Anionic polymerization of MMA and renewable methylene butyrolactones by resorbable potassium salts

Article abstract of DOI:10.1002/pola.24628

Three resorbable potassium salts of hydride (K[H]), enolate Me ₂Ci￡/C(OⁱPr)OK (K[E]), and allyl K[1,3-(SiMe₃)₂C₃H₃] (K[A]) have been investigated for controlled anionic polymerization of methyl methacrylate (MMA) and its cyclic analogs, naturally renewable methylene butyrolactones including α-methylene-I3-butyrolactone (MBL) and I3-methyl-α- methylene-I3-butyrolactone (MMBL). When used alone at ambient temperature in toluene, these salts exhibit no (K[H]) to low (K[A]) to modest (K[E]) polymerization activity. Mixing of K[H] and Al(C₆F₅) ₃ leads to the formation of an "ate" complex, K ⁺[HAl(C₆F₅)₃]^-, which has been structurally characterized by X-ray diffraction; this complex has a high polymerization activity producing atactic PMMA, but addition of another equiv of Al(C₆F₅)₃ further enhances both the rate and the efficiency of the polymerization, now producing syndiotactic PMMA with a narrow molecular weight (MW) distribution of 1.04. The K[H]/2Al(C ₆F₅)₃ system also exhibits high activity for polymerization of (M)MBL. In sharp contrast, addition of Al(C₆F ₅)₃ to K[A] shuts down the polymerization at various temperatures. The most active, controlled, and syndioselective polymerization system in this series is K[E]/2Al(C₆F₅)₃. Accordingly, the polymerization control and kinetics of this most effective system have been examined in more detail.

Full text of DOI:10.1002/pola.24628

Anionic Polymerization of MMA and Renewable Methylene
Butyrolactones by Resorbable Potassium Salts
YANGJIAN HU, LAURA O. GUSTAFSON, HONGPING ZHU,* EUGENE Y.-X. CHEN
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872
Received 21 January 2011; accepted 8 February 2011
DOI: 10.1002/pola.24628
Published online 7 March 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Three resorbable potassium salts of hydride (K[H]),
enolate Me2C¼C(OiPr)OK (K[E]), and allyl K[1,3-(SiMe3)2C3H3]
(K[A]) have been investigated for controlled anionic polymer-
ization of methyl methacrylate (MMA) and its cyclic analogs,
naturally renewable methylene butyrolactones including a-
methylene-c-butyrolactone (MBL) and c-methyl-a-methylene-c-
butyrolactone (MMBL). When used alone at ambient tempera-
ture in toluene, these salts exhibit no (K[H]) to low (K[A]) to
modest (K[E]) polymerization activity. Mixing of K[H] and
Al(C6F5)3 leads to the formation of an ‘‘ate’’ complex,
Kþ[HAl(C6F5)3]ꢀ, which has been structurally characterized by
X-ray diffraction; this complex has a high polymerization activ-
ity producing atactic PMMA, but addition of another equiv of
Al(C6F5)3 further enhances both the rate and the efficiency of
the polymerization, now producing syndiotactic PMMA with a
narrow molecular weight (MW) distribution of 1.04. The K[H]/
2Al(C6F5)3 system also exhibits high activity for polymerization
of (M)MBL. In sharp contrast, addition of Al(C6F5)3 to K[A]
shuts down the polymerization at various temperatures. The
most active, controlled, and syndioselective polymerization
system in this series is K[E]/2Al(C6F5)3. Accordingly, the poly-
merization control and kinetics of this most effective system
C
V
have been examined in more detail.
2011 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 49: 2008–2017, 2011
KEYWORDS: anionic polymerization; catalysis; methylene
butyrolactone; organometallic catalysts; potassium initiator;
renewable monomer
INTRODUCTION Calling for gradually replacing existing
petroleum-based polymeric materials with those derived
from naturally occurring, renewable resources in a techno-
logically and economically competitive fashion has stimulated
increasing attention to sustainability-related research
directed at examining the possibility of replacing petroleum-
based raw materials by renewable feedstocks for the produc-
tion of polymeric materials in large commodity and specialty
chemicals markets.1 In this context, naturally renewable
in MBL also imparts significant enhancements in the materi-
als properties of the resulting PMBL, due to the conforma-
tional rigidity of the polymer chain through incorporation of
the butyrolactone moiety. For example, the glass-transition
temperature (Tg) of PMBL produced by the radical polymer-
ization is 195 ꢂC,7 which is about 90 ꢂC higher than that of
atactic PMMA. Additionally, compared to PMMA, PMBL exhib-
its increased optical properties and resistance to solvent,
heat, and scratch.8
methylene
butyrolactones,
a-methylene-c-butyrolactone
Polymerization of MBL has been investigated predominately
using various free radical polymerization mechanisms,5,6,7,9
but several other types of mechanisms, including group-
transfer polymerization,10 anionic polymerization,7 and coor-
dination polymerization by metallocene complexes (vide
infra) have also been explored. MBL has been copolymerized
with various comonomers5 such as MMA,11 styrene,9(d),12
methoxystyrene,13 and vinyl thiophenes.14 In comparison,
the polymerization of MMBL has been studied to a much
lesser extent. Nevertheless, MMBL has also been free radi-
cally copolymerized with styrene and MMA,15 polymerized
by free radical emulsion polymerization,16,17 and polymer-
ized by free-radical, anionic, and group-transfer polymeriza-
tion methods, which required long reaction times (2 to
44 h), often at low temperatures, achieving low to high, but
(MBL) and c-methyl-a-methylene-c-butyrolactone (MMBL),
are of particular interest in exploring the prospects of substi-
tuting the petroleum-based methacrylate monomers for spe-
cialty chemicals production.2 MBL, or tulipalin A, is a natural
substance found in tulips, and the MBL ring is an integral
building block of many (ꢁ10% known) natural products,3
whereas its c-methyl derivative, MMBL, is readily prepared
via a two-step process from the cellulosic biomass-derived
levulinic acid.4 As the cyclic analog of MMA (Scheme 1), MBL
exhibits greater reactivity in free radical polymerization5
than MMA, attributable to the presence of both the nearly
planar five-membered lactone ring (which provides reso-
nance stabilization for the active radical species) and the
higher energy exocyclic C¼¼C double bond (as a result of the
ring strain and the fixed s-cis conformation).6 The cyclic ring
*Present address: Department of Chemistry, Ximen University, China.
Correspondence to: E. Y.-X. Chen (E-mail: eugene.chen@colostate.edu)
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 2008–2017 (2011) 2011 Wiley Periodicals, Inc.
2008
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ARTICLE
polymerize MMBL in a low monomer-to-initiator ratio of 60
at ꢀ78 ꢂC for 2 h,18 but neither report gave MW and MWD
characteristics of the resulting polymers. On the other hand,
anionic polymerization by organolithium initiators has
become the most widely adopted methodology to polymerize
polar vinyl monomers, especially conjugated acrylic mono-
mers, to technologically important polar-functionalized vinyl
polymers.24 An important challenge in anionic polymeriza-
tion using such initiators has been the extensive and contin-
ued quests for effective additives or catalysts that can render
these multiple-site anionic active species to exhibit high
polymerization activities and degrees of polymerization con-
trol preferably at ambient temperature or above.25 In this
context, we showed that bulky strong Lewis acids such as
MeAl(BHT)2 (BHT ¼ 2,6-di-tert-butyl-4-methylphenolate) can
convert the hexameric isopropyl a-lithioisobutyrate initiator
into the monomeric lithium ester enolaluminate propagating
species, Liþ[Me2C¼¼C(OiPr)OAlMe(BHT)2]ꢀ, thereby, effecting
the single-site anionic polymerization with a high degree of
polymerization control at ambient temperature.26 Accord-
ingly, the central objective of this study was to examine the
characteristics of anionic polymerization of (M)MBL mono-
mers, along with MMA, using three potassium salts of
hydride (K[H]), enolate Me2C¼¼C(OiPr)OK (K[E]), and allyl
K[1,3-(SiMe3)2C3H3] (K[A]), as well as their combinations
with strong and bulky aluminum Lewis acids, MeAl(BHT)2
and Al(C6F5)3. The reasons for choosing the potassium-based
initiators for this study are threefold. First, potassium-based
initiators have received much less attention as initiators for
anionic polymerization in general25 and never been investi-
gated for polymerizations of renewable (M)MBL monomers,
the focus of the current work. Second, potassium-based ini-
tiators may possess some advantages over analogous organo-
lithium initiators because the larger potassium cation should
change ion-pairing and solvation dynamics of the anionic
propagating species and its counterion, thus impacting prop-
agation rates and even polymerization stereochemistry.25,27
Third, unlike lithium salts, potassium salts are typically non-
toxic (resorbable) as the potassium ion belongs to the
human metabolism.
SCHEME 1 Renewable methylene butyrolactones (M)MBL as
cyclic analogs of MMA and comparisons between their derived
polymers. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
never complete conversions, with unknown polymerization
and polymer molecular weight (MW) characteristics.18
Building-off of our extensive work on the metal-mediated
coordination polymerization of polar vinyl monomers such
as (meth)acrylates and (meth)acrylamides,19 we have
recently developed the rapid and controlled coordination–
addition polymerization of (M)MBL by metallocene com-
plexes with high activity and other new types of polymeriza-
tion processes for (M)MBL. For example, the coordination
polymerization of (M)MBL in dimethylformamide (DMF) by
the divalent Cp* Sm(THF, tetrahydrofuran)2 catalyst is fast,
2
with TOF (turn-over frequency ¼ mol of monomer consumed
per mol catalyst or initiator per h) approaching 3000 hꢀ1, ef-
ficient (initiator efficiency, I*, approaching 100%), and con-
trolled, leading to P(M)MBL with relatively narrow MW dis-
tributions (MWDs) and their diblock copolymers with MMA
or with each other.20 Half-sandwich dialkyl rare-earth metal
complexes incorporating a disilylated indenyl ligand, (1,3-
(SiMe3)2C9H5)M(CH2SiMe3)2(THF) (M ¼ Sc, Y, Dy, Lu), are
even more (10 times) active for polymerization of MMBL in
DMF, achieving quantitative monomer conversion in <1 min
with a 0.2 mol % catalyst loading, which gives a high TOF of
>30,000 hꢀ1, and producing PMMBL with MW ranging from
a medium Mn of 1.89 ꢃ 104 Da to a high Mn of 1.63 ꢃ 105
Da.21 We also reported the high-speed and living polymeriza-
tion of (M)MBL catalyzed by silylium ions R3Siþ.22 Atactic
PMMBL produced exhibits a Tg of 225 ꢂC, representing a Tg
enhancement of ꢁ120 ꢂC over the Tg of the typical atactic
PMMA with a comparable MW. Most recently, we developed
an extremely active (M)MBL polymerization system, based
on classical or frustrated Lewis pairs consisting of a bulky
EXPERIMENTAL
Materials and Methods
All syntheses and manipulations of air- and moisture-sensi-
tive materials were carried out in flamed Schlenk-type glass-
ware on a dual-manifold Schlenk line, a high-vacuum line, or
in an argon or nitrogen-filled glovebox. HPLC-grade organic
solvents were sparged extensively with nitrogen during fill-
ing of the solvent reservoir 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. HPLC-
grade DMF was degassed, dried over CaH2 overnight,
followed by vacuum transfer (not by distillation). NMR sol-
vents CDCl3 and DMSO-d6 were dried over activated Davison
4-Å molecular sieves, and NMR spectra were recorded on a
Varian Inova 300 MHz, a Varian Inova 400 MHz, or an Inova
t
phosphine (e.g., Bu3P) or bulky N-heterocyclic carbene base
and the strong acid Al(C6F5)3, exhibiting an exceptional TOF
of >48,000 hꢀ1 23
.
It becomes readily apparent that reports on the anionic poly-
merization of renewable (M)MBL monomers are scarce.
There were two reports that mentioned the anionic polymer-
ization, one of which used C6H5MgBr and 9-fluorenyllithium
to polymerize MBL in a low monomer-to-initiator ratio of 43
7
n
ꢂ
at typically ꢀ78 C for 6 h, whereas the other used BuLi to
ANIONIC POLYMERIZATION OF MMA AND RENEWABLE (M)MBL, HU ET AL.
2009

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
500 MHz spectrometer. Chemical shifts for 1H and 13C spec-
tra were referenced to internal solvent resonances and are
reported as parts per million relative to tetramethylsilane.
The structure was refined by full-matrix least squares on F2
for all reflections. All atoms were located by difference
Fourier synthesis and refined anisotropically except for one
hydrogen atom, which was refined isotropically. Owing to the
inclusion of potassium as a heavy atom in this ionic
compound, the residual electronic density of 2.228 e Åꢀ3
appeared a little high. CCDC-807949 contains the supplemen-
tary crystallographic data for this article. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Monomers MBL and MMBL were purchased from TCI Amer-
ica, whereas methyl methacrylate (MMA), n-butyl acrylate (n-
BA), potassium hydride, potassium bis(trimethylsilyl)amide,
and methyl isobutyrate were purchased from Aldrich. The liq-
uid chemicals were degassed, dried over CaH2 overnight, fol-
lowed by vacuum distillation, whereas MMA was further puri-
fied by titration with neat tri(n-octyl)aluminum (Strem
Chemical) to a yellow end point,28 followed by vacuum distil-
lation. The purified monomers were stored in brown glass
bottles over activated Davison 4 Å molecular sieves (for n-
BA) in a ꢀ30 ꢂC freezer inside the glovebox. Butylated
hydroxytoluene (BHT-H, 2,6-di-tert-butyl-4-methylphenol)
was purchased from Aldrich and was recrystallized from hex-
anes before use. Tris(pentafluorophenyl)borane, B(C6F5)3,
was obtained as a research gift from Boulder Scientific Co.
and was further purified by recrystallization from hexanes
at ꢀ30 ꢂC. Tris(pentafluorophenyl)alane, Al(C6F5)3, as a
0.5ꢄtoluene adduct based on the elemental analysis for the
vacuum-dried sample, was prepared from the exchange
reaction of B(C6F5)3 and AlMe3 in a 1:3 toluene/hexanes sol-
vent mixture in quantitative yield according to a literature
X-ray crystal structural data for Kþ[HAl(C6F5)3]ꢀ: C18HAlF15K,
Mr ¼ 568.27, T ¼ 173(2) K, k ¼ 0.71073 Å, crystal dimensions
0.54 ꢃ 0.39 ꢃ 0.31 mm3, monoclinic, C2/c, a ¼ 21.2204 (5) Å,
b ¼ 12.1699(3) Å, c ¼ 15.7115(4) Å, b ¼ 109.5400(10)ꢂ, V ¼
3823.82(16) Å3, Z ¼ 8, qcalcd ¼ 1.974 Mg mꢀ3, h range for data
collection ¼ 1.96–33.24ꢂ, 22,601 reflections collected, 7278
unique (Rint ¼ 0.0238), zero restraints, goodness-of-fit on F2 ¼
1.099, final R1 ¼ 0.0803 and wR2 ¼ 0.2287 with I > 2r(I), and
residual electron density extremes ¼ 2.228 and ꢀ0.483 eÅ–3.
General Polymerization Procedures
Polymerizations were performed either in 25-mL flame-dried
Schlenk flasks interfaced to the dual-manifold Schlenk line
for runs using external temperature bath, or in 30-mL glass
reactors inside the glovebox for ambient temperature (ca.
procedure.29 Literature procedures were used to prepare
30
ꢂ
MeAl(BHT)2
and potassium 1,3-bis(trimethylsilyl)allyl
25 C) runs. In a typical polymerization procedure, a prede-
K[1,3-(SiMe3)2C3H3] (K[A]).31 Potassium isopropyl isobutyrate
Me2C¼¼C(OiPr)OK (K[E]) was freshly prepared, following typi-
cal procedures established for the preparation of alkali metal
isobutyrate ester enolates,32 from the reaction of isopropyl
isobutyrate and potassium bis(trimethylsilyl)amide (0.5 M in
termined amount of the potassium initiator (11.7 lmol for
runs with a [MMA]/[K] ratio of 400, or 10.3 lmol for runs
with a [(M)MBL]/[K] ratio of 400) was dissolved in a solvent
(4.5 mL toluene or 3.0 mL DMF). With vigorous stirring,
monomer (MMA, 0.50 mL, 4.68 mmol; MBL, 403 mg, 4.11
mmol; or MMBL, 461 mg, 4.11 mmol) was quickly added to
the above initiator solution via syringe to start the polymer-
ization. For polymerizations by the potassium initiator com-
bined with aluminum Lewis acids, Al(C6F5)3 or MeAl(BHT)2
was premixed with the initiator in toluene for 5 min before
addition of monomer. For (M)MBL polymerizations in DMF
or dichloromethane (DCM), toluene (291 lL, 2.74 mmol) was
added as an internal standard, to the reaction mixture. 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’’ CDCl3 stabi-
lized by 250 ppm of BHT-H; the quenched aliquots were
later analyzed by 1H NMR to obtain the percent monomer
conversion data. The polymerization was immediately
quenched after the removal of the aliquot by addition of 5
mL 5% HCl-acidified methanol. The quenched mixture was
precipitated into 100 mL of methanol, stirred for 1 h, fil-
tered, washed with methanol, and dried in a vacuum oven at
ꢂ
toluene) at 0 C and isolated as a solid after filtering, washing
with hexanes, and drying in vacuo. 1H NMR (C6D6, 23 ꢂC): d
4.11 (sept, J ¼ 6.3 Hz, 1H, OCHMe2), 1.91 (s, 3H, ¼¼CMe2),
1.71 (s, 3H, ¼¼CMe2), 1.18 (d, 6H, J ¼ 6.3 Hz, CHMe2).
Isolation of ‘‘Ate’’ Complex Kþ[HAl(C6F5)3]ꢀ
A suspension containing Al(C6F5)3ꢄtoluene (115 mg, 0.20
mmol), and KH (160 mg, 4.0 mmol) in precooled toluene
(5 mL, ꢀ30 ꢂC) was stirred for about 10 min. On filtration,
the filtrate was kept at ambient temperature for 1 week af-
ter which colorless crystals suitable for X-ray diffraction
studies were formed and isolated (80 mg, 70% yield). The
in situ generated solution of KH and Al(C6F5)3 is soluble in
hydrocarbon solvents such as benzene, toluene, and cyclo-
hexane. However, the slowly grown crystals of the titled
complex have poor solubility in such solvents. 19F NMR
(C6D6, 23ꢂC): d ꢀ123.2 (d, 6F, o-F), ꢀ155.3 (t, 3F, p-F),
ꢀ162.2 (m, 6F, m-F).
ꢂ
50 C overnight to a constant weight.
X-Ray Crystallographic Analysis of Kþ[HAl(C6F5)3]ꢀ
Single crystals of the complex were quickly covered with a
layer of Paratone-N oil (Exxon, dried, and degassed at 120
ꢂC/10ꢀ6 Torr for 24 h) after decanting the mother liquor.
A crystal was then mounted on a thin glass fiber and trans-
ferred 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 library.33
The ratio of [MMA]0/[MMA]t at a given time t was deter-
mined by integration of the peaks for MMA (5.2 and
6.1 ppm for the vinyl signals; 3.4 ppm for the OMe signal)
and PMMA (centered at 3.4 ppm for the OMe signals)
according to [MMA]0/[MMA]t ¼ 2A3.4/3A5.2þ6.1, where A3.4 is
the total integrals for the peaks centered at 3.4 ppm (typi-
cally in the region 3.2ꢀ3.6 ppm) and A5.2þ6.1 is the total
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ARTICLE
TABLE 1 Selected Results of Polymerization of MMA by K[H] and K[H]/ER3 (E 5 Al, B)a
Convb
(%)
Mn
MWDc
I*d
[rr]b
[mr]b
[mm]b
c
Run No.
Initiator (I)
(kg molꢀ1
)
(Mw/Mn)
(%)
(%)
(%)
(%)
1
2
3
4
5
6
7
8
9
a
K[H]
0
69
–
–
–
–
–
–
K[H]/1MeAl(BHT)2
K[H]/2MeAl(BHT)2
K[H]/1Al(C6F5)3
K[H]/2Al(C6F5)3
K[HAl(C6F5)3]
57.1
90.4
67.0
77.7
21.3
50.3
–
1.25
1.13
1.04
1.04
1.44
1.04
–
48
44
60
52
148
80
–
76.2
76.2
74.2
76.5
26.6
69.5
–
22.7
21.1
23.0
21.3
51.7
26.5
–
1.1
2.7
2.8
1.2
21.7
4.0
–
100
100
100
79
K[HAl(C6F5)3]/Al(C6F5)3
K[H]/1B(C6F5)3
K[H]/2B(C6F5)3
100
0
0
–
–
–
–
–
–
c
Carried out in toluene at ambient temperature in a [MMA]/[I] ratio of
400 for 2 h.
Monomer conversion and polymer tacticity measured by 1H or 13C
Determined by GPC relative to PMMA standards.
d
Initiator efficiency (I*)
W(monomer) ꢃ [monomer]/[initiator] ꢃ conversion% þ MW of chain-
end groups.
¼
Mn(calcd)/Mn(exptl), where Mn(calcd)
¼
b
M
NMR.
integrals for both peaks at 5.2 and 6.1 ppm. For (M)MBL
polymerizations, monomer percent conversions were calcu-
lated by comparing the integration of the vinyl protons of
the unreacted monomer to the methyl protons of toluene
added as internal standard.
chromatography (GPC) analyses carried out at 40 ꢂC and a
flow rate of 1.0 mL minꢀ1, with CHCl3 as the eluent for
PMMA or with DMF for PMBL and PMMBL, on a Waters Uni-
versity 1500 GPC instrument coupled with a Waters RI de-
tector and equipped with four PLgel 5-lm mixed-C columns
(Polymer Laboratories; linear range of MW
¼
200–
1
ꢂ
H NMR (CDCl3, 300 MHz, 23 C) for PMMA: d 3.60 (s, OMe),
2.05 (d, CH2), 1.99–1.90 (m, CH2), 1.82 (s, CH2), 1.48 (d,
CH2), 1.22 (s, CH3, mm), 1.02 (s, CH3, mr), 0.85 (s, CH3, rr).
2,000,000). The instrument was calibrated with 10 PMMA
standards, and chromatograms were processed with Waters
Empower software (version 2002). 1H NMR spectra for the
analysis of PMMA microstructures were recorded in CDCl3 at
50 ꢂC and analyzed according to the literature methods,34
whereas tacticities of PMBL,10 and PMMBL18 were measured
1
ꢂ
H NMR (DMSO-d6, 300 MHz, 100 C) for PMBL: d 4.34 (b.s,
2H, OCH2), 2.24-1.99 (m, 4H, CH2, CH2). 13C NMR (DMSO-d6,
125 MHz, 100 ꢂC) for PMBL: d 179 (C¼¼O), 64.36 (OCH2),
44.22, 43.90, 43.74 (quaternary carbon, rr, mr, mm), 41.89–
40.58 (main-chain CH2, unresolved tetrads), 30.47 (b-CH2).
1H NMR (DMSO-d6, 300 MHz, 100 ꢂC) for PMMBL: d 4.64
(b.s, 1H, CH), 2.31 (b.s, 2H, CH2), 1.99 (b.s, 2H, CH2), 1.39
(b.s, 3H, CH3). 13C NMR (DMSO-d6, 125 MHz, 100 ꢂC) for
PMMBL: d 178 (C¼¼O), 72.65 (OCH), 46.48, 46.15, 45.80
(quaternary carbon, rr, mr, mm), 43.05 (b-CH2), 40.53, 39.19,
37.69 (main-chain CH2, rr, mr, mm), 19.46 (CH3). DEPT
experiments were used to remove the DMSO signals in the
13C NMR experiments.
13
ꢂ
by C NMR in DMSO-d6 at 100 C.
RESULTS AND DISCUSSION
Polymerization by K[H] and K[H]/AlR3
Alkali metal (Li, Na, K) hydrides, in combination with com-
mon alkyl aluminum AlR3, have been used as initiators for
the so-called ‘‘retarded’’ anionic polymerization of styrene
and conjugated dienes in toluene or cyclohexane.35 We won-
dered if such hydrides could also initiate the anionic poly-
merization of MMA and (M)MBL. Potassium hydride K[H] is
insoluble in toluene, and its suspension in this solvent exhib-
ited no activity for MMA polymerization at ambient tempera-
ture (run 1, Table 1). On the other hand, premixing K[H]
with 1 equiv of MeAl(BHT)2 in toluene at ambient tempera-
ture generated a homogenous solution that is active for
MMA polymerization, achieving 69% monomer conversion in
2 h with a [MMA]:[KH] ratio of 400; the PMMA produced
had a modest syndiotacticity of 76.2% rr (run 2, Table 1).
Increasing the amount of MeAl(BHT)2 to 2 equiv relative to
K[H] brought about a quantitative monomer conversion
under the same conditions; the polymer tacticity remained
the same, but the polymer MWD was narrower (1.13 for run
3 vs. 1.25 for run 2). However, both runs had <50% initiator
efficiencies, presumably caused by K[H] weighing errors due
to its rather small formula weight and insolubility in toluene
Polymerization Kinetics
Kinetic experiments were carried out in a stirred glass reac-
tor at ambient temperature (ca. 25 C) inside the argon-filled
ꢂ
glovebox using the procedure already described above and
[MMA]0/[KE]0 ratios of 400:1, 800:1, 1200:1, and 1600:1,
where [MMA]0 ¼ 0.936 M, [KIPIB]0 ¼ 2.34, 1.17, 0.78, and
0.585mM in 4.5 mL of toluene þ MMA solutions. At appro-
priate time intervals, 0.2-mL aliquots were withdrawn from
the reaction mixture using syringe and quickly quenched
into 1-mL septum-sealed vials containing 0.6 mL of undried
‘‘wet’’ CDCl3 mixed with 250 ppm BHT-H. The quenched ali-
quots were analyzed by 1H NMR to obtain monomer
conversions.
Polymer Characterizations
Polymer number-average MWs (Mn) and MW distributions
(MWD
¼
Mw/Mn) were measured by gel permeation
ANIONIC POLYMERIZATION OF MMA AND RENEWABLE (M)MBL, HU ET AL.
2011

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
a
TABLE 2 Selected Results of Polymerization of (M)MBL by K[H] and K[H] 1 AlR3
Convb
(%)
TOF
(hꢀ1
Mn
MWDc
c
Monomer
(M)
Time
(h)
Run No.
Initiator (I)
)
(kg/mol)
(Mw/Mn)
10
11
12
13
14
15
a
MBL
K[H]
24
24
4
0
0
–
–
d
MBL
K[H]/2MeAl(BHT)2
K[H]/2Al(C6F5)3
K[H]
8.4
100
0
1.4
100
0
88.0
52.0
–
1.33
MBL
1.87
–
MMBL
MMBL
MMBL
24
24
0.83
K[H]/2MeAl(BHT)2
K[H]/2Al(C6F5)3
12.2
100
2.0
482
63.0
209e
1.68
1.56
d
Carried out in DMF at ambient temperature (ꢁ25 ꢂC) in a [M]/[I] ratio
A clear bimodal MWD; for a lower MW peak, Mn ¼ 4.90 kg/mol, Mw
/
of 400.
Mn ¼ 1.22.
b
e
Conversion measured by 1H NMR.
There exists a small amount of a lower MW tail (Mn ¼ 12.0 kg/mol,
c
Determined by GPC relative to PMMA standards.
M
w/Mn ¼ 1.10).
(so that a stock solution could not be made for a more accu-
rate handling of this reagent in very small quantities).
and 13). Addition of 2 equiv of MeAl(BHT)2 brought about
some activity: TOF ¼ 1.4 hꢀ1 for MBL polymerization (run
11) and 2.0 hꢀ1 for MMBL polymerization (run 14). Signifi-
cantly, the K[H]/2Al(C6F5)3 system is far more active, with
TOF approaching 100 hꢀ1 for MBL polymerization (run 12).
The MMBL polymerization by K[H]/2Al(C6F5)3 was even
more impressive and achieved a high TOF of 482 hꢀ1 (run
15), producing high MW, atactic PMMBL (Mn ¼ 209 kg
molꢀ1; MWD ¼ 1.56; mr ¼ 49.9%, mm ¼ 10.7%, rr ¼
39.4%).
Replacing MeAl(BHT)2 with a more strongly Lewis acidic
Al(C6F5)3 further enhances the polymerization activity (run 4
vs. run 2) and improves the polymerization control (MWD ¼
1.04 for runs 4 and 5) while keeping the PMMA syndiotactic-
ity largely unchanged. The polymerization with the isolated
‘‘ate’’ complex Kþ[HAl(C6F5)3]ꢀ, derived from the reaction of
K[H] and Al(C6F5)3 in toluene (see Experimental), was also
examined (run 6). This polymerization was also effective but,
interestingly, the resulting PMMA is now essentially atactic
(51.7% mr); when an additional equiv of Al(C6F5)3 was
added, the expected syndiotactic PMMA was produced
(69.5% rr, run 7).
Given the fact that Kþ[HAl(C6F5)3]ꢀ itself is active for poly-
merization, presumably with the hydride being the initiating
moiety, and that the presence of excess of Al(C6F5)3 drasti-
cally accelerates the polymerization, the current polymeriza-
tion by K[H]/2AlR3 can be reasonably proposed to proceed
through an activated-monomer anionic polymerization
mechanism.26
Other two aspects of the polymerization by K[H] are worth
mentioning. First, substituting the alane with the borane con-
gener, B(C6F5)3 (1 or 2 equiv) completely shuts down the
polymerization (runs 8 and 9). This observation is consistent
with the polymerization behavior of other classical organo-
In light of the high activity of K[H] when combined with
Al(C6F5)3, we examined the fundamental reaction and deter-
mined the product to be the anticipated ‘‘ate’’ complex
Kþ[HAl(C6F5)3]ꢀ, the structure of which has been confirmed
by X-ray diffraction analysis (see Experimental). Figure 1
highlights the geometry of the hydridoaluminate anion, fea-
t
lithium initiators, such as BuLi and Me2C¼¼C(OiPr)OLi, when
combined with the alane and borane Lewis acids;36 the re-
markable differences, in terms of the observed high activity
and control with the alane additive versus the inactivity with
the borane additive, were attributed to the inability of the
lithium enolborate/borane pair to effect the bimolecular,
activated-monomer anionic polymerization as does the lith-
ium enolaluminate/alane pair.19 Second, the K[H]/2AlR3 ini-
tiator system is also effective for polymerization of n-butyl
acrylate in toluene or even in cyclohexane. For example, the
polymerization by K[H]/2MeAl(BHT)2 achieved quantitative
monomer conversions at ambient temperature in 2 h, pro-
ducing high MW P(n-BA), Mn ¼ 374 kg molꢀ1 (MWD ¼
1.87), and 713 kg molꢀ1 (MWD ¼ 1.68), in toluene and
cyclohexane, respectively.
turing
a slightly distorted tetrahedron with a sum of
CAAlAC angles of 336.66ꢂ, whereas Figure 2 depicts the
coordination environment of the potassium cation and a 3-D
molecular packing structure formed on the basis of the ionic
interactions. As in the case of zirconocenium–hydridoborate
[Cp*ZrH]þ[HB(C6F5)3]ꢀ ion pair in that two F atoms, rather
2
than the H atom, of the borate anion are coordinated to the
cation,37 the K cation in Kþ[HAl(C6F5)3]ꢀ is coordinated to
eight aryl F atoms and one H atom of the anion, thus giving
rise to a total coordination number of 9 for Kþ (Fig. 2). The
KAF bonds span from the shortest KAF26 distance of
2.670(2) Å to the longest KAF34 distance of 3.116(2) Å, and
the KAH contact has a distance of 2.688(2) Å.
Having established the effectiveness of the K[H]/2AlR3 initia-
tor system for MMA polymerization at ambient temperature,
we turned our attention to the polymerization of renewable
(M)MBL, the results of which were summarized in Table 2.
Consistent with the above findings, no activity was observed
for polymerizations up to 24 h using K[H] alone (runs 10
Polymerization by K[E] and K[E]/AlR3
We first examined MMA polymerization in toluene by potas-
sium enolate Me2C¼¼C(OiPr)OK (K[E]), which showed a
2012
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
broad MWD of 3.21 (run 16, Table 3). Interestingly, this tac-
ticity is substantially different from the isotacticity observed
for the MMA polymerization in toluene by the enolate with
the much smaller lithium counterion.36(b) Addition of 2 equiv
of MeAl(BHT)2 significantly enhanced the rate of the poly-
merization, now reaching a TOF of 80 hꢀ1 and also produced
syndiotactic PMMA with rr ¼ 72% (run 17). The rate
enhancement was even more dramatic by Al(C6F5)3; thus,
the K[E]/2Al(C6F5)3 system gave a high TOF of 1600 hꢀ1
,
producing high MW, syndiotactic PMMA (Mn ¼ 100 kg
molꢀ1; MWD ¼ 1.23; rr ¼ 79.3%; run 18). In comparison,
polymerizations of MBL (runs 19 and 20) and MMBL (runs
21 and 22) are slower but, nonetheless, the K[E]/2Al(C6F5)3
initiator system can achieve quantitative monomer conver-
sion in 24 h for MBL polymerization (run 20) or 4 h for
MMBL polymerization (run 22). Noteworthy is the loss of
tacticity control over the polymerizations of MBL and MMBL.
We also investigated the degree of control of the MMA poly-
merization over polymer MW by percentage monomer con-
version and by the [MMA]/ [K] ratio. As can be seen from
Figure 3, the Mn of the resulting PMMA increased linearly
with the MMA conversion (R2 ¼ 0.997) for the MMA poly-
merization by K[E]/2Al(C6F5)3 in toluene at 25 ꢂC with a
[MMA]:[K] ratio of 800:1, while MWD varies narrowly from
1.10 to 1.15 for the entire conversion range. Polymerizations
with other [MMA]/[K] ratios exhibited similarly linear plots.
The ability of the current initiator system to control the
polymer MW was reaffirmed by the experiments varying the
[MMA]/[K] ratio. Hence, a plot of Mn of PMMA versus
the [MMA]/[K] ratio (400–1600) also gave a straight line
(Fig. 4), demonstrating the control over Mn by adjusting the
monomer-to-initiator ratio.
FIGURE 1 X-ray crystal structure of ‘‘ate’’ complex
Kþ[HAl(C6F5)3]ꢀ, with thermal ellipsoids drawn at the 50% prob-
ꢂ
˚
ability. Selected bond lengths [A] and angels [ ]: Al(1)AH(1)
1.59(5), Al(1)AC(11) 2.016(3), Al(1)AC(21) 2.024(3), Al(1)AC(31)
2.015(3),
K(1)AF(25)
3.039(3),
K(1)AF(26)
2.670(2);
C(11)AAl(1)AC(21) 111.70(12), C(11)AAl(1)AC(31) 107.55(11),
C(21)AAl(1)AC(31) 111.41(12), C(11)AAl(1)AH(1) 106.7(17),
C(21)AAl(1)AH(1)
108.9(16),
C(31)AAl(1)AH(1)
110.5(17),
F(25)AK(1)AF(26) 55.13(6). [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
modest activity for MMA polymerization at ambient tempera-
ture with a TOF of 15.4 hꢀ1 (i.e., 92.4% conversion in 24 h)
and produced essentially atactic (52.5% mr) PMMA with a
FIGURE 2 Molecular structure of Kþ[HAl(C6F5)3]ꢀ, with isotropic ellipsoids drawn at ARAD of 0.6 for the adjusted the radius of Kþ
for clarity. The left picture highlights the coordination environment of Kþ, whereas the picture on the right shows a 3-D molecular
packing structure.
ANIONIC POLYMERIZATION OF MMA AND RENEWABLE (M)MBL, HU ET AL.
2013

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
a
TABLE 3 Selected Results of Polymerization of MMA and (M)MBL by K[E] and K[E] 1 AlR3
Convb
(%)
TOF
(hꢀ1
Mn
MWDc
[rr]b
[mr]b
[mm]b
c
Monomer
(M)
Time
(h)
Run No.
Initiator (I)
)
(kg/mol)
(Mw/Mn)
(%)
(%)
(%)
16
17
18
19
20
21
22
a
MMA
MMA
MMA
MBL
K[E]
24
5
92.4
100
15
80
39.0
85.0
100
3.21
1.36
1.23
1.32
1.84
1.76
1.79
25.1
72.0
79.3
n.d.
34.7
n.d.
47.1
52.5
26.3
19.1
n.d.
30.9
n.d.
43.4
22.4
1.7
K[E]/2MeAl(BHT)2
K[E]/2Al(C6F5)3
K[E]
0.25
24
24
24
4
100
1,600
4
1.6
23.2
100
5.60
28.0
37.0
28.0
n.d.
34.4
n.d.
9.5
MBL
K[E]/2Al(C6F5)3
K[E]
17
MMBL
MMBL
72.2
100
12
K[E]/2Al(C6F5)3
100
c
Carried out in toluene (MMA) or DMF (MBL and MMBL) at ambient
Determined by GPC relative to PMMA standards.
temperature (ꢁ25 ꢂC) in a [M]/[I] ratio of 400; n.d. ¼ not determined.
b
Monomer conversion and polymer tacticity measured by 1H or 13C
NMR.
Kinetic experiments that used the [MMA]0/[K]0 ratio ranging
from 200 to 1600 revealed that the MMA polymerization by
K[E]/2Al(C6F5)3 in toluene at 25 ꢂC follows zero-order
kinetics in [MMA] for all the [MMA]/[K] ratios investigated
(Fig. 5). A zero-order dependence on monomer concentration
in this activated-monomer polymerization suggests that the
rate determining step is the conjugate Michael addition of
the monomer activated by the alane into the polymer chain
(i.e., the CAC bond forming step), whereas the recapture of
the aluminum catalyst coordinated to the penultimate poly-
mer chain end by the incoming monomer is relatively fast
(Scheme 2). This mechanism resembles that of the bimolecu-
lar, activated-monomer anionic polymerization by the lithium
enolaluminate/alane pair,36(a),19,38 or by nonbridged group 4
metallocene/metallocenium pair.39
with 2 equiv of Al(C6F5)3 (run 25). On the other hand, K[ꢂA]
was found to be active for polymerization of MMBL at 25 C
(run 26) and more active at 0 ꢂC, achieving a quantitative
monomer conversion (run 27) and producing atactic PMMBL
(15.4% mm, 50.5% mr, 34.1% rr). Interestingly, unlike other
potassium or lithium-based anionic initiators, addition of 2
equiv of Al(C6F5)ꢂ3 to K[A] completely shut down the poly-
ꢂ
ꢂ
merization at 25 C (run 28), 0 C, or ꢀ78 C. This phenom-
enon could be attributed to the irreversible formation of an
‘‘ate’’ complex K[AAl(C6F5)3], as shown by 19F NMR (C6D6,
23ꢂC) chemical shifts at d ꢀ121.1, ꢀ123.0 (m, 6F, o-F),
ꢀ157.7 (t, 3F, p-F) and ꢀ163.5 (m, 6F, m-F) for the alumi-
nate formation,26,36 which inhibits the allyl anion to function
as an effective initiator. Consistent with this hypothesis, addi-
tion of only 0.5 equiv of Al(C6F5)3 rendered an active system
for polymerization of MMBL at ambient temperature, achiev-
ing 75% monomer conversion in 24 h.
Polymerization by K[A] and K[A]/Al(C6F5)3
Although potassium allyl species K[1,3-(SiMe3)2C3H3] (K[A])
was reported to be highly active for MMA polymerization in
CONCLUSIONS
31
ꢂ
toluene at 0 C, it showed no activity for polymerization of
MBL in DMF (in which the polymer is soluble) at either
This study has investigated the characteristics of anionic
polymerization of MMA and renewable (M)MBL monomers
using three resorbable potassium salts of hydride K[H],
enolate K[E], and allyl K[A], as well as their combinations
ꢂ
ꢂ
25 C and 0 C (runs 23 and 24, Table 4) or in combination
FIGURE 3 Plot of Mn of PMMA vs monomer conversion for the
polymerization of MMA by K[E]/2Al(C6F5)3 in toluene at 25 ꢂC
with a [MMA]:[K] of 800:1. MWD ¼ 1.10 to 1.15 for the entire
conversion range.
FIGURE 4 Plot of Mn of PMMA versus the [MMA]/[K] ratio for
the polymerization of MMA by the K[E]/2Al(C6F5)3 in toluene at
25 ꢂC, achieving quantitative monomer conversion for all runs.
2014
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
FIGURE 5 Zero-order kinetic plots of [M]t/[M]0 versus time for the polymerization of MMA by K[E]/2Al(C6F5)3 in toluene at 25 ꢂC.
Left: [M]/[I] ¼ 400/1; right: [M]/[I] ¼ 1600/1.
teristics can also be achieved by a simple mixing of K[H]
with 2 equiv of Al(C6F5)3. Significantly, the K[H]/2Al(C6F5)3
system is also highly active for polymerizations of (M)MBL,
achieving high TOF’s of 100 hꢀ1 and 482 hꢀ1 for polymeriza-
tions of MBL and MMBL, respectively.
Second, the potassium enolate K[E] itself exhibits a modest
activity for MMA polymerization in toluene at ambient tem-
perature, producing atactic PMMA with a broad MWD of 3.2.
Addition of 2 equiv of MeAl(BHT)2 enhances the rate of the
polymerization by five times and also produces syndiotactic
PMMA (rr ¼ 72%). More dramatic rate enhancement is
observed for the K[E]/2Al(C6F5)3 system that reaches a TOF
of 1600 hꢀ1, representing a rate enhancement of over 100
times. This system produces syndiotactic PMMA (rr ¼
79.3%); the degree of polymerization control of this system
has been demonstrated by linear relationships between poly-
mer Mn and percentage monomer conversion while main-
taining a narrow MWD range from 1.10 to 1.15 for the entire
conversion range as well as between Mn and the [mono-
mer]/[initiator] ratio. Kinetic experiments showed that the
polymerization by K[E]/2Al(C6F5)3 in toluene at 25 ꢂC
follows zero-order kinetics in [monomer], suggesting that
the rate determining step of this activated-monomer poly-
merization is the conjugate Michael addition of the monomer
activated by the alane into the polymer chain, whereas the
recapture of the aluminum catalyst coordinated to the
SCHEME 2 Proposed propagation ‘‘catalysis’’ cycle for the poly-
merization by K[E]/2AlR3.
with bulky and strong aluminum Lewis acids MeAl(BHT)2
and Al(C6F5)3. Key findings of this study are summarized as
follows.
First, while K[H] itself is ineffective for polymerization of
MMA and (M)MBL in toluene at ambient temperature, the
K[H]/2AlR3 system exhibits high activity for the activated-
monomer polymerization of such monomers and even n-
butyl acrylate. In particular, when combined with 1 equiv of
Al(C6F5)3, the ‘‘ate’’ complex Kþ[HAl(C6F5)3]ꢀ, derived from
the reaction of K[H] and Al(C6F5)3 in toluene and structur-
ally characterized by X-ray diffraction analysis, polymerizes
MMA rapidly and efficiently, producing syndiotactic PMMA
with a narrow MWD of 1.04. Similar polymerization charac-
a
TABLE 4 Selected Results of Polymerization of (M)MBL by K[A] and K[A] 1 Al(C6F5)3
Conv.b
(%)
Mn
MWDc
c
Monomer
(M)
Temp
(ꢂC)
Time
(h)
Run No.
Initiator (I)
(kg/mol)
(Mw/Mn)
23
24
25
26
27
28
a
MBL
K[A]
25
0
24
24
24
24
24
24
<1
0
–
–
MBL
K[A]
–
–
MBL
K[A]/2Al(C6F5)3
K[A]
25
25
0
0
–
–
MMBL
MMBL
MMBL
52.8
>99
0
27.0
33.0
–
1.49
1.42
–
K[A]
K[A]/2Al(C6F5)3
25
Carried out in DMF in a [M]/[I] ratio of 400.
Conversion measured by 1H NMR.
b
c
Determined by GPC relative to PMMA standards.
ANIONIC POLYMERIZATION OF MMA AND RENEWABLE (M)MBL, HU ET AL.
2015

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
14 Trumbo, D. L. Polym Bull 1991, 26, 271–275.
penultimate polymer chain end by the incoming monomer is
relatively fast. Relative to the MMA polymerization, polymer-
izations of (M)MBL by this system are slower but can none-
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hours (4 h for MMBL).
15 Cockburn, R. A.; McKenna, T. F. L.; Hutchinson, R. A. Macro-
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