CD₂Cl₂) was added to this tube via

pipette at ambient temperature, and the colorless mixture was

allowed to react for 10 min before analysis by NMR, which

showed the clean and quantitative formation of zwitterion 1 as

This journal is © The Royal Society of Chemistry 2012

Dalton Trans., 2012, 41, 9119–9134 | 9121

t

solution of Bu₃P (0.1 mmol) in 4 mL hexanes on an equimolar

Bruker SHELXTL program library²⁶and reﬁned by full-matrix

least-squares on F²for all reﬂections. All non-hydrogen atoms

were reﬁned with anisotropic displacement parameters, whereas

hydrogen atoms were included in the structure factor calculations

at idealized positions. Selected crystallographic data for

2: C₃₅H₃₄AlF₁₅O₂P, monoclinic, space group P2₁/n, a =

15.9769(3) Å, b = 12.0995(2) Å, c = 18.1623(3) Å, β = 93.4810

solution of MMA·Al(C₆F₅)₃in 1 mL CH₂Cl₂at −30 °C, were

quickly covered with a layer of Paratone-N oil (Exxon, dried and

degassed at 120 °C/10⁻⁶Torr for 24 h) after decanting the

mother liquor. A crystal was then mounted on a thin glass ﬁber

and transferred into the cold nitrogen stream of a Bruker

SMART CCD diffractometer. The structure was solved by direct

methods and reﬁned using the Bruker SHELXTL program

library²⁶and reﬁned by full-matrix least-squares on F²for all

reﬂections. All non-hydrogen atoms were reﬁned with anisotro-

pic displacement parameters, whereas hydrogen atoms were

included in the structure factor calculations at idealized positions.

A disordered CH₂Cl₂solvent molecule was found in the crystal

lattice, and the Platon Squeeze program was applied to take care

(10)°, V = 3504.52(11) Å³, Z = 4, D_calcd= 1.572 Mg m⁻³

,

GOF = 1.013, R₁= 0.0442 [I > 2σ(I)], wR₂= 0.1483. CCDC

865898 contains the supplementary crystallographic data.

Generation and NMR data of phosphonium enolaluminate

^tBu₃P–CH₂C(Me)vC(OMe)O–AlCl₃(3). A Teﬂon-valve-sealed

J. Young-type NMR tube was charged with AlCl₃(21.4 mg,

0.16 mmol) and 0.3 mL of CD₂Cl₂. MMA (16.1 mg,

0.16 mmol) was added to this tube via pipette at ambient temp-

erature, and the colorless mixture was allowed to react for

10 min before analysis by NMR, which showed the clean and

quantitative formation of MMA·AlCl₃adduct. To this colorless

solution was added P^tBu₃(32.5 mg, 0.16 mmol) and 0.3 mL of

CD₂Cl₂. Two isomers A (major) and B (minor) in a 2 : 1 ratio

were formed as predominant products. This reaction also pro-

duced other unidentiﬁed minor species, presumably a result of

competing decomposition of 3 because it has limited stability at

RT. A mixture containing phosphonium enolaluminate 3, free

P^tBu₃, and some other unidentiﬁable species was formed after

sitting the solution at RT for 36 h.

of the solvent void (203 Å³) in the structure reﬁnement. The Bu

t

group in 1 was disordered and treated in part (C24 to C35,

59.20%; C24A to C35A, 40.80%). One perﬂuorophenyl ring

(C13 to C18, F11 to F15, 60.36%; C13A to C18A, F11A to

F15A, 39.64%) was disordered and the SAME command was

implemented to restrain the bond distances and angles of the dis-

ordered perﬂuorophenyl ring. Selected crystallographic data for

ˉ

1: C₃₅H₃₅AlF₁₅O₂P, triclinic, space group P1, a = 10.7749(4) Å,

b = 12.9668(4) Å, c = 15.2829(5) Å, α = 92.432(2)°, β =

97.632(2)°, γ = 111.204(2)°, V = 1963.59(11) Å³, Z = 2, D_calcd

=

1.405 Mg m⁻³, GOF = 1.115, R₁= 0.0488 [I > 2σ(I)], wR₂=

0.1545. CCDC 865899 contains the supplementary crystallo-

graphic data.

¹H NMR (CD₂Cl₂, 23 °C) for the MMA·AlCl₃adduct: δ 6.79

(s, 1H, CH₂v), 6.28 (s, 1H, CH₂v), 4.28 (s, 3H, OMe), 2.09 (s,

1

3H,vCMe). H NMR (CD₂Cl₂, 23 °C) for 3A: δ 3.63 (s, 3H,

Generation, NMR data, and crystal structure of MBL-based

OMe), 3.22 (d, J = 10.2 Hz, 2H, PCH₂), 1.83 (s, br, 3H,vCMe),

phosphonium enolaluminate ^tBu₃P–MBL–Al(C₆F₅)₃(2)

t

1.61 (d, J = 13.5 Hz, 27H, Bu); 3B: δ 3.60 (s, 3H, OMe), 3.24

Zwitterion 2 derived from the reaction of MBL with P^tBu₃and

toluene·Al(C₆F₅)₃was carried out in the same manner as that

(d, J = 10.8 Hz, 2H, PCH₂), 1.80 (d, J = 1.2 Hz, 3H,vCMe),

t

1.62 (d, J = 13.5 Hz, 27H, Bu). ³¹P NMR (CD₂Cl₂, 23 °C) for

1

described for 1. H NMR (CD₂Cl₂, 300 MHz, 23 °C): δ 4.12 (t,

3A: δ 49.8 (s, P^tBu₃); 3B: δ 52.3 (s, P^tBu₃).

2H, J = 8.5 Hz, CH₂O), 3.19 (d, 2H, J = 8.8 Hz, CH₂P), 2.67 (t,

t

2H, J = 8.5 Hz, CH₂CH₂O), 1.56 (d, 27H, J = 13.5, Bu₃). ¹³C

General polymerization procedures. MMA polymerizations

were performed either in 30 mL oven-dried glass reactors inside

the glovebox under ambient conditions (ca. 25 °C) or in 25 mL

oven-dried Schlenk ﬂasks interfaced to the dual-manifold

Schlenk line for runs carried out at other temperatures. A pre-

determined amount of a Lewis acid (LA), such as MMA·Al

(C₆F₅)₃, (toluene)_{0.5 or 1.0}·Al(C₆F₅)₃, AlCl₃, AlEt₃, AlMe₃, Al

(C₆F₅)₂Cl et al., was ﬁrst dissolved in the monomer MMA

(1.00 mL, 9.35 mmol) and 5 mL of solvent (CH₂Cl₂or toluene)

inside a glovebox, and the polymerization was started by rapid

addition of a solution of a Lewis base (LB), such as phosphines

PR₃, a chiral phosphines, NHCs, amines, superbases et al., in

4 mL of solvent (CH₂Cl₂or toluene) via a gastight syringe to

the above LA + MMA solution under vigorous stirring. The

amount of the monomer (M) and the [LA]/[LB] ratio (2/1) were

ﬁxed for all polymerizations, whereas the [M]/[LB] ratio was

varied in some experiments. After the measured time interval, a

0.2 mL aliquot was taken from the reaction mixture via syringe

and quickly quenched into a 4 mL vial containing 0.6 mL of

undried “wet” CDCl₃stabilized by 250 ppm of BHT-H; the

3

NMR (CD₂Cl₂, 23 °C): δ 163.5 (d, J_C–P= 10.0 Hz, OC

(OMe)v), 150.3 (dm, J_C–F= 231 Hz), 141.0 (dm, J_C–F

246 Hz), 136.9 (dm, J_C–F= 250 Hz), 66.44, 61.32 (d, J_C–P

1

=

1

2

1

4.6 Hz), 39.29 (d, J_C–P= 26.7 Hz, CMe₃), 34.06, 30.12

(CMe₃), 19.73 (d, J_C–P= 37.9 Hz, CH₂P). ¹⁹F NMR (CD₂Cl₂,

1

282 MHz, 23 °C): δ −123.5 (m, 6F, o-F), −157.9 (t, J_F–F

=

19.3 Hz, 3F, p-F), −164.6 (m, 6F, m-F). ³¹P NMR (CD₂Cl₂,

23 °C): δ 47.4 (s, P^tBu₃).

The molecular structure of 2 has been conﬁrmed by single-

crystal X-ray diffraction analysis. A 20 mL glass vial was

charged with toluene·Al(C₆F₅)₃(0.16 mmol), MBL

(0.16 mmol), and 4 mL toluene, while another vial was charged

with P^tBu₃(0.16 mmol) and 10 mL hexanes. The two vials were

cooled to −30 °C and the solution of P^tBu₃was layered on the

mixture of toluene·Al(C₆F₅)₃and MBL via pipette at low temp-

erature. The vial was stored in the freezer for one week and crys-

tals of 2 were formed. The crystals were quickly covered with a

layer of Paratone-N oil (Exxon, dried and degassed at 120 °C/

10⁻⁶Torr for 24 h) after decanting the mother liquor. A crystal

was then mounted on a thin glass ﬁber and transferred into the

cold nitrogen stream of a Bruker SMART CCD diffractometer.

The structure was solved by direct methods and reﬁned using the

1

quenched aliquots were later analyzed by H NMR to obtain the

percent monomer conversion data. The procedures for obtaining

the monomer conversion data vs. reaction time were detailed in

9122 | Dalton Trans., 2012, 41, 9119–9134

This journal is © The Royal Society of Chemistry 2012

literature.^25,27aThe polymerization was immediately quenched

after the removal of the aliquot by addition of 5 mL 5% HCl-aci-

diﬁed methanol. The quenched mixture was precipitated into

100 mL of methanol, stirred for 1 h, ﬁltered, washed with metha-

nol, and dried in a vacuum oven at 50 °C overnight to a constant

weight.

Results and discussion

Generation and characterization of Al(C₆F₅)₃-based zwitterionic

active species

Reaction of MMA·Al(C₆F₅)₃with P^tBu₃at RT in C₆D₆or

CD₂Cl₂cleanly generates the corresponding zwitterionic phos-

phonium enolaluminate, ^tBu₃P–CH₂C(Me)vC(OMe)O–Al

(C₆F₅)₃(1), as two isomers (E/Z) in a 7 : 3 ratio (Scheme 1).¹⁶

This addition pattern is similar to additions of FLP to non-polar

substrates, such as 1,4-addition of FLPs to 1,3-dienes³⁵and

1,2-addition of FLPs to terminal alkynes,³⁶as well as

1,4-additions of the ethylene-bridged intramolecular frustrated

P/B (phosphine/borane) Lewis pair to ynones.³⁷Zwitterion 1

can be readily characterized by NMR (here using the major

isomer for illustration) for the phosphonium cation [^tBu₃P⁺]³⁵

Polymerization of other monomers was performed using the

same procedure already described for the MMA polymerization

except for DEVP. In a typical DEVP polymerization procedure,

the reactor was charged with a stir bar, a LB (P^tBu₃, PPh₃, or

I^tBu), 200 equiv. of DEVP (2 mL, 0.013 mol), and 2 mL of

CH₂Cl₂. The polymerization was started by rapid addition of

(toluene)_0.5·Al(C₆F₅)₃(32.8 mg, 55.1 mmol) under vigorous stir-

ring. The polymerization was quenched at the times speciﬁed in

the polymerization table, by pouring the polymer solution into

100 mL of MeOH, and stirred for 1 h. Prior to quenching, a

0.2 mL aliquot was withdrawn from the solution and quenched

into septum sealed vials containing 0.6 mL of undried “wet”

[δ 49.0 (s) ppm in ³¹P NMR; 1.52 (d, Bu) in H NMR], for the

enolaluminate anion [OAl(C₆F₅)₃]^{− 25,38}[δ −123.4 (m, 6F, o-F),

−157.8 (t, 3F, p-F), −164.4 (m, 6F, m-F) in ¹⁹F NMR], and for

the remaining ester enolate moiety –CH₂C(Me)vC(OMe)O–

[δ 3.50 (s, 3H, OMe), 3.19 (d, 2H, PCH₂), 1.63 (s, 3H,vCMe)

t

1

CHCl₃and analyzed by H NMR for monomer conversion data.

Oily polymers were collected by decanting the solvents and

dried in a vacuum oven at 50 °C to a constant weight.

1

in H NMR], as depicted in Fig. 1. The molecular structure (the

E isomer) of 1 has been conﬁrmed by X-ray diffraction analysis,

as shown in Fig. 2. It is worth noting that phosphonium enola-

luminate 1 starts to undergo decomposition after sitting in sol-

ution at RT for 2 h, forming a mixture containing 1, free P^tBu₃,

and other two unidentiﬁable species, but clearly no free

MMA·Al(C₆F₅)₃or Al(C₆F₅)₃present, after 20 h. Addition of

excess MMA to a freshly generated solution of phosphonium

enolaluminate 1 led to immediate polymerization, but the

polymerization with an additional equiv. of Al(C₆F₅)₃is much

more rapid due to monomer activation (vide infra).

Polymer characterizations. Polymer number-average molecu-

lar weights (M_n) and molecular weight distributions (MWD =

M_w/M_n) were measured by gel permeation chromatography

(GPC) analyses carried out at 40 °C and a ﬂow rate of 1.0 mL

min⁻¹, with DMF (for PMBL and PMMBL samples) or CHCl₃

(for PMMA and other polymer samples) as the eluent, on a

Waters University 1500 GPC instrument coupled with a Waters

RI detector and equipped with four PLgel 5 μm mixed-C

columns (Polymer Laboratories; linear range of molecular

weight = 200–2 000 000). The instrument was calibrated with 10

PMMA standards, and chromatograms were processed with

Waters Empower software (version 2002). ¹H NMR and ¹³C

NMR spectra for the analysis of PMMA^27,28and PMMBL²⁹

microstructures were recorded and analyzed according to the lit-

erature methods.

Intriguingly, no reaction took place between PMes₃and

MMA·Al(C₆F₅)₃at RT, although PMes₃/Al(C₆F₅)₃can be

described as an FLP.¹⁶Importantly, this observation correlates to

the non-polymerization activity of this FLP in the presence of

excess MMA (vide infra). On the other hand, PPh₃and

Al(C₆F₅)₃cleanly form a CLP adduct, Ph₃P·Al(C₆F₅)₃, but this

adduct formation did not quench its reactivity toward polymeriz-

ation, as this CLP is still highly active for MMA polymerization

(vide infra); it is worth noting that the stoichiometric reaction of

PPh₃and MMA·Al(C₆F₅)₃never went to completion, and the

formed zwitterionic species, Ph₃P–CH₂C(Me)vC(OMe)O–

Al(C₆F₅)₃(two isomers in a 1 : 1 ratio), started to decompose at

RT in less than 1 h.¹⁶As NHCs, such as I^tBu and IMes, are

strong bases, they react with C₇H₈·Al(C₆F₅)₃at RT in benzene to

form cleanly stable adducts, I^tBu·Al(C₆F₅)₃and IMes·Al(C₆F₅)₃,

and the reaction between I^tBu or IMes with MMA·Al(C₆F₅)₃

readily generates the corresponding zwitterionic imidazolium

enolaluminate active species, I^tBu–CH₂C(Me)vC(OMe)O–

Al(C₆F₅)₃or IMes–CH₂C(Me)vC(OMe)O–Al(C₆F₅)₃, which

also correlates to the observed high polymerization activity of

the I^tBu/Al(C₆F₅)₃and IMes/Al(C₆F₅)₃pairs in the presence of a

large excess of MMA.¹⁶In contrast, there is no reaction between

I^tBu and B(C₆F₅)₃at −60 °C in toluene (i.e., forming a FLP),³⁹

but at RT the same reaction yields a zwitterionic product with B

(C₆F₅)₃being attached to the 4-position of the imidazole hetero-

cycle.⁴⁰TPT, a weaker nucleophile than I^tBu or IMes, but still a

strong base, reacts with MMA·Al(C₆F₅)₃at RT to give a mixture

Computational details. All the density functional theory

(DFT) calculations were performed using the Gaussian09

package.³⁰Geometry optimizations were performed using the

BP86 GGA functional of Becke and Perdew³¹with the standard

split-valence basis set with a polarization function of Ahlrichs

and co-workers (SVP keyword in Gaussian).³²Geometry optim-

izations were performed without symmetry constraints. Tran-

sition states (TSs) were approached through a linear transit

procedure using the forming C–C bond as the reaction coordi-

nate. Full TS searches were started from the maxima along the

linear transit paths. All the geometries discussed in this work

were conﬁrmed as minima or TSs by frequency calculations.

The reported energies have been obtained via single point

energy calculations with the BP86 and M06³³functionals with

the triple-ζ basis set with one polarization function developed by

Ahlrichs (TZVP keyword in Gaussian09). Solvent effects have

been included in this single point energy calculation, based on

the polarizable continuum solvation model PCM using toluene

as the solvent.³⁴

This journal is © The Royal Society of Chemistry 2012

Dalton Trans., 2012, 41, 9119–9134 | 9123

Scheme 1 Al(C₆F₅)₃-based classical Lewis pair (CLP) and frustrated Lewis pair (FLP) examples and their reaction with conjugated polar vinyl

monomers such as MMA leading to zwitterionic species that are active for polymerization.

t

Fig. 1 ¹H, ¹⁹F, and ³¹P NMR spectra (CD₂Cl₂, 23 °C) of Bu₃P–CH₂C(Me)vC(OMe)O–Al(C₆F₅)₃(1) as two isomers: major isomer A and minor

isomer B.

containing the zwitterionic species, PMMA, and other minor

species. Nevertheless, addition of excess MMA to this mixture

rapidly converted all monomer to PMMA.

enolaluminate 2 is active for MBL polymerization, but this

polymerization becomes much more rapid with an additional

equiv. of Al(C₆F₅)₃(vide infra).

Likewise, replacing MMA with the cyclic MBL having ﬁxed

s-cis-conformation in the reaction with the P^tBu₃/Al(C₆F₅)₃pair

affords the corresponding zwitterionic phosphonium enolalumi-

Polymerization of MMA by Al(C₆F₅)₃/LB (LB = phosphines

and NHCs) pairs. Control runs using Al(C₆F₅)₃(as a toluene or

MMA adduct), phosphines (P^tBu₃, PMes₃, and PPh₃), and

NHCs (I^tBu, IMes, and TPT) individually for polymerization of

MMA (200–800 equiv.) at RT in toluene yielded no polymer for-

mation up to 24 h. (Note that TPT catalyzes tail-to-tail

t

nate as a single isomer (³¹P NMR: δ 47.4), Bu₃P–MBL–

Al(C₆F₅)₃(2). This zwitterion has been characterized spectro-

scopically by NMR (Fig. 3) and also structurally by X-ray dif-

fraction analysis (Fig. 4). Also noteworthy is that phosphonium

9124 | Dalton Trans., 2012, 41, 9119–9134

This journal is © The Royal Society of Chemistry 2012

dimerization of MMA in this or other solvents or in bulk at

80 °C.⁴¹) Premixing C₇H₈·Al(C₆F₅)₃with P^tBu₃at RT for

10 min in either a 1 : 1 or 2 : 1 [LA]/[LB] ratio, followed by

addition of MMA (800 equiv.), also led to no monomer con-

sumption up to 24 h, due to the FLP-induced C–H activation

and other decomposition of this Lewis pair.¹⁶On the other hand,

addition of MMA (800 equiv.) to the preformed zwitterionic

phosphonium enolaluminate active species 1 by pre-treating the

alane–monomer adduct, MMA·Al(C₆F₅)₃, with this phosphine

LB in toluene at RT for 10 min (vide infra), brought about rapid

polymerization, consuming all monomer in 60 min and yielding

high molecular weight (MW) polymer (M_n= 2.83 × 10⁵

g

mol⁻¹, polydispersity index (PDI) = M_w/M_n= 1.42; run 1,

Table 1). The resulting PMMA exhibited a syndiotacticity of

75.5% rr. The same polymerization was much more rapid in a

2 : 1 [LA]/[LB] ratio, which converted all monomer to high MW

polymer in 4 min (M_n= 3.97 × 10⁵g mol⁻¹, MWD = 1.52, rr =

73.5%, run 2), thus giving a high turn-over frequency (TOF) of

1.2 × 10⁴h⁻¹. A more convenient procedure by simply premix-

ing C₇H₈·Al-(C₆F₅)₃with MMA in toluene (i.e., generating

MMA·Al(C₆F₅)₃in situ), followed by addition of the base to

Fig. 2 X-ray crystal structure of ^tBu₃P–CH₂C(Me)vC(OMe)O–

Al(C₆F₅)₃(1) with thermal ellipsoids drawn at the 50% probability.

Selected bond lengths (Å) and angles (°): Al(1)–O(1), 1.7513(11);

O(1)–C(19), 1.3167(16); C(19)–O(2), 1.3718(18); C(19)–C(20),

1.345(2); C(20)–C(23), 1.5155(19); C(23)–P(1), 1.8487(15); Al(1)–

O(1)–C(19), 142.69(11); O(1)–C(19)–C(20), 126.13(14); O(1)–C(19)–

O(2), 116.33(13); O(2)–C(19)–C(20), 117.52(12); C(19)–C(20)–C(23),

118.56(13); C(20)–C(23)–P(1), 124.86(11).

Fig. 3 ¹H, ¹⁹F, and ³¹P NMR spectra (CD₂Cl₂, 23 °C) of ^tBu₃P–MBL–Al(C₆F₅)₃(2).

This journal is © The Royal Society of Chemistry 2012

Dalton Trans., 2012, 41, 9119–9134 | 9125

start the polymerization, also led to a highly active polymeriz-

ation system, achieving quantitative monomer conversion in

7 min and yielding similarly high MW polymer (M_n= 3.15 ×

10⁵g mol⁻¹, PDI = 1.72, rr = 73.6%, run 3). The same polymer-

ization carried out at 0 °C was still highly effective (100% con-

version in 1 h), producing PMMA with higher syndiotacticity of

77.9% rr (run 4). The PMMA with high syndiotacticity of 88.1%

rr can be produced at −78 °C, at expense of polymerization

activity (consuming all 80 equiv. of MMA in 100 min). Chan-

ging solvent polarity from relatively non-polar toluene (ε = 2.38)

to more polar CH₂Cl₂(ε = 8.93)⁴²did not noticeably affect the

polymerization activity, although there was some variation in

polymer MW (run 5 vs. 1, 6 vs. 2).

Having achieved high polymerization activity with the Al

(C₆F₅)₃/P^tBu₃pair using appropriate procedures (i.e., by avoid-

ing direct contact of the FLP in absence of monomer or by pre-

forming zwitterion 1), we examined the effectiveness of other

sterically encumbered bases known to form FLPs with B(C₆F₅)₃.

As suggested from the no reaction between PMes₃and MMA·

Al(C₆F₅)₃at RT (vide supra), the Al(C₆F₅)₃/PMes₃pair is

indeed ineffective for MMA polymerization in toluene or

CH₂Cl₂, up to 24 h (runs 7 and 8, Table 1). Interestingly, the Al

(C₆F₅)₃/PPh₃pair exhibits exceptional activity (TOF = 4.8 × 10⁴

h⁻¹, run 9), despite the fact that PPh₃forms a classic acid–base

adduct with the alane.¹⁶However, the polymer produced by this

pair had a bimodal MW distribution (MWD) consisting of

∼41% higher MW fraction (M_n= 3.88 × 10⁵g mol⁻¹, PDI =

1.04) and ∼59% lower MW fraction (M_n= 1.33 × 10⁵g mol⁻¹

,

PDI = 1.05), characteristic of co-existing two types of active

species with rather similar catalytic activity; this polymerization

behavior is consistent with the incomplete reaction of PPh₃with

MMA·Al(C₆F₅)₃and instability of the resulting zwitterion (vide

supra).

Excitingly, the Al(C₆F₅)₃/NHC pairs are not only extremely

active for MMA polymerization (runs 10–14), they also produce

PMMA with narrow and unimodal MWDs. Thus, the Al(C₆F₅)₃/

IMes pair consumed all 800 equiv. of MMA in less than 1 min,

giving a high TOF of >4.8 × 10⁴h⁻¹(run 10). In comparison,

the polymerization activity of Al(C₆F₅)₃/I^tBu is ∼15 times lower,

but the resulting polymer MW is ∼20 times higher (M_n= 5.25 ×

10⁵g mol⁻¹, PDI = 1.43, run 11) than that produced by IMes

(M_n= 2.66 × 10⁴g mol⁻¹, PDI = 1.77, run 10). The polymeriz-

ation by Al(C₆F₅)₃/I^tBu in CH₂Cl₂performed similarly to that in

toluene (run 12 vs. 11). Likewise, the polymerization by

Al(C₆F₅)₃/TPT in either toluene (run 13) or CH₂Cl₂(run 14) is

highly active, achieving quantitative monomer conversion in less

than 1 min. An interesting and welcoming feature of the

Al(C₆F₅)₃/TPT pair is that it produced structurally more deﬁned

polymers with considerably narrower MWDs (PDI = 1.17, run

13; PDI = 1.23, run 14) than those achieved by Al(C₆F₅)₃/IMes

and Al(C₆F₅)₃/I^tBu pairs.

Fig. 4 X-ray crystal structure of ^tBu₃P–MBL–Al(C₆F₅)₃(2) with

thermal ellipsoids drawn at the 50% probability. Selected bond lengths

(Å) and angles (°): Al(2)–O(1), 1.7599(11); O(1)–C(19), 1.3073(17);

C(19)–O(2), 1.3666(18); C(19)–C(20), 1.342(2); C(20)–C(23),

1.496(2); C(23)–P(1), 1.8332(15); Al(2)–O(1)–C(19), 138.81(10);

O(1)–C(19)–C(20), 128.77(13); O(1)–C(19)–O(2), 115.97(13); O(2)–

C(19)–C(20), 115.27(13); C(19)–C(20)–C(23), 120.81(13); C(20)–

C(23)–P(1), 123.32(11).

Table 1 Selected results of MMA polymerization by Al(C₆F₅)₃/LB pairs ^a

Conv.^bTOF

10⁻⁴M_n

PDI^c[rr]^d

(M_w/M_n) (%)

[mr]^d[mm]^d

c

LA

LB

(1 eq.)

Temp Time

(°C) (min) (%)

Run no. (adduct)

[LA]/[LB] Solvent

(h⁻¹

)

(g mol⁻¹

)

(%)

1

2

3

4

5

6

7

8

Al·MMA P^tBu₃

1 : 1

2 : 1

1 : 1

2 : 1

Toluene 25

60

4

7

60

4

1440

1

100

0

800

12 000 39.7

6840

800

28.3

1.42

1.52

1.72

2.39

1.41

1.47

—

75.5 23.1

73.5 25.0

73.6 24.7

77.9 20.9

75.8 22.6

75.0 23.0

1.4

1.5

1.7

1.2

1.6

2.0

—

Al·TOL

P^tBu₃

31.5

13.8

38.0

Toluene

CH₂Cl₂

toluene

CH₂Cl₂

0

Al·MMA P^tBu₃

25

12 000 36.9

Al·MMA PMes₃2 : 1

0

—

0

—

9

Al·MMA PPh₃

Al·MMA IMes

Al·MMA I^tBu

Al·MMA TPT

2 : 1

Toluene 25

100

48 000 38.8; 13.3 1.04

73.0 25.4

72.7 25.7

75.1 23.3

74.3 24.3

71.6 26.7

74.2 23.7

1.6

1.4

1.7

2.1

10

11

12

13

14

1

48 000 2.66

3200

24 000 15.9

24 000 12.5

1.77

1.43

1.34

1.17

1.23

15

1

52.5

60.0

CH₂Cl₂

Toluene 25

CH₂Cl₂25

25

1

^aConditions: [MMA]/[LB] = 800, except for runs 13 and 14 where LB = TPT and [MMA]/[TPT] = 400; 10 mL total solution volume. ^bMonomer

1

conversions measured by H NMR. ^cNumber-average molecular weight (M_n) and polydispersity index (PDI) determined by GPC relative to PMMA

standards. ^dPolymer methyl triads (rr, mr, mm) measured by ¹H NMR.

9126 | Dalton Trans., 2012, 41, 9119–9134

This journal is © The Royal Society of Chemistry 2012

monomers by Al(C₆F₅)₃/LB pairs, the result of which were sum-

marized in Table 2. First, we investigated the effectiveness of

Al(C₆F₅)₃/LB pairs for polymerization of renewable monomers

MBL and γ-MMBL, cyclic analogs of MMA. Despite being a

heterogeneous process (due to insolubility of the resulting

polymer in the reaction medium), the polymerization of MBL by

Al(C₆F₅)₃/P^tBu₃in CH₂Cl₂with [MBL]/[LB] = 800 achieved

greater than 90% polymer yield in 1 h (run 1, Table 2). The

PMBL produced had a medium M_nof 4.48 × 10⁴g mol⁻¹and a

relatively high PDI of 2.18. Likewise, the Al(C₆F₅)₃/I^tBu pair is

also quite effective for MBL polymerization, but the M_n(which

contained a small, ∼4% low MW tail peak) of the resulting

PMBL is ∼4 times higher than that by Al(C₆F₅)₃/P^tBu₃(run 2

vs. 1, Table 2). Owing to good solubility of PMMBL in CH₂Cl₂,

the polymerization of MMBL by such LPs is homogeneous and

highly effective. Speciﬁcally, the polymerization by Al(C₆F₅)₃/

P^tBu₃achieved quantitative monomer conversion in 10 min,

giving a high MW, essentially atactic polymer (M_n= 1.92 × 10⁵

g mol⁻¹, PDI = 2.28, mr = 47.0%, run 3, Table 2). All LPs with

three NHC bases are highly active for MMBL polymerization,

with a high TOF of 4.8 × 10⁴h⁻¹(runs 4–6, Table 2), but M_nof

1.39 × 10⁵g mol⁻¹of the PMMBL produced by I^tBu is about

twice of that produced by IMes, and the polymer also exhibits a

much more narrow MWD (PDI = 1.15 for run 4, vs. PDI = 1.42

for run 5). The bimodal behavior of the LP with PPh₃is once

again manifested in the MMBL polymerization, resulting in the

polymer product with ∼43% high MW fraction and ∼57% low

MW fraction (run 7).

Fig. 5 Plots of monomer conversion (%) vs. reaction time (min) for

MMA polymerization in toluene at RT by the Al(C₆F₅)₃/P^tBu₃pair with

two different procedures and LA catalyst concentrations. Line ▲: pro-

cedure A, [MMA]/[LA]/[LB] = 800/2/1; [MMA]₀= 0.935 M, [MMA·Al

(C₆F₅)₃] = 2.34 mM, [P^tBu₃] = 1.17 mM. Line ●: procedure B, [MMA]/

[LA]/[LB] = 800/1.8/1. Line ■: procedure B, [MMA]/[LA]/[LB] =

800/2/1.

Concentrating on the highly active Al(C₆F₅)₃/P^tBu₃pair for

polymerization of MMA, we examined kinetic proﬁles of the

polymerization in toluene at RT using two different procedures

and the LA catalyst concentrations. Speciﬁcally, in procedure A,

2 equiv. of MMA·Al(C₆F₅)₃was dissolved in 800 equiv. MMA

and toluene, followed by addition of 1 equiv. P^tBu₃in toluene

solution to start the polymerization. As can be seen from Fig. 5

(▲ line), after an initial slow induction period (∼2 min), the

monomer conversion increased almost linearly with reaction

time till completion of the reaction, thus showing a zero-order

dependence on [MMA]; this type of polymerization kinetics was

also observed in the MMA polymerization by other nucleophile/

electrophile pairs, such as zirconocene enolate/zirconocenium

cation,⁴³enolaluminate anion/organoaluminum,⁴⁴and silyl

ketene acetal/silylium cation⁴⁵pairs. On the other hand, in pro-

cedure B, 1 equiv. MMA·Al(C₆F₅)₃was ﬁrst premixed with 1

equiv. P^tBu₃in toluene for 10 min (to form the zwitterionic

species 1), followed by addition of another equiv. MMA·Al

(C₆F₅)₃+ 800 equiv. of MMA, and the resulting polymerization

system was considerably faster (by 55%); hence, after now a

much shorter induction period (∼1 min), the kinetic plot showed

a linear increase of monomer conversion with time, see Fig. 5

(■ line). There also showed a signiﬁcant effect of the LA con-

centration on the polymerization rate. For example, using the

same procedure (B) but lowering the total amount of the alane

from 2 equiv. to 1.8 equiv., the polymerization was slowed by

23%, see Fig. 5 (● line); consistent with this observation,

increasing the amount of the alane to 2.5 equiv., the polymeriz-

ation became faster by approximately 50%. These results

showed that this polymerization is strongly catalyzed by LA,

which is consistent with the proposed bimolecular, activated

monomer propagation mechanism (vide infra).

Neither the Al(C₆F₅)₃/P^tBu₃pair nor the Al(C₆F₅)₃/IMes pair

exhibited any reactivity toward polymerization of a bulky metha-

crylate, FMA, up to 24 h (runs 8–9, Table 2). Moving to an acry-

late monomer, polymerization of ⁿBA by Al(C₆F₅)₃/P^tBu₃

achieved 33% conversion in 1 h (run 10); however, there was no

further signiﬁcant increase in conversion, even after 24 h at

which point the conversion was still only 35%, indicating sub-

stantial catalyst deactivation in this acrylate polymerization. On

the other hand, polymerization of an acrylamide monomer,

DMAA, by Al(C₆F₅)₃/P^tBu₃with [DMAA]/[LB] = 800 achieved

quantitative monomer conversion in 1 min, affording a high

TOF of 4.8 × 10⁴h⁻¹and a high MW polymer with M_n= 2.93 ×

10⁵g mol⁻¹, PDI = 1.43 (run 11, Table 2). Likewise, the

Al(C₆F₅)₃/I^tBu pair is also highly active for DMAA polymeriz-

ation, producing PDMAA with even higher MW of M_n= 3.69 ×

10⁵g mol⁻¹and lower PDI of 1.28 (run 12). In comparison, the

polymerization of DPAA by Al(C₆F₅)₃/P^tBu₃([DPAA]/[LB] =

200) was much slower, requiring 24 h to achieve quantitative

monomer conversion. Nonetheless, the PDPAA produced has a

high MW of M_n= 3.57 × 10⁵g mol⁻¹and PDI = 1.31 (run 13,

Table 2). Polymerization of a vinyl phosphonate monomer,

DEVP, by Al(C₆F₅)₃/P^tBu₃was active but sluggish (run 14),

while the polymerization by Al(C₆F₅)₃/IMes was signiﬁcantly

faster (run 15).

Moving onto cyclic esters and lactones, Al(C₆F₅)₃/P^tBu₃

(0.125 mol% catalyst) exhibited moderate activity toward polymer-

ization of ε-CL, achieving 58% conversion after 20 h (run 16,

Table 2). On the other hand, none of the ﬁve-membered lactones

investigated, including γ-BL, γ-VL, and α-AL, were polymerized

by the current LPs, such as Al(C₆F₅)₃/P^tBu₃and Al(C₆F₅)₃/IMes,

under current ambient conditions (runs 17–20, Table 2).

Scope of monomer. To expand the utility of LPP in polymer

synthesis, we examined the polymerization of 11 other

This journal is © The Royal Society of Chemistry 2012

Dalton Trans., 2012, 41, 9119–9134 | 9127

Table 2 Polymerization results by Al(C₆F₅)₃/LB pairs^a

Run no.

Monomer (M)

LB (1 eq.)

[M]/[LB]

Solvent

Time (min)

Conv.^b(%)

TOF (h⁻¹

)

10⁻⁴M_n^c(g mol⁻¹

)

PDI^c(M_w/M_n)

1

2

3

4

5

6

7

8

MBL

P^tBu₃

I^tBu

800

400

800

400

800

200

800

400

CH₂Cl₂

Toluene

CH₂Cl₂

Toluene

60

10

1

0.5

1

1440

60

(90.5)

(85.5)

100

0

760

704

4800

48 000

0

4.48

16.3 (>96%)

19.2

13.9

6.28

7.35 (>95%)

13.9; 5.01

—

n.d.

29.3

36.9

35.7

n.d.

2.79

7.37

—

2.18

1.28

2.28

1.15

1.42

1.22

1.03; 1.04

—

γ-MMBL

FMA

P^tBu₃

I^tBu

IMes

TPT

PPh₃

P^tBu₃

IMes

P^tBu₃

I^tBu

9

FMA

0

—

10

11

12

13

14

15

16

17

18

19

20

ⁿBA

33.0

100

79.3

75.6

58.0

0

264

48 000

32 000

67

6

10

23

0

n.d.

1.43

1.28

1.31

n.d.

2.11

2.76

—

DMAA

DPAA

DEVP

ε-CL

γ-BL

γ-VL

α-AL

1

1.5

300

1740

960

1200

120

P^tBu₃

IMes

P^tBu₃

IMes

P^tBu₃

0

—

0

^aConditions: [LA]/[LB] = 2; room temperature (∼25 °C); n.d. = not determined. ^bMonomer conversions measured by ¹H NMR; values in parentheses

were isolated yields. ^cNumber-average molecular weight (M_n) and polydispersity index (PDI) determined by GPC relative to PMMA standards;

values (percentages) in parentheses were the percentage of the major peak when the other minor shoulder peak showed up on the GPC trace.

Table 3 Polymerization results by LA/LB pairs^a

Conv.^b

(%)

TOF

10⁻⁴M_n

PDI^c

(M_w/M_n)

[rr]^d

(%)

c

LA

(2 eq.)

LB

(1 eq.)

Monomer

(M)

Temp

(°C)

Time

(min)

Run no.

[M]/[LB]

Solvent

(h⁻¹

)

(g mol⁻¹

)

1

2

3

4

5

6

7

8

B(C₆F₅)₃

MeAl(BHT)₂

AlMe₃

AlEt₃

AlCl₃

ClAl(C₆F₅)₂

MAO

P^tBu₃

PPh₃

P^tBu₃

IMes

P^tBu₃

IMes

none

P^tBu₃

I^tBu

MMA

400

800

400

40

Toluene

CH₂Cl₂

Toluene

DMF

25

–78

25

–78

25

1440

600

1440

1080

0

—

Trace

97.3

100

66.7

100

32.8

36.4

22.5

45.4

30.6

93.2

95.6

0

∼0

1.6

1.7

22

80

5.5

6.1

3.8

7.6

n/a

15

16

0

1.17

3.86

2.55

27.3

1.84 (98%)

0.52

1.64 (98%)

2.47

1.35 (99%)

0.79 (99%)

—

2.25

1.60

1.91

2.12

1.42

1.43

1.36

2.06

1.29

1.26

1.27

—

67.8

87.5

68.9

91.1

n.d.

n/a

—

40

800

400

n.a.

400

800

200

9

MMA

10

11

12

13

14

15

16

17

18

γ-MMBL

MMA

γ-MMBL

MAO

Et₃Si⁺

P^tBu₃

IMes

I^tBu

ⁱPr₃Si⁺

0

—

C₆₀

^aConditions: [LA]/[LB] = 2, except for runs by MAO (12–14) where 100 equiv. was used. Silylium ions R₃Si⁺[B(C₆F₅)₄]⁻were generated in situ

from R₃SiH + Ph₃C[B(C₆F₅)₄]. n/a = not applicable; n.d. = not determined. ^bMonomer conversions measured by ¹H NMR. ^cNumber-average

molecular weight (M_n) and polydispersity index (PDI) determined by GPC relative to PMMA standards; values (percentages) in parentheses were the

percentage of the major peak when the other minor (trace) shoulder peak showed up on the GPC trace. ^dPolymer methyl triads (rr, mr, mm) measured

by ¹H NMR.

P^tBu₃, for MMA polymerization, but observed none to negli-

Scope of Lewis acids and bases. As described above, LPs

based on the strong LA Al(C₆F₅)₃are highly active for polymer-

ization of conjugated polar alkenes such as methacrylates, acryl-

amides, and α-methylene-γ-butyrolactones. An interesting

fundamental and practical question is whether this LA could be

replaced by other more common LAs without compromising

polymerization activity. To address this question, we initially

examined common LAs such as B(C₆F₅)₃, MeAl(BHT)₂, and

AlMe₃, in combination with the highly effective LB partner,

gible polymer formation, up to 24 h (runs 1–3, Table 3). The

ﬁnding of the inactivity of the B(C₆F₅)₃/P^tBu₃pair, even with

another equiv. of B(C₆F₅)₃, is intriguing because MMA·B

(C₆F₅)₃readily reacts with P^tBu₃to form zwitterionic phos-

t

phonium enolborate Bu₃P–CH₂C(Me)vC(OMe)O–B(C₆F₅)₃.¹⁶

This observation is reminiscent of our previous ﬁndings regard-

ing the high activity of enolaluminate vs. inactivity of enolborate

species toward conjugate-addition polymerization, attributed to

9128 | Dalton Trans., 2012, 41, 9119–9134

This journal is © The Royal Society of Chemistry 2012

the inability of the enolborate/borane pair to effect the bimolecu-

lar, activated-monomer anionic polymerization as does the enola-

luminate/alane pair.^9,38With a 10-fold increase in the LP loading

(2.5 mol%), the AlEt₃/PPh₃pair is active for MMA polymeriz-

ation at both RT and −78 °C, albeit the TOF numbers were low

(<2 h⁻¹, runs 4–5), which is consistent with the results reported

by Hatada et al.⁴⁶The syndiotacticity of the PMMA produced at

−78 °C, 87.5% rr, is considerably higher than that produced at

RT, 67.8% rr, so is the polymer molecular weight (run 5 vs. 4).

For LPP of MMA, AlCl₃is much more effective than AlR₃.

Thus, even in a low catalyst loading of 0.125 mol%, the AlCl₃/

P^tBu₃pair exhibited modest activity in MMA polymerization,

achieving 66.7% conversion in 24 h (run 6). Kinetic proﬁling of

this polymerization showed that 66.4% conversion was already

achieved within 1.5 min and there was no noticeable increase in

conversion thereafter (up to 24 h), the observation of which indi-

cated rapid catalyst deactivation, as indicated by the instability of

the AlCl₃-derived zwitterionic species (see Experimental).

Keeping this in mind, we ran the same polymerization at −78 °C

and achieved quantitative monomer conversion in 10 h (run 7).

More signiﬁcantly, the MW of the PMMA produced at −78 °C

(M_n= 2.73 × 10⁵g mol⁻¹) is more than 10 times higher than

that produced at RT (run 7 vs. 6). Impressively, this low tempera-

ture polymerization produced highly syndiotactic PMMA

(91.1% rr).

Compared to Al(C₆F₅)₃, ClAl(C₆F₅)₂based LPs are much less

active for both polymerizations of MBL and γ-MMBL (runs

8–11, Table 3). Speciﬁcally, the ClAl(C₆F₅)₂/LB pairs achieved

low monomer conversions of <50%, even after extended times

(14 h). Based on kinetic proﬁling experiments, there was sub-

stantial catalyst deactivation after an initial stage of polymeriz-

ation (1 min), after which no signiﬁcant increase in monomer

conversion occurred. Considering methylaluminoxane (MAO) as

a potent Lewis acidic activator for oleﬁn polymerization by

metal-based catalysts,⁴⁷we examined MAO-based LPs for

γ-MMBL polymerization. In absence of a LB, MAO (100

equiv.) itself is active for γ-MMBL (400 equiv.) polymerization

at RT, achieving 30.6% conversion after 24 h (run 12). With an

additional of 1 equiv. of P^tBu₃, the conversion was enhanced to

93.2% in the same period, producing the polymer with a rela-

tively low MW of M_n= 7.89 × 10³g mol⁻¹(run 13, Table 3).

Using an NHC base, the MAO/I^tBu pair performed similarly in

this polymerization (run 14 vs. 13, Table 3).

run 1, Table 4) than P^tBu₃(TOF = 1.2 × 10⁴h⁻¹). In addition,

the polymer produced exhibited a bimodal MWD, with M_n=

3.70 × 10⁵g mol⁻¹, PDI = 1.13 (56%) and M_n= 1.29 × 10⁶g

mol⁻¹, PDI = 1.01 (44%). CP-2, a benzene-bridged diphosphine,

showed a 3.5-fold activity enhancement over CP-1 (run 2 vs. 1),

but it is still less active than the simple P^tBu₃. Impressively,

CP-3, an ethylene-bridged diphosphine, is 4 times more active

than P^tBu₃, achieving a high TOF of 4.8 × 10⁴h⁻¹(run 3,

Table 4). The PMMA produced also had a high MW of M_n=

2.74 × 10⁵g mol⁻¹and a unimodal MWD (PDI = 1.40). The

analogous 2,3-butane-bridged chiral diphosphine, CP-4, is also

highly active, achieving a high TOF of 1.6 × 10⁴h⁻¹and a high

MW of M_n= 3.68 × 10⁵g mol⁻¹(PDI = 1.48, run 4, Table 4). In

all cases, the syndiotacticity of PMMA produced by these chiral

LB/LA pairs was about 74–75% rr, showing no enhancement

over the PMMA produced by the achiral Al(C₆F₅)₃/P^tBu₃pair.

Superbases, P₄-^tBu and P₂-^tBu (Chart 1), were also investi-

gated as LBs for LPP of MMA. The Al(C₆F₅)₃/P₄-^tBu pair

exhibited the highest activity amongst all LPs investigated in this

study, giving the highest TOF of 9.6 × 10⁴h⁻¹(i.e., consuming

all 800 equiv. of monomer in 30 s!) and also producing high

MW PMMA with M_n= 2.12 × 10⁵g mol⁻¹and narrow MWD

of PDI = 1.34 (run 5, Table 4). The Al(C₆F₅)₃/P₂-^tBu pair is also

highly active with TOF = 4.8 × 10⁴h⁻¹(run 6, Table 4). On the

other hand, the LPs of Al(C₆F₅)₃with two tertiary amines, NPh₃

and NEtⁱPr₂, showed no activity toward polymerization of MMA

up to 24 h (runs 7–8). The Al(C₆F₅)₃/NPh₃pair is also inactive

for the polymerization of the more reactive monomer γ-MMBL

(run 9), but the Al(C₆F₅)₃/NEtⁱPr₂pair exhibits good activity for

polymerization of γ-MMBL, achieving quantitative conversion

in 5 h and giving a TOF of 160 h⁻¹(run 10, Table 4).

Computational studies of zwitterionic active species formation,

chain initiation and propagation

This section focuses on the mechanistic aspects of Al(C₆F₅)₃-

based LPs as catalysts in MMA polymerization. The LB/Al

(C₆F₅)₃pairs, LB = phosphines (P^tBu₃, PMes₃and PPh₃) and

NHC (IMes), were examined for their formation of the active

species upon reaction with the monomer MMA as well as sub-

sequent MMA addition to the formed zwitterionic active species.

Zwitterion formation and stability. We started from MMA

coordination to the Al atom of Al(C₆F₅)₃to form the alane-acti-

vated monomer adduct [Al]–MMA. Next, we considered the

reaction of the adduct with different LBs to form the zwitterionic

active species [Al]–MMA–LB. The energetics of these steps is

shown in Scheme 2. Here we report the energy values obtained

with both BP86 and M06 functionals. The former is known to

underestimate binding energy but offers a balanced approach

when the binding of different large ligands is compared. Conver-

sely, the latter predicts much more reasonable binding energies,

but has some tendency to overestimate the binding energy of

large ligands in solution.⁴⁹

Despite the unfavorable entropic contribution due to MMA

coordination to Al(C₆F₅)₃, formation of the [Al]–MMA adduct,

which was previously isolated and structurally characterized,^9,25

is strongly favored, with a binding energy of the alane to MMA

of 25.7 and 38.2 kcal mol⁻¹at the BP86 and M06 levels,

Interestingly, strongly Lewis acidic silylium ions R₃Si⁺-

[B(C₆F₅)₄]⁻, generated in situ from the reaction of R₃SiH +

Ph₃C[B(C₆F₅)₄], are inactive as LAs for LPP of MMA polymer-

ization in either toluene or CH₂Cl₂at RT, when combined with

P^tBu₃(runs 15–16, Table 3). We also examined C₆₀, a known

Lewis acid,⁴⁸in combination with NHC bases, for LPP of MMA

and γ-MMBL; no polymerization was observed up to 24 h in

different solvents (runs 17–18, Table 3).

Turning our attention to the scope of Lewis bases and consid-

ering the success of the Al(C₆F₅)₃/P^tBu₃pair for LPP of various

types of monomers, we were interested in the potential of chiral

chelating phosphines for enhancements in activity and/or stereo-

selectivity in LPP. To this end, four chiral phosphines (Chart 1)

were selected and investigated for MMA polymerization as LB

components of LPs with Al(C₆F₅)₃. In comparison, CP-1, a

chiral biNAP diphosphine, is much less active (TOF = 224 h⁻¹

,

Dalton Trans., 2012, 41, 9119–9134 | 9129

Table 4 Polymerization results by Al(C₆F₅)₃/LB pairs^a

Run no. LB (1 eq.) Monomer (M) [M]/[LB] Solvent Time (min) Conv.^b(%) TOF (h⁻¹

)

10⁻⁴M_n^c(g mol⁻¹

)

PDI^c(M_w/M_n) [rr]^d(%)

1

2

3

4

5

6

7

8

9

10

CP-1

CP-2

CP-3

MMA

800

400

800

Toluene 210

Toluene 60

98.0

98.3

100

0

100

224

786

48 000

16 000

96 000

48 000

0

160

37.0 (56%)

25.8

27.4

36.8

21.2

4.82

—

9.77

1.13

1.83

1.40

1.48

1.34

1.64

—

1.90

74.7

73.8

73.7

74.2

72.4

72.8

—

n/a

Toluene

1

3

CP-4

P₄-^tBu

P₂-^tBu

NPh₃

Toluene 0.5

CH₂Cl₂1440

CH₂Cl₂300

NEtⁱPr₂

NPh₃

γ-MMBL

NEtⁱPr₂

^aConditions: [LA]/[LB] = 2, LA = Al(C₆F₅)₃as a toluene adduct; RT (∼25 °C). n/a = not applicable. ^bMonomer conversions measured by ¹H NMR.

^cNumber-average molecular weight (M_n) and polydispersity index (PDI) determined by GPC relative to PMMA standards; values (percentages) in

parentheses were the percentage of the major peak when the other minor shoulder peak showed up on the GPC trace. ^dPolymer methyl triads (rr, mr,

mm) measured by ¹H NMR.

Scheme 2 Energetics (kcal mol⁻¹) of zwitterion formation with different LBs, as predicted by the BP86 and M06 functionals. Separated MMA,

Al(C₆F₅)₃and LB is assumed as the reference system at 0 kcal mol⁻¹

.

respectively (Scheme 2). This result was expected considering

the extremely high Lewis acidity of Al(C₆F₅)₃.^7,11,50The ener-

getics of the zwitterion formation largely depends on the LB and

the functional considered. In fact, as shown in Scheme 2,

binding of IMes to the [Al]–MMA adduct is strongly favored,

with the zwitterionic species [Al]–MMA–IMes more than

50 kcal mol⁻¹below the starting species, with a net gain of

roughly 26 kcal mol⁻¹from the [Al]–MMA adduct, while phos-

phines bind much less strongly to the [Al]–MMA adduct, with

the binding strongly inﬂuenced by the functional. For instance,

the BP86 functional predicts an energy gain of only 0.4 for

P^tBu₃, whereas binding of PPh₃and PMes₃to the [Al]–MMA

adduct is unfavored by 1.2 and 19.4 kcal mol⁻¹, respectively.

Considering an unfavorable entropic contribution, not considered

in the present case, the [Al]–MMA–PR₃zwitterion is predicted

to be unstable at the BP86 level. Differently, a sizeable zwitter-

ion energy formation is predicted by the M06 functional, with

the binding of all the LBs to the [Al]–MMA moiety being

roughly 15 kcal mol⁻¹stronger. Consequently, even including an

unfavorable entropic contribution, formation of [Al]–MMA–PR₃

Fig. 6 DFT optimized geometry of the Al(C₆F₅)₃–MMA–PMes₃

zwitterion.

PMes₃zwitterion can be easily explained considering the opti-

mized geometry shown in Fig. 6, where repulsive steric inter-

action between the methyl substituents on the mesityl rings and

the phenyl groups of Al(C₆F₅)₃, due to the short distances

between these groups, are responsible for the zwitterion

instability.

t

(R = Bu, Ph) zwitterions is favored with the M06 functional,

If similar stability of the [Al]–MMA–PPh₃and [Al]–MMA–

P^tBu₃zwitterionic species was expected, there would be no

meaningful steric interaction between the alane and the base that

can justify the different stability of the zwitterions derived from

IMes and PR₃. Indeed, the energy gain associated with addition

of the small PMe₃to the [Al]–MMA adduct is 7.4 and 14.6 kcal

mol⁻¹at the BP86 and M06 levels, which indicates that steric

repulsion somewhat destabilizes the binding of the larger P^tBu₃

while PMes₃is unbound to the adduct, in agreement with exper-

iments. It is worth noting that the relative stability of the phos-

phine zwitterion species predicted by both the BP86 and the

M06 functionals is in good agreement with the experimental

results, which showed somewhat greater stability of the P^tBu₃

zwitterion species relative to the PPh₃zwitterion as well as no

zwitterion formation in the case of PMes₃and no polymerization

activity by the Mes₃P/Al(C₆F₅)₃pair. The instability of the

9130 | Dalton Trans., 2012, 41, 9119–9134

Table 5 Natural bond orbital and bond strength analysis of [Al]–

MMA–PPh₃and [Al]–MMA–IMes zwitterionic species

[Al]–MMA–PPh₃[Al]–MMA–IMes

Bond length C_MMA–LB (Å)

Bond order C_MMA–LB

Bond length O_MMA–Al (Å)

Bond order O_MMA–Al

Charge on Al(C₆F₅)₃(electrons) −0.21

Charge on MMA (electrons)

Charge on LB (electrons)

1.94

0.83

1.83

0.32

1.50

1.04

1.83

0.33

−0.21

−0.60

0.81

−0.70

0.91

and PPh₃phosphines with the BP86 functional, and that the

main source of stabilization with the M06 functional is disper-

sive in nature (at the M06 level P^tBu₃binds more strongly than

PMe₃to [Al]–MMA). Analysis of the bond strength and charge

distribution on PPh₃and IMes zwitterions clearly show that the

higher stability of the NHC zwitterion mainly arises from elec-

tronic effects (see Table 5). As shown in Table 5, for both the

PPh₃and IMes zwitterions the distance between the coordinated

C atom of MMA and the coordinating P or C atom of the PPh₃

and IMes is close to a classical P–C or C–C σ bond, 1.84 and

1.54 Å, respectively. However, the C_NHC–C_MMAbond order is

1.04 in case of IMes, whereas, in case of the P–C_MMAbond with

PPh₃, the order is lower, around 0.83. This result indicates that

the MMA–IMes interaction in the zwitterion has a very strong

covalent character, while the MMA–PPh₃interaction in the

zwitterion is more dative in nature. The Al–MMA interaction in

the zwitterion, instead, is quite similar for both PPh₃and IMes.

Natural Population Analysis (NPA) shows a slightly larger

charge separation for the [Al]–MMA–PPh₃with a greater posi-

tive charge on PPh₃, compared to the charge on IMes in the cor-

responding zwitterionic species. This analysis is consistent with

a greater ionic character of the C_MMA–PPh₃bond relative to the

Scheme 3 Energetics (kcal mol⁻¹) of the ﬁrst MMA addition to a pre-

formed zwitterion with the BP86 and the M06 functionals.

C

_MMA–IMes bond.

The weaker stability of the PR₃-based zwitterions could be

related to the experimentally observed bimodal molecular weight

distribution of the PMMA obtained when the Ph₃P/Al(C₆F₅)₃

pair is employed as catalyst.¹⁶The formation of more than one

active species could just arise from the decomposition of the

[Al]–MMA–PPh₃zwitterion.

Fig. 7 Transition state geometries of MMA addition to the Al(C₆F₅)₃–

MMA–PPh₃zwitterion proceeding via the monometallic (a) and bime-

tallic (b) mechanisms.

MMA chain initiation. We move now to the addition of the

ﬁrst MMA molecule to the active [Al]–MMA–LB species for

LB = PPh₃, P^tBu₃and IMes, considering the two most probable

mechanisms shown in Scheme 3. In the ﬁrst pathway the zwitter-

ion attacks an Al-free MMA molecule (monometallic mechan-

ism, Scheme 3a), while in the second the zwitterion attacks an

Al-activated MMA molecule, namely the preformed alane–

monomer adduct (bimetallic mechanism, Scheme 3b). The

energy barriers obtained with the BP86 and M06 functionals for

the two mechanisms are also reported in Scheme 3.

The most stable transition states for MMA addition along the

two reaction pathways shown in Scheme 3 are compared in

Fig. 7 for the case of PPh₃-based zwitterionic species. Although

the bimetallic TS displays a more crowded geometry, see the

short distance between the LB and the Al(C₆F₅)₃moiety coordi-

nated to the monomer in Fig. 7b, the numbers in the Scheme 3

show clearly that whatever LB is considered, the bimetallic

mechanism is strongly favored, with an energy barrier roughly

15–20 kcal mol⁻¹lower than the energy barrier calculated for

the monometallic mechanism. Both the BP86 and the M06 func-

tionals performed consistently in this case.

The most notable difference between the M06 and BP86

results is that the BP86 functional, for both mechanisms, pre-

dicted energy barriers roughly 10–15 kcal mol⁻¹higher than the

corresponding energy barriers predicted by the M06 functional.

Furthermore, the M06 functional predicted a negligible or even

negative (LB = PPh₃and IMes) energy barrier in case of the

bimetallic mechanism. This result is expected based on the pre-

viously discussed differences between the two functionals, with

a tendency of the M06 functional to overestimate the dispersive

interactions, which are generated at the level of the TS by the

close proximity of the zwitterions and the monomer.⁵¹However,

Dalton Trans., 2012, 41, 9119–9134 | 9131

away from the reacting atoms by the added methylene unit on

the growing chain, mainly contributes to the reduced energy

barrier for MMA addition.

Conclusions

Highly active and effective LPP systems have been achieved

with classical and frustrated LPs based on the strong Lewis acid

Al(C₆F₅)₃. Such CLPs and FLPs are especially effective for

polymerizing conjugated polar alkenes such as MMA, acryl-

amides and especially renewable α-methylene-γ-butyrolactones

MBL and γ-MMBL, into high MW polymers with relatively

narrow MWDs. Investigations into the scope of LAs show that

although some common LAs can also be utilized for LPP,

Al(C₆F₅)₃-based LPs are far more active and effective than other

LA-based LPs. On the other hand, the scope of LBs is not

limited, and several types of LBs can also be utilized for estab-

lishing highly active and effective LPP systems employing

Al(C₆F₅)₃. Such highly effective LBs include (listed here only

MMA polymerization data for comparison): achiral phosphine

P^tBu₃(TOF = 1.2 × 10⁴h⁻¹M_n= 3.97 × 10⁵g mol⁻¹, PDI =

1.52), chiral ethylene-bridged diphosphine CP-3 (TOF = 4.8 ×

10⁴h⁻¹, M_n= 2.74 × 10⁵g mol⁻¹, PDI = 1.40), NHC bases

IMes (TOF = 4.8 × 10⁴h⁻¹, M_n= 2.66 × 10⁴g mol⁻¹, PDI =

1.77), I^tBu (TOF = 3.2 × 10³h⁻¹, M_n= 5.25 × 10⁵g mol⁻¹, PDI

= 1.43), and TPT, as well as the superbase P₄-^tBu, which is most

active (TOF = 9.6 × 10⁴h⁻¹, M_n= 2.12 × 10⁵g mol⁻¹, PDI =

1.34). These LPs are also highly active and effective for

polymerization of γ-MMBL, observing the similar effect of the

NHC base structure on the resulting polymer MW (e.g., the MW

of the polymer by I^tBu is much higher than that by IMes). In

terms of stereoselectivity of these LPP systems, the polymers

produced at RT are typically atactic (PMMBL with ∼47% mr) or

syndio-rich (PMMA with ∼70–75% rr). The use of chiral chelat-

ing phosphine bases did not noticeably affect the stereoselectiv-

ity. However, highly syndiotactic PMMA with 91.% rr can be

produced by chiral or achiral LPs at −78 °C.

Mechanistic studies have identiﬁed and structurally character-

ized zwitterionic phosphonium and imidazolium enolaluminate

species, formed by the reaction of the monomer·Al(C₆F₅)₃

adduct with P^tBu₃and NHCs, respectively. Such zwitterions

were proposed to be the active species for the polymerization

because they rapidly polymerize the subsequently added

monomer. Kinetic studies have revealed that the MMA polymer-

ization by the most representative LP of the current systems,

^tBu₃P/Al(C₆F₅)₃is zero-order in monomer concentration after an

initial induction period and the polymerization is signiﬁcantly

catalyzed by the LA, conforming to a bimolecular, activated

monomer propagation mechanism.

Computational work has examined the energetics of the zwit-

terionic active species formation as well as chain initiation and

propagation fundamental steps involved in LPP. Consistent with

experimental results, formation of the zwitterion is favored with

all the LBs considered, with the exception of PMes₃, due to the

bulkiness of the mesityl groups. Analysis of the chain initiation

step indicated that the bimetallic mechanism, with addition of a

LA-activated MMA molecule to the zwitterion, is clearly

favored over the monometallic mechanism involving addition of

a non-activated MMA. Analysis of a model of the MMA

Scheme 4 Energetics (kcal mol⁻¹) of the generic MMA addition to a

preformed zwitterion with the BP86 and the M06 functionals.

of paramount relevance here is the calculated preference for the

bimetallic mechanism, whatever functional is considered, which

is in agreement with the experimental data showing that the rate

of MMA polymerization increases signiﬁcantly by increasing the

[Al]/LB ratio, and that the MMA reaction starts with a negligible

induction period when the preformed zwitterion and [Al]–MMA

adduct is employed (vide supra).

MMA chain propagation step. In this section we describe the

reaction proﬁle for MMA addition in a propagation step in the

case of a long PMMA chain. The generic propagating active

species was simulated by adding a methylene unit between the

LB and the last inserted MMA molecule with a coordinated [Al]

moiety, see Scheme 4. The addition of the methylene unit breaks

connection between the LB and the [Al] through π-bonds,

leaving only a σ-bond connection. The energy proﬁles for the

mono- and the bimetallic MMA additions using PPh₃, P^tBu₃and

IMes LBs are schematically shown in Scheme 4.

As anticipated, also in the chain growth step addition of the

[Al]–MMA adduct is strongly favored, compared to the addition

of non-activated MMA for all the LP systems considered. In

fact, the numbers in Scheme 4 show a clear preference for the

bimetallic mechanism with both functionals considered.⁵²Com-

parison between the initiation step, Scheme 3, and the generic

chain growth step, Scheme 4, indicates that in the generic chain

growth step there is a reduction of roughly 10 kcal mol⁻¹in the

energy barriers for both the mono and the bimetallic mechan-

isms, although this decrease is somewhat smaller in case of the

M06 functional. The reduction of repulsive steric interaction

between the Al(C₆F₅)₃moiety coordinated to the monomer and

the LB bonded to the growing chain, the latter being pushed

9132 | Dalton Trans., 2012, 41, 9119–9134

J. M. Farrell, C. C. Brown, Z. M. Heiden, G. C. Welch and M. Ullrich,

Inorg. Chem., 2011, 50, 12338–12348; (c) D. Chen, Y. Wang and

J. Klankermayer, Angew. Chem., Int. Ed., 2010, 49, 9475–9478; (d) Z.

M. Heiden and D. W. Stephan, Chem. Commun., 2011, 47, 5729–5731;

(e) D. Chen, Y. Wang and J. Klankermayer, Angew. Chem., Int. Ed.,

2010, 49, 9475–9478; (f) G. Erős, H. Mehdi, I. Pápai, T. A. Rokob,

P. Király, G. Tárkányi and T. Soós, Angew. Chem., Int. Ed., 2010, 49,

6559–6563; (g) A. J. M. Miller, J. A. Labinger and J. E. Bercaw, J. Am.

Chem. Soc., 2010, 132, 3301–3303; (h) A. W. Ashley, A. L. Thompson

and D. O’Hare, Angew. Chem., Int. Ed., 2009, 48, 9839–9843; (i) K.

V. Axenov, G. Kehr, R. Fröhlich and G. Erker, J. Am. Chem. Soc., 2009,

131, 3454–3455.

propagation step conﬁrmed the preference for the bimetallic

mechanism, with an overall reduction of the addition energy

barrier, which is a consequence of the reduced steric stress

induced by the more spatially separated LA and LB centers. The

main difference between the NHC and the PR₃based LPP

systems is in the relative stability of the zwitterion, while the be-

havior in MMA addition is remarkably similar.

Acknowledgements

5 For selected recent examples on new reactivity/reaction development with

FLPs, see: (a) B.-H. Xu, C. M. Mömming, R. Fröhlich, G. Kehr and

G. Erker, Chem.–Eur. J., 2012, 18, 1826–1830; (b) A. J. P. Cardenas, B.

J. Culotta, T. H. Warren, S. Grimme, A. Stute, R. Fröhlich, G. Kehr and

G. Erker, Angew. Chem., Int. Ed., 2011, 50, 7567–7571; (c) A.

M. Chapman, M. F. Haddow and D. F. Wass, J. Am. Chem. Soc., 2011,

133, 8826–8829; (d) G. Ménard and D. W. Stephan, Angew. Chem., Int.

Ed., 2011, 50, 8396–8399; (e) X. Zhao and D. W. Stephan, J. Am. Chem.

Soc., 2011, 133, 12448–12450; (f) C. M. Mömming, G. Kehr,

R. Fröhlich and G. Erker, Chem. Commun., 2011, 47, 2006–2007;

(g) G. Ménard and D. W. Stephan, J. Am. Chem. Soc., 2010, 132, 1796–

1797; (h) M. Alcarazo, C. Gomez, S. Holle and R. Goddard, Angew.

Chem., Int. Ed., 2010, 49, 5788–5791; (i) A. Berkefeld, W. E. Piers and

M. Parvez, J. Am. Chem. Soc., 2010, 132, 10660–10661; ( j) C.

M. Mömming, G. Kehr, B. Wibbeling, R. Fröhlich, B. Schirmer,

S. Grimme and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 2414–2417.

6 E. Y.-X. Chen, in e-EROS Encyclopedia of Reagents for Organic Syn-

thesis, John Wiley & Sons, Ltd, West Sussex, UK, 2011, DOI: 10.1002/

047084289X.rn01382.

7 E. Y.-X. Chen, W. J. Kruper, G. Roof and D. R. Wilson, J. Am. Chem.

Soc., 2001, 123, 745–746.

8 E. Y.-X. Chen and K. A. Abboud, Organometallics, 2000, 19, 5541–

5543.

9 A. D. Bolig and E. Y.-X. Chen, J. Am. Chem. Soc., 2001, 123, 7943–

7944.

10 D. Chakraborty and E. Y.-X. Chen, Macromolecules, 2002, 35, 13–15.

11 G. S. Hair, A. H. Cowley, R. A. Jones, B. G. McBurnett and A. Voigt,

J. Am. Chem. Soc., 1999, 121, 4922–4923.

This work was supported by the National Science Foundation

(CHE 1150792) for the study carried out at Colorado State Uni-

versity. MGJ thanks NSF-REU (CHE 1004924) for support of

summer research. LC thanks the HPC team of Enea (www.enea.

it) for using the ENEA-GRID and the HPC facilities CRESCO

(www.cresco.enea.it) in Portici, Italy. We thank Boulder Scien-

tiﬁc Co. for the research gifts of B(C₆F₅)₃and [Ph₃C][B(C₆F₅)₄],

and Dr. Brian Newell for the help on the X-ray structural

analysis.

References

1 For recent reviews, see: (a) D. W. Stephan and G. Erker, Angew. Chem.,

Int. Ed., 2010, 49, 46–76; (b) G. Erker, Dalton Trans., 2011, 40, 7475–

7483.

2 (a) J. S. J. McCahill, G. C. Welch and D. W. Stephan, Angew. Chem., Int.

Ed., 2007, 46, 4968–4971; (b) P. A. Chase, G. C. Welch, T. Jurca and D.

W. Stephan, Angew. Chem., Int. Ed., 2007, 46, 8050–8053; (c) G.

C. Welch and D. W. Stephan, J. Am. Chem. Soc., 2007, 129, 1880–1881;

(d) P. Spies, G. Erker, G. Kehr, K. Bergander, R. Fröhlich, S. Grimme

and D. W. Stephan, Chem. Commun., 2007, 5072–5074; (e) G. C. Welch,

R. R. S. Juan, J. D. Masuda and D. W. Stephan, Science, 2006, 314,

1124–1126.

12 (a) E. Y.-X. Chen, Chem. Rev., 2009, 109, 5157–5214; (b) E. Y.-X. Chen,

Dalton Trans., 2009, 8784–8793.

3 For selected recent examples on activation of small molecules by FLPs,

see: (a) F. Bertini, V. Lyaskovskyy, B. J. J. Timmer, F. J. J. de Kanter,

M. Lutz, A. W. Ehlers, J. C. Slootweg and K. Lammertsma, J. Am.

Chem. Soc., 2012, 134, 201–204; (b) A. Schäfer, M. Reiβmann,

A. Schäfer, W. Saak, D. Haase and T. Müller, Angew. Chem., Int. Ed.,

2011, 50, 12636–12638; (c) Z. Lu, Z. Cheng, Z. Chen, L. Weng, Z. H. Li

and H. Wang, Angew. Chem., Int. Ed., 2011, 50, 12227–122231;

(d) S. Kronig, E. Theuergarten, D. Holschumacher, T. Bannenberg, C.

G. Daniliuc, P. G. Jones and M. Tamm, Inorg. Chem., 2011, 50, 7344–

7359; (e) W. E. Piers, A. J. V. Marwitz and L. G. Mercier, Inorg. Chem.,

2011, 50, 12252–12262; (f) X. Zhao and D. W. Stephan, Chem.

Commun., 2011, 47, 1833–1835; (g) O. Ekkert, G. Kehr, R. Fröhlich and

G. Erker, J. Am. Chem. Soc., 2011, 133, 4610–4616;

(h) A. J. V. Marwitz, J. L. Dutton, L. G. Mercier and W. E. Piers, J. Am.

Chem. Soc., 2011, 133, 10026–10029; (i) R. C. Neu, E. Otten, A. Lough

and D. W. Stephan, Chem. Sci., 2011, 2, 170–176; ( j) C. Appelt,

H. Westenberg, F. Bertini, A. W. Ehlers, J. C. Slootweg, K. Lammertsma

and W. Uhl, Angew. Chem., Int. Ed., 2011, 50, 3925–3928;

(k) S. Grimme, H. Kruse, L. Goerigk and G. Erker, Angew. Chem., Int.

Ed., 2010, 49, 1402–1405; (l) B. Inés, S. Holle, R. Goddard and

M. Alcarazo, Angew. Chem., Int. Ed., 2010, 49, 8389–8391; (m) C.

M. Mömming, E. Otten, G. Kehr, R. Fröhlich, S. Grimme, D. W. Stephan

and G. Erker, Angew. Chem., Int. Ed., 2009, 48, 6643–6646; (n) M.

A. Dureen, G. C. Welch, T. M. Gilbert and D. W. Stephan, Inorg. Chem.,

2009, 48, 9910–9917; (o) M. A. Dureen, T. M. Gilbert and D.

W. Stephan, Chem. Commun., 2008, 4303–4305; (p) P. A. Chase and D.

W. Stephan, Angew. Chem., Int. Ed., 2008, 47, 7433–7437;

(q) D. Holschumacher, T. Bannenberg, C. G. Hrib, P. G. Jones and

M. Tamm, Angew. Chem., Int. Ed., 2008, 47, 7428–7432; (r) V. Sumerin,

F. Schulz, M. Nieger, M. Leskela, T. Repo and B. Rieger, Angew. Chem.,

Int. Ed., 2008, 47, 6001–6003.

13 (a) Y. Zhang, L. O. Gustafson and E. Y.-X. Chen, J. Am. Chem. Soc.,

2011, 133, 13674–13684; (b) Y. Hu, X. Xu, Y. Zhang, Y. Chen and E. Y.-

X. Chen, Macromolecules, 2010, 43, 9328–9336; (c) G. M. Miyake, S.

E. Newton, W. R. Mariott and E. Y.-X. Chen, Dalton Trans., 2010, 39,

6710–6718; (d) R. A. Cockburn, T. F. L. McKenna and R.

A. Hutchinson, Macromol. Chem. Phys., 2010, 211, 501–509;

(e) J. Mosnáček and K. Matyjaszewski, Macromolecules, 2008, 41,

5509–5511.

14 H. M. R. Hoffman and J. Rabe, Angew. Chem., Int. Ed. Engl., 1985, 24,

94–110.

15 (a) L. E. Manzer, ACS Symp. Ser., 2006, 921, 40–51; (b) L. E. Manzer,

Appl. Catal. A: Gen., 2004, 272, 249–256.

16 Y. Zhang, G. M. Miyake and E. Y.-X. Chen, Angew. Chem., Int. Ed.,

2010, 49, 10158–10162.

17 Y. Zhang, G. M. Miyake and E. Y.-X. Chen, Angew. Chem., Int. Ed.,

2012, 51, 2465–2469.

18 Group transfer polymerization of MMA initiated by silyl ketene acetals

and catalyzed by nucleophilic NHCs, IⁱPr and I^tBu, was recently reported

in: (a) J. Raynaud, Y. Gnanou and D. Taton, Macromolecules, 2009, 42,

5996–6005; (b) J. Raynaud, A. Ciolino, A. Baceiredo, M. Destarac,

F. Bonnette, T. Kato, Y. Gnanou and D. Taton, Angew. Chem., Int. Ed.,

2008, 47, 5390–5393; (c) M. D. Scholten, J. L. Hedrick and R.

M. Waymouth, Macromolecules, 2008, 41, 7399–7404.

19 Y. C. Kim, M. Jeon and S. Y. Kim, Macromol. Rapid Commun., 2005,

26, 1499–1503.

20 (a) D. Enders, K. Breuer, G. Raabe, J. Runsink, J. H. Teles, J. P. Melder,

K. Ebel and S. Brode, Angew. Chem., Int. Ed. Engl., 1995, 34, 1021–

1023; (b) D. Enders, K. Breuer, U. Kallfass and T. Balensiefer, Synthesis,

2003, 1292–1295.

21 A. P. Shreve, R. Mülhaupt, W. Fultz, J. Calabrese, W. Robbins and

S. Ittel, Organometallics, 1988, 7, 409–416.

22 D. Chakraborty and E. Y.-X. Chen, Inorg. Chem. Commun., 2002, 5,

698–701.

4 For selected recent examples on catalytic hydrogenation by FLPs, see:

(a) G. Erős, K. Nagy, H. Mehdi, I. Pápai, P. Nagy, P. Király, G. Tárkányi

and T. Soós, Chem.–Eur. J., 2012, 18, 574–585; (b) D. W. Stephan,

S. Greenberg, T. W. Graham, P. Chase, J. J. Hastie, S. J. Geier,

Dalton Trans., 2012, 41, 9119–9134 | 9133

23 (a) M. Bochmann and S. J. Lancaster, J. Organomet. Chem., 1992, 434,

C1–C5; (b) J. C. W. Chen, W.-M. Tsai and M. D. Rausch, J. Am. Chem.

Soc., 1991, 113, 8570–8571.

24 (a) S. Feng, G. R. Roof and E. Y.-X. Chen, Organometallics, 2002, 21,

832–839; (b) C. H. Lee, S. J. Lee, J. W. Park, K. H. Kim, B. Y. Lee and

J. S. Oh, J. Mol. Catal. A: Chem., 1998, 132, 231–239; (c) P. Biagini,

G. Lugli, L. Abis and P. Andreussi, U.S. Pat., 5,602269, 1997.

25 A. Rodriguez-Delgado and E. Y.-X. Chen, J. Am. Chem. Soc., 2005, 127,

961–974.

26 SHELXTL, Version 6.12, Bruker Analytical X-ray Solutions, Madison,

WI, 2001.

27 (a) A. Rodriguez-Delgado and E. Y.-X. Chen, Macromolecules, 2005, 38,

2587–2594; (b) A. D. Bolig and E. Y.-X. Chen, J. Am. Chem. Soc., 2004,

126, 4897–4906.

28 (a) A. Rodriguez-Delgado, W. R. Mariott and E. Y.-X. Chen, Macromol-

ecules, 2004, 37, 3092–3100; (b) F. A. Bovey and P. A. Mirau, NMR of

Polymers, Academic Press, San Diego, CA, 1996; (c) R. Subramanian,

R. D. Allen, J. E. McGrath and T. C. Ward, Polym. Prepr. (Am. Chem.

Soc., Div. Polym. Chem.), 1985, 26, 238–240.

29 (a) G. M. Miyake, Y. Zhang and E. Y.-X. Chen, Macromolecules, 2010,

43, 4902–4908; (b) J. Suenaga, D. M. Sutherlin and J. K. Stille, Macro-

molecules, 1984, 17, 2913–2916.

30 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,

J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson,

H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov,

J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota,

R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,

H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro,

M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov,

R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S.

S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J.

E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,

R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,

J. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth,

P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J.

B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, GAUSSIAN 09,

Gaussian, Inc., Wallingford, CT, 2009.

31 (a) A. D. Becke, Phys. Rev. A, 1988, 38, 3098–3100; (b) J. P. Perdew,

Phys. Rev. B, 1986, 33, 8822–8824; (c) J. P. Perdew, Phys. Rev. B, 1986,

34, 7406–7406.

32 F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys., 2005, 7, 3297–

3305.

33 (a) Y. Zhao and D. G. Truhlar, Theor. Chem. Acc., 2008, 120, 215–241;

(b) Y. Zhao and D. G. Truhlar, J. Chem. Phys., 2006, 125, 194101–

194118.

34 (a) J. Tomasi, B. Mennucci and R. Cammi, Chem. Rev., 2005, 105,

2999–3094; (b) V. Barone and M. Cossi, J. Phys. Chem. A, 1998, 102,

1995–2001.

35 M. Ullrich, K. S.-H. Seto, A. J. Lough and D. W. Stephan, Chem.

Commun., 2009, 2335–2337.

36 M. A. Dureen and D. W. Stephan, J. Am. Chem. Soc., 2009, 131, 8396–

8397.

37 B.-H. Xu, G. Kehr, R. Fröhlich, B. Wibbeling, B. Schirmer, S. Grimme

and G. Erker, Angew. Chem., Int. Ed., 2011, 50, 7183–7186.

38 Y. Ning, H. Zhu and E. Y.-X. Chen, J. Organomet. Chem., 2007, 692,

4535–4544.

39 P. A. Chase and D. W. Stephan, Angew. Chem., Int. Ed., 2008, 47, 7433–

7437.

40 D. Holschumacher, T. Bannenberg, C. G. Hrib, P. G. Jones and

M. Tamm, Angew. Chem., Int. Ed., 2008, 47, 7428–7432.

41 (a) A. T. Biju, M. Padmanaban, N. E. Wurz and F. Glorius, Angew.

Chem., Int. Ed., 2011, 50, 8412–8415; (b) S.-I. Matsuoka, Y. Ota,

A. Washio, A. Katada, K. Ichioka, K. Takagi and M. Suzuki, Org. Lett.,

2011, 13, 3722–3725.

42 Direct contact of DCM with Al(C₆F₅)₃must be avoided as it results in

decomposition leading to formation of [ClAl(C₆F₅)₂]₂: D. Chakraborty

and E. Y.-X. Chen, Inorg. Chem. Commun., 2002, 5, 698–701. On the

other hand, The Al(C₆F₅)₃-monomer adducts (either isolated or in situ

generated) are inert towards DCM.

43 (a) Y. Ning, M. J. Cooney and E. Y.-X. Chen, J. Organomet. Chem.,

2005, 690, 6263–6270; (b) G. Stojcevic, H. Kim, N. J. Taylor, T.

B. Marder and S. Collins, Angew. Chem., Int. Ed., 2004, 43, 5523–5526;

(c) Y. Li, D. G. Ward, S. S. Reddy and S. Collins, Macromolecules, 1997,

30, 1875–1883.

44 E. Y.-X. Chen, Dalton Trans., 2009, 8784–8793.

45 (a) Y. Zhang and E. Y.-X. Chen, Macromolecules, 2008, 41, 36–42;

(b) Y. Zhang and E. Y.-X. Chen, Macromolecules, 2008, 41, 6353–6360.

46 Some activity (TOF < 2 h^–1) was observed for MMA polymerization by

R₃P–Et₃Al at low temperatures (−78 °C or −93 °C): T. Kitayama,

E. Masuda, M. Yamaguchi, T. Nishiura and K. Hatada, Polym. J., 1992,

24, 817–827.

47 E. Y.-X. Chen and T. J. Mark, Chem. Rev., 2000, 100, 1391–1434.

48 (a) J. Iglesias-Sigüenza and M. Alcarazo, Angew. Chem., Int. Ed., 2012,

51, 2–4; (b) H. Li, C. Risko, J. H. Seo, C. Campbell, G. Wu, J.-L. Brédas

and G. C. Bazan, J. Am. Chem. Soc., 2011, 133, 12410–12413.

49 (a) H. Jacobsen and L. Cavallo, ChemPhysChem, 2012, 13, 562–569;

(b) S. Torker, D. Merki and P. Chen, J. Am. Chem. Soc., 2008, 130,

4808–4814; (c) D. Benitez, E. Tkatchouk and W. W. Goddard, Chem.

Commun., 2008, 6194–6196; (d) Y. Zhao and D. G. Truhlar, Org. Lett.,

2007, 9, 1967–1970.

50 (a) A. Y. Timoshkin and G. Frenking, Organometallics, 2008, 27, 371–

380; (b) K. Vanka, M. S. W. Chan, C. C. Pye and T. Ziegler, Organo-

metallics, 2000, 19, 1841–1849.

51 We have also tested the M05-2X functional in the calculation of the ﬁrst

transition state energy barrier. This functional is largely used in describ-

ing the reactivity with the FLP systems: (a) F. Huang, G. Lu, L. Zhao,

H. Li and Z.-X. Wang, J. Am. Chem. Soc., 2010, 132, 12388–12396;

(b) T. A. Rokob, A. Hamza and I. Pàpai, J. Am. Chem. Soc., 2009, 131,

10702–10710. The energy barrier for the bimetallic mechanism for the

IMes-based system increases to 0.2 kcal mol⁻¹with the M05-2X func-

tional with respect to −1.7 kcal mol⁻¹with the M06 functional.

52 For all systems the energy barriers calculated at BP86 level for the bime-

tallic mechanism are comparable to the energy barrier we calculated for

MMA addition with metallocene catalysts, see: (a) Y. Ning, L. Caporaso,

A. Correa, L. O. Gustafson, L. Cavallo and E. Y.-X. Chen, Macromol-

ecules, 2008, 41, 6910–6919; (b) L. Caporaso and L. Cavallo, Macromol-

ecules, 2008, 41, 3439–3445; (c) L. Caporaso, J. Gracia-Budrìa and

L. Cavallo, J. Am. Chem. Soc., 2006, 128, 16649–16654.

9134 | Dalton Trans., 2012, 41, 9119–9134

Article Doi

Lewis pair polymerization by classical and frustrated Lewis pairs: Acid, base and monomer scope and polymerization mechanism

DOI: 10.1039/c2dt30427a

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Article abstract of DOI:10.1039/c2dt30427a

Full text of DOI:10.1039/c2dt30427a

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