Silylium dual catalysis in living polymerization of methacrylates via in situ hydrosilylation of monomer

Article abstract of DOI:10.1002/pola.27641

Silylium ions ("R₃Si⁺") are found to catalyze both 1,4-hydrosilylation of methyl methacrylate (MMA) with R₃SiH to generate the silyl ketene acetal initiator in situ and subsequent living polymerization of MMA. The living c

Full text of DOI:10.1002/pola.27641

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
Silylium Dual Catalysis in Living Polymerization of Methacrylates
via In Situ Hydrosilylation of Monomer
Tieqi Xu,1 Eugene Y.-X. Chen2
1State Key Laboratory of Fine Chemicals, College of Chemistry, Dalian University of Technology, No. 2 Linggong Road,
Dalian 116024, China
2Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872
Correspondence to: T. Xu (E-mail: tqxu@dlut.edu.cn) or E. Y.-X. Chen (E-mail: eugene.chen@colostate.edu)
Received 10 February 2015; accepted 16 March 2015; published online 13 April 2015
DOI: 10.1002/pola.27641
ABSTRACT: Silylium ions (“R3Si1”) are found to catalyze both
1,4-hydrosilylation of methyl methacrylate (MMA) with R3SiH
to generate the silyl ketene acetal initiator in situ and subse-
quent living polymerization of MMA. The living characteristics
of the MMA polymerization initiated by R3SiH (Et3SiH or
Me2PhSiH) and catalyzed by [Et3Si(L)]1[B(C6F5)4]– (L 5 toluene),
which have been revealed by four sets of experiments, enabled
the synthesis of the polymers with well-controlled Mn values
(identical or nearly identical to the calculated ones), narrow
MMA 1 Me2PhSiH 1 [Et3Si(L)]1[B(C6F5)4]– (0.5 mol%) by 1H
NMR provided clear evidence for in situ generation of the cor-
responding SKA, Me2C@C(OMe)OSiMe2Ph, via the proposed
“Et3Si1”-catalyzed 1,4-hydrosilylation of monomer through
“frustrated Lewis pair” type activation of the hydrosilane in the
form of the isolable silylium-silane complex, [Et3SiAHASiR3]1
–
C
V
[B(C6F5)4] . 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
Polym. Chem. 2015, 53, 1895–1903
-
molecular weight distributions (D 5 1.05–1.09), and well
defined chain structures {HA[MMA]nAH}. The polymerization is
highly efficient too, with quantitative or near quantitative initia-
tion efficiencies (I* 5 96–100%). Monitoring of the reaction of
KEYWORDS: living polymerization; methacrylate; hydrosilylation;
group-transfer polymerization; silylium catalysts; methyl
methacrylate
reaction of a silyl ketene acetal [RSKA, Me2C@C(OMe)OSiR3]
with a catalytic amount of [Ph3C][B(C6F5)4], catalyzes living
anionic-addition polymerization of acrylic monomers such as
methyl methacrylate (MMA) at room temperature (RT), pro-
ducing poly(methyl methacrylate) (PMMA) with medium to
high molecular weights (Mn > 105 g/mol) and narrow molec-
INTRODUCTION Silylium ions,1 denoted as “R3Si1” herein,
are highly reactive species and coordinated/stabilized in the
condensed phase by even very weak bases or donors such as
weakly coordinating anions, solvent molecules, monomers/
substrates, or moieties in the ligand R itself. Such reactive
species have been shown attractive utilities in organic syn-
thesis2 and catalysis.3 Silylium ions have also been utilized
to catalyze polymerization reactions for polymer synthesis.
In this context, Olah et al.4 and others5,6 have realized the
cationic ring-opening polymerization (ROP) of cyclosiloxanes
mediated predominantly by trisilyloxonium ions generated
by reacting R3SiH with [Ph3C][B(C6F5)4] in the presence of
siloxanes. Olah et al.7 have also achieved the ROP of 4-, 6-,
and 7-membered lactones using silylium-generating reagents.
Manners, Reed and coworkers8 have reported the ROP of
cyclic chlorophosphazene trimer [N3P3Cl6] catalyzed by sily-
lium ions paired with weakly coordinating halogenated car-
borane anions.
-
ular weight distributions (MWD’s, defined by D 5 Mw/
Mn 5 1.04–1.12).9 Subsequent structure–reactivity relation-
ship studies have revealed a remarkable selectivity of RSKA
for monomer structure.10 Specifically, the small “Me3Si1”
(0.05 mol %) is highly active and efficient for MMA polymer-
ization, but it shows poor activity and efficiency for the poly-
merization of sterically less demanding, active a-proton-
containing acrylates such as n-butyl acrylate (nBA). In con-
trast, the larger “iBu3Si1” catalyst exhibits low activity for
the polymerization of MMA, but exceptional activity, effi-
ciency, and control for the polymerization of nBA, achieving
quantitative nBA conversion in 1 min at 25 ꢀC and giving a
very high catalyst turn-over frequency (TOF) of 1.2 3 105
h21 10
Although this silylium-catalyzed addition polymeriza-
.
To realize unique utilities of “R3Si1” in catalyzing polymer-
ization reactions other than a ROP process, we9 have demon-
strated that “R3Si1,” generated either externally by
{R3SiH 1 [Ph3C][B(C6F5)4]}, or more intriguingly, by in situ
tion uses RSKA, which are commonly employed as initiators
in the group-transfer polymerization (GTP),11 both chain ini-
tiation and propagation involved in the silylium-catalyzed
C
V
2015 Wiley Periodicals, Inc.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 1895–1903
1895

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
the more advanced GTP systems using the RSKA initiator and
additionally employing different combinations of a Lewis acid
and
a
Me3Si-containing reagent, such as [Me3SiOTf 1
B(C6F5)3],23 [Me3SiI 1 HgI2],24 or [Me3SiI 1 RAl(OAr)2],25 are
considerably more active than the typical GTP system using no
such combinations, which may indicate their possible involve-
ment of the silylium-catalyzed process similar to what has
been demonstrated for the RSKA 1 [Ph3C][B(C6F5)4]9,10 and
RSKA 1 [H(Et2O)2][B(C6F5)4] systems.19
The above overviewed “R3Si1”-catalyzed, GTP-type polymer-
ization systems have one thing in common: they all utilize
the preformed SKA as initiator. As the SKA initiator is sensi-
tive to moisture and protic impurities, it would be desirable
to develop an advanced GTP process without directly
employing SKA. In this context, in situ generation of the SKA
initiator via 1,4-hydrosilylation of a,b-unsaturated esters
(acrylic monomers) using a hydrosilane (R3SiH)26 offers an
attractive approach. Piers et al. reported the first B(C6F5)3-
catalyzed hydrosilylation of aromatic aldehydes, ketones, and
esters,27 as well as 1,4-hydrosilylation of a,b-unsaturated
ketones.28 Mechanistic studies revealed that this hydrosilyla-
tion proceeds through the so-called frustrated Lewis pair
(FLP)29 activation of the hydrosilane in the form of an unsta-
ble intermediate [R3Si–H!B(C6F5)3], rather than activation
of the more basic carbonyl function in the form of the more
stable classical Lewis adduct [>C@O!B(C6F5)3].30 Accord-
ingly, we hypothesized that this facile B(C6F5)3-catalyzed
hydrosilylation of carbonyl substrates could provide a new,
convenient GTP process for polymerization of acrylic mono-
mers directly using R3SiH, instead of the sensitive SKA. In
fact, while our work on this front was still in progress, Kaku-
chi and coworkers reported the living/controlled GTP of
acrylate nBA by the in situ generated SKA initiator through
SCHEME 1 Proposed catalytic cycle for in situ generation of
the RSKA initiator through the “Et3Si1”-catalyzed 1,4-hydrosi-
lyation of MMA with hydrosilane R3SiH. The anion [B(C6F5)4]–
was omitted for clarity. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
polymerization, as outlined in Scheme 1, are different than
those fundamental steps of the conventional GTP. Further-
more, the silylium-catalyzed polymerization offers advantages
over the conventional GTP in terms of its ability to readily
produce high MW poly(methacrylate)s and also effects living
polymerization of acrylates under ambient temperature and
low catalyst loading conditions.9,10 The silylium-catalyzed GTP
method has also enabled high-speed, living polymerization of
renewable a-methylene-c-butyrolactones, producing homo and
random or block copolymers with controlled low to high MW
5
-
(Mn up to 5.43 3 10 kg/mol) and narrow MWD’s (D 5 1.01–
n
B(C6F5)3-catalyzed hydrosilylation of BA with R3SiH.31 How-
1.06).12 Covalently linked dinuclear, bifunctional silylium-SKA
active species have also been developed to circumvent the
limitation of the bimolecular, activated monomer propagation
mechanism imposed on polymerizations under highly dilute
initiator or catalyst conditions and on the stereochemical con-
trol of polymerization.13
ever, in our hands this R3SiH/B(C6F5)3 system exhibited no
activity for polymerization of less reactive, sterically bulkier
methacrylate monomers such as MMA.32 Subsequently, we
found that the system consisting of the stronger silylium
Lewis acid, R3SiH/[Et3Si(L)]1[B(C6F5)4]– (L 5 toluene or
Et3SiH), enables the first successful example of the silylium-
catalyzed living methacrylate polymerization by the in situ
generated SKA initiator through the “R3Si1”-catalyzed 1,4-
hydrosilylation of MMA with R3SiH. This contribution
presents a full account of this study.
Besides [Ph3C][B(C6F5)4], several additional types of activators
have also been used to deliver the silylium catalyst in situ
while activating RSKA. Kakuchi and coworkers14 utilized a
strong BrØnsted acid, trifluoromethanesulfonimide (HNTf2),15
to activate RSKA for in situ generation of Me3SiNTf2,16 leading
to the silylium-catalyzed living polymerization of MMA, and to
activate dialkylaminosilyl enol ether for the living polymeriza-
tion of N,N-dimethylacrylamide.17 Kakuchi et al.18 also reported
EXPERIMENTAL
Materials, Reagents, and Methods
n
Me3SiNTf2-catalyzed living GTP of BA with functional initiators
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 nitrogen during filling solvent res-
ervoirs and then dried by passage through activated alumina
(for Et2O, THF, and CH2Cl2) followed by passage through Q-5
and/or terminators to produce well-defined end-functionalized
polymers. We19 employed strong BrØnsted acids paired with
weakly
coordinating
perfluorophenylborate
anions,20
[H(Et2O)2][B(C6F5)4],21 [HN(Me2)Ph][B(C6F5)4],22 and a strong
BrØnsted acid bearing a chiral disulfonimide counteranion,2d to
deliver the R3Si1 catalyst that effects high-speed living poly-
merization with a unique “self-repair” features in the presence
of water.19 Also worth noting here are the observations that
1896
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 1895–1903

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
supported copper catalyst (for toluene and hexanes) stain-
less steel columns. Benzene-d6 and toluene-d8 were dried
over sodium/potassium alloy and vacuum-distilled or fil-
tered, whereas CDCl3 was dried over activated Davison 4 Å
molecular sieves. HPLC-grade fluorobenzene and 1,2-difluor-
obenzene were degassed and dried over CaH2 overnight, fol-
lowed by vacuum distillation (CaH2 was removed before
distillation). NMR spectra were recorded on a Varian Inova
300 (300 MHz, 1H; 75 MHz, 13C; 282 MHz, 19F) or a Varian
400 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 SiMe4, whereas 19F
NMR spectra were referenced to external CFCl3.
Conversely, the borane-catalyzed 1,4-hydrosilylation of MMA
in a 1:1:0.1 ratio of MMA/Me2PhSiH/B(C6F5)3 at 20 ꢀC in
CDCl3, following the above procedure, afforded cleanly the
corresponding Me2PhSKA without contamination with the
unconsumed silane and/or the oligomeric species (Fig. 5),
because there are no competing oligomerization/polymeriza-
tion reactions for the R3SiH/B(C6F5)3 system under the cur-
rent conditions.32
General Polymerization Procedures
Polymerizations were performed in 20 mL oven-dried glass
reactors inside the glovebox under ambient conditions (ca.
20 ꢀC). Two different polymerization procedures were
employed. In the first procedure, which was adopted by the
majority of this study, initiator Et3SiH or Me2PhSiH and
monomer MMA (0.20 g) were premixed in a predetermined
ratio in 1.0 mL C6H5F for 5 min, followed by rapid addition
of the catalyst solution of [Et3Si(L)]1[B(C6F5)4]–
(L 5 toluene) in 1.0 mL C6H5F to start the polymerization. In
the second procedure, which was used for some comparative
or control runs, a predetermined amount of MMA (0.20 g)
was first dissolved in 1.0 mL of solvent inside a glovebox,
and the polymerization was started by rapid addition of a
measured amount of the corresponding mixture of catalyst
and initiator in 1.0 mL of C6H5F via a gastight syringe to the
MMA solution under vigorous stirring. In both procedures,
after the measured time interval, a 0.1 mL aliquot was taken
from the reaction mixture via syringe and quickly quenched
into a 2-mL vial containing 0.6 mL of “wet” CDCl3 stabilized
by 250 ppm of BHT-H; the quenched aliquots were later ana-
MMA was purchased from Sigma-Aldrich Co.; it was first
degassed and dried over CaH2 overnight, followed by vac-
uum distillation. Further purification of MMA involved titra-
tion with neat tri(n-octyl) aluminum to a yellow end point,
followed by distillation under reduced pressure. The purified
monomer was stored in a brown bottle and stored inside a
glovebox freezer at 230 ꢀC. Triethylsilane (Et3SiH) and
dimethylphenylsilane (Me2PhSiH) were purchased from TCI.
These silanes were first degassed and dried over CaH2 over-
night, followed by vacuum distillation. Butylated hydroxyto-
luene (BHT-H) was purchased from Sigma-Aldrich and
recrystallized from hexanes prior to use. Tris(pentafluoro-
phenyl)borane B(C6F5)3 and trityl tetrakis(pentaflurophenyl)-
borate [Ph3C][B(C6F5)4] were purchased from Beijing
Innochem Co.; the borane was further purified by recrystalli-
zation from hexanes at 230 ꢀC. Literature procedures were
employed or modified for the preparation of the following
1
lyzed by H NMR to obtain the percent monomer conversion
1r
silylium complexes: [Et3Si(toluene)]1[B(C6F5)4]–
and
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, filtered, washed
with methanol, and dried in a vacuum oven at 50 ꢀC over-
night to a constant weight.
[Et3Si–H–SiEt3]1[B(C6F5)4]21c
.
In Situ Generation of Me2C@C(OMe)OSiMe2Ph (Me2PhSKA)
by Silylium-Catalyzed 1,4-Hydrosilylation of MMA
In an argon-filled glovebox, an NMR tube was charged with
10 mg (0.1 mmol) of MMA, 13.6 mg (0.1mmol) Me2PhSiH,
and 0.5 mL of C7D8. A 0.5 mL fluorobenzene solution of
[Et3Si(L)]1[B(C6F5)4]– (L 5 toluene) (0.50 l mol) was slowly
added to this NMR tube via a syringe at RT, and the resulting
reaction mixture was analyzed immediately by NMR. In the
1H NMR spectrum of the mixture, the signals at d 5.49 and
6.09 ppm characteristic of the @CH2 protons for MMA com-
pletely disappeared and those signals at d 4.58 and 0.41
ppm characteristic of H and Me protons for Me2PhSiH dimin-
ished greatly (but not completely disappeared, vide infra),
whereas the signals at d 3.31 and 0.46 ppm characteristic of
the OMe and SiMe2 protons for Me2PhSKA26b appeared. It is
noted here that Me2PhSiH was not completely consumed
because this silylium-catalyzed hydrosilylation is accompa-
nied by the competing reaction of the relatively fast silylium-
catalyzed oligomerization/polymerization that takes place as
soon as Me2PhSKA is generated. This competition resulted in
imbalanced consumption between MMA and Me2PhSiH; thus,
this reaction afforded predominantly the desired Me2PhSKA,
along with a small amount of the unconsumed Me2PhSiH
and oligomeric SKAs (Fig. 4).
Monomer conversions {1–[M]t/[M]0} were obtained from 1H
NMR analysis of aliquots. The ratio of [MMA]0 to [MMA]t at
a given time t, [M]0/[M]t, was determined by integration of
the peaks for MMA (5.2 and 6.1 ppm for the vinyl signals;
3.4 ppm for the OMe signal in CDCl3) and PMMA (centered
at 3.4 ppm for the OMe signals in CDCl3) according to [M]0/
[M]t 5 2A3.4/3A5.216.1, where A3.4 is the total integrals for
the peaks centered at 3.4 ppm (typically in the region
3.223.6 ppm) and A5.216.1 is the total integrals for both
peaks at 5.2 and 6.1 ppm.
Polymer Characterizations
Polymer number-average molecular weights (Mn) and MWDs
-
(D 5 Mw/Mn) were measured by gel peꢀrmeation chromatog-
raphy (GPC) analyses carried out at 35 C and a flow rate of
1.0 mL/min with DMF as the eluent, on a Agilent 1260 GPC
instrument coupled with a Waters RI detector and equipped
with two PL gel 5 lm mixed-C columns (Polymer Laborato-
ries; linear range of molecular weight 5 200–2,000,000). The
instrument was calibrated with 10 PMMA standards, and
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 1895–1903
1897

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
TABLE 1 Selected Results of MMA Polymerization by Et3SiH/[Et3Si(L)]1[B(C6F5)4]– (L 5 toluene)a
b
c
c
d
e
-
D
Run no.
Time (min)
conv. (%)
Mn (kg/mol)
(Mw/Mn)
Mn (calcd) (kg/mol)
I* (%)
1
2
3
4
5
6
7
8
9
4
9
3.50
4.10
7.20
10.1
13.4
15.7
20.0
30.5
42.0
1.05
1.05
1.05
1.08
1.06
1.06
1.05
1.07
1.09
3.52
4.05
7.25
10.1
13.6
15.7
20.1
30.5
40.1
101
99
5
10
18
25
34
39
50
76
100
10
15
20
25
30
50
70
101
100
101
100
101
100
96
a
c
p 5 RT (ꢂ20 ꢀC),
Mn and D determined by gel-permeation chromatography (GPC) rela-
-
Conditions: solvent 5 C6H5F (2.0 mL),
T
[MMA]0 5 1.0
M
(0.2 mmol), [MMA]0/[Et3SiH]0/[Et3Si(L)]1[B(C6F5)4]– 5
tive to PMMA standards in DMF.
d
400:1:1.
b
Mn(calcd) 5 MW(MMA) 3 [MMA]0/[I]0 3 conversion% 1 MW of chain-
Monomer conversions measured by 1H NMR.
end groups, where [I]0 5 [Et3SiH]0 and chain ends 5 2 H.
e
Initiation efficiency (I*) 5 Mn(calcd)/Mn(exptl).
chromatograms were processed with Agilent Empower soft-
ware (version 2002).
Et3SiH 1 [Ph3C][B(C6F5)4] in toluene, followed by extensive
washing with toluene and hexanes (to remove any possible
excess Et3SiH and/or [Ph3C][B(C6F5)4]), the isolated pure
complex, [Et3Si(toluene)]1[B(C6F5)4]–,1r exhibited no poly-
merization activity. In contrast, when the silylium complex
was actually impure (i.e., containing a small amount of
Et3SiH) or prepared from the reaction using excess Et3SiH,1r,u
it polymerized MMA rapidly. Finally, the isolated pure
silylium-silane complex, [Et3Si–H–SiEt3]1[B(C6F5)4]–,1c,f rap-
idly polymerized MMA. Overall, the above control experi-
ments clearly showed that the active polymerization system
was brought about by the combination of Et3SiH and
[Et3Si(L)]1[B(C6F5)4]–, or in the form of a single component
silylium-silane complex, [Et3Si–H–SiEt3]1[B(C6F5)4]–. From a
practical point of view, the polymerization by R3SiH/
[Et3Si(L)]1[B(C6F5)4]– (L 5 toluene) is more convenient and
controlled, which was the protocol adopted for this study.
Low molecular-weight MMA oligomers produced by the cata-
lyst system Et3SiH/[Et3Si(L)]1[B(C6F5)4]– (L5 toluene) in flu-
orobenzene at RT were analyzed by matrix-assisted laser
desorption/ionization time-of-flight mass spectroscopy
(MALDI-TOF MS). The experiment was performed on a Micro
MX MALDI-TOF mass spectrometer (Waters) operated in a
positive ion and reflector mode using a 1.2 kV accelerating
voltage. The sample (1 lL in THF) was mixed with 1 lL of a-
cyano-4-hydroxycinnamic acid (CHCA, 10 mg/ml in THF). The
mixture was spotted on the MALDI target and allowed to air
dry. External calibration was done using a peptide calibration
mixture (4 to 6 peptides) on a spot adjacent to the sample.
RESULTS AND DISCUSSION
Characteristics of MMA Polymerization by R3SiH/
Table 1 summarizes the results of the MMA polymerization
by Et3SiH/[Et3Si(L)]1[B(C6F5)4]– (L 5 toluene) in a 400/1/1
ratio (i.e., with a silylium catalyst loading of 0.25 mol % rela-
tive to monomer). It can be seen from the table, the poly-
merization went smoothly to completion in less than 70 min
in a living/controlled manner. The monomer conversion
increased gradually with time and the Mn of the polymer
increased linearly with monomer conversion (Fig. 1), while
the MWD remained narrow throughout the entire course of
[Et3Si(L)]1[B(C6F5)4]–
At the outset, we carried out necessary control MMA poly-
merization runs using all the reagents involved in the pres-
ent system (i.e., silane, activator, silylium complex), either an
individual reagent alone or when combined together in dif-
ferent ratios. Although other solvents may also be appropri-
ate for this polymerization system involving the highly
reactive and sensitive silylium ions, we settled to fluoroben-
zene (C6H5F) as the solvent due to the suitable solubility
and stability of “R3Si1“ in this solvent. Thus, control MMA
polymerization runs at RT using hydrosilane (Et3SiH or
Me2PhSiH) or the activator [Ph3C][B(C6F5)4] alone yielded
no polymer formation up to 24 h. The 1:1 mixture of
Et3SiH 1 [Ph3C][B(C6F5)4] in toluene after being stirred for
10 min, followed by addition of MMA, also exhibited no poly-
merization activity, but the 2:1 mixture brought about rapid
polymerization. Next, the polymerization using the isolated
silylium complex, [Et3Si(L)]1[B(C6F5)4]–, yielded two con-
trasting results, depending on how the complex was pre-
pared; when it was prepared from the 1:1 reaction of
-
the polymerization (D 5 1.05–1.09). The measured Mn values
were identical or nearly identical to those calculated ones
based on the monomer-to-initiator ratio ([MMA]0/[Et3SiH]0)
and conversion data, thereby affording essentially quantita-
tive initiation efficiencies (I* ꢁ 96%) for all conversion points
analyzed in this study (see Table 1). These results high-
lighted the high degree of control and efficiency possessed
by this silylium-catalyzed polymerization system.
The living characteristics of the silylium-catalyzed polymer-
ization were further revealed by the results obtained from
investigating effects of the monomer-to-initiator ratio on the
1898
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 1895–1903

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
Interestingly, under the same conditions (RT, C6H5F, 0.25
mol% loading of [Et3Si(L)]1[B(C6F5)4]–), the MMA polymeriza-
tion with Me2PhSiH was noticeably faster than that using
Et3SiH but the degree of polymerization control is more or
less the same. Thus, the monomer conversion after 50 min
was 76.2% with Et3SiH (run 8, Table 1), whereas the conver-
sion was 100% with Me2PhSiH, at which time the PMMA pro-
4
-
duced had Mn 5 4.23 3 10 g/mol and D 5 1.08, which were
nearly identical to those values of the PMMA produced by
Et3SiH at quantitative monomer conversion (run 9, Table 1).
"Et3Si1"-Catalyzed Hydrosilylation of MMA for In Situ
Generation of RSKA
The SKA initiator Me2C5C(OMe)OSiMe2Ph (Me2PhSKA) can be
prepared by the Rh-catalyzed 1,4-hydrosilylation of MMA
with Me2PhSiH.26b To demonstrate in situ generation of the
initiator Me2PhSKA in the current polymerization system
starting with Me2PhSiH/[Et3Si(L)]1[B(C6F5)4]– (L 5 toluene),
-
FIGURE 1 Plots of PMMA Mn and D values versus monomer
conversion. Data taken from Table 1.
we
carried
out
the
model
reaction
of
MMA 1 Me2PhSiH 1 [Et3Si(L)]1[B(C6F5)4]– in
a
1:1:0.005
resulting polymer Mn values and from the chain-extension
experiments. Thus, when the [MMA]0/[Et3SiH]0 ratio was
varied from 100:1, 200:1, 300:1, to 400:1, the Mn value of
ratio at 20 ꢀC in fluorobenzene 1 toluene-d8. The 1H NMR
spectrum obtained immediately after mixing of these
reagents clearly showed the formation of Me2PhSKA [d 7.64
(m, 2H, Ph), 7.39 (m, 3H, Ph), 3.42 (s, 3H, OMe), 1.57 (s, 3H,
5CMe2), 1.51 (s, 3H, 5CMe2), 0.48 (s, 6H, SiMe2)], along
with a small amount of the unconsumed Me2PhSiH and oli-
gomeric SKAs (Fig. 4). The inevitable presence of the latter
two types of species from this reaction is due to the rela-
tively fast silylium-catalyzed oligomerization/polymerization
that takes place as soon as Me2PhSKA is generated, which
competes with the hydrosilylation reaction; hence, such com-
peting reaction products were unavoidable regardless of the
the resulting PMMA increased linearly from Mn 5 1.04 3 104
4
-
-
g/mol (D 5 1.10, I* 5 96%), 2.04 3 10 g/mol (D 5 1.06,
4
-
I* 5 98%), 3.05 3 10 g/mol (D 5 1.07, I* 5 98%), to 4.20 3
104 g/mol (D 5 1.09, I* 5 96%), respectively, (Fig. 2). Worth
-
noting also are the nearly quantitative initiation efficiencies
and narrow MWD’s achieved for all the ratio runs (vide
supra) by this polymerization method. Furthermore,
a
sequential chain-extension experiment was performed by
addition of another 200 equiv of MMA to the end of the
polymerization that started with a [MMA]0/[Et3SiH]0 ratio of
200:1, which produced PMMA with Mn 5 4.32 3 104 g/mol
ꢀ
reaction conditions (from RT to 280 C in toluene-d8 or flu-
orobenzene 1 toluene-d8), the catalyst loading (from 0.1 to
-
and D 5 1.07 (Fig. 3); this Mn was approximately double of
10 mol %), and the nature of hydrosilanes (Et3SiH or
the MW of the PMMA produced by the first sequence
4
-
(Mn 5 2.13 3 10 g/mol, D 5 1.09) and agreed well with the
Mn of the PMMA produced by the polymerization directly
using a [MMA]0/[Et3SiH]0 ratio of 400:1.
FIGURE 3 Overlay of GPC traces of the PMMA samples pro-
duced by the sequential addition of MMA. A: PMMA obtained
after consuming the first 200 equiv of MMA (Mn 5 2.13 3 104 g/
-
mol, D 5 1.09); B: PMMA obtained after consuming the second
200 equiv of MMA (Mn 5 4.32 3 104 g/mol, D 5 1.07). [Color fig-
-
ure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIGURE 2 Plot of PMMA Mn versus [MMA]0/[Et3SiH]0 ratio.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 1895–1903
1899

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 4 1H NMR spectrum for in situ generation of Me2PhSKA
from the 1:1:0.005 ratio reaction of MMA/Me2PhSiH/
[Et3Si(L)]1[B(C6F5)4]– (L 5 toluene) at 20 ꢀC in fluorobenze-
ne 1 toluene-d8. Unlabeled peaks were attributed to a small
amount of the unconsumed Me2PhSiH (marked with an *),
oligomers, and NMR solvents.
FIGURE 6 Zero-order plot of [M]t/[M]0 versus time for polymer-
ization of MMA ([MMA]0 5 0.909
M in C6H5F) by Et3SiH/
[Et3Si(L)]1[B(C6F5)4]– (L 5 toluene) at 20 ꢀC in a MMA/Et3SiH/
Me2PhSiH). Nonetheless, in all cases the in situ generation of
[Et3Si(L)]1[B(C6F5)4]– ratio of 400:1:1.
RSKA was clearly confirmed.
activation of the hydrosilane by the Lewis acid,30 now in the
form of the isolable silylium-silane complex, [Et3Si–H–
SiR3]1[B(C6F5)4]–,1c,f is outlined in Scheme 1.
In contrast, 1,4-hydrosilylation of MMA catalyzed by
B(C6F5)3 generates cleanly the corresponding RSKA without
contamination with the unconsumed silane and/or the oligo-
meric species. For example, the 1:1:0.1 ratio reaction of
Me2PhSiH 1 MMA 1 B(C6F5)3 at 20 ꢀC in CDCl3 afforded
cleanly Me2PhSKA (Fig. 5), thanks to the fact that the R3SiH/
B(C6F5)3 system exhibited no activity for polymerization of
MMA under the current conditions;32 therefore, there was no
imbalanced consumption between MMA and R3SiH and no
oligomer formation either. This result also provided insight
into why the R3SiH/B(C6F5)3 system exhibited no activity in
MMA polymerization, which is due to the inability of this
borane Lewis acid to promote subsequent repetitive itera-
tions of Mukaiyama-Michael addition of the SKA to monomer
in the chain propagation cycle.
Kinetics and Mechanism of Polymerization by R3SiH/
[Et3Si(L)]1[B(C6F5)4]–
Our previous detailed kinetic studies on the acrylic poly-
merization catalyzed by silylium ions “R3Si1,” generated by
in situ oxidative activation of RSKA with a catalytic amount
of [Ph3C][B(C6F5)4], showed that such silylium-catalyzed liv-
ing anionic-addition polymerization of acrylics followed
first-order kinetics with respect to the catalyst [R3Si1] and
initiator [RSKA] concentrations, but zero-order kinetics with
respect to monomer [M] concentration.9,10,12 Noteworthy is
that such polymerization kinetics (i.e., zero-order depend-
ence on [M] and first- order dependence on initiator and
Lewis acid catalyst concentrations) were also observed in
the GTP-type polymerization catalyzed by zirconocenium
cations.33 Such kinetics are consistent with the propagation
mechanism in that the CAC bond forming step via intermo-
lecular Mukaiyama-Michael addition of the propagating spe-
cies to the Lewis acid-activated monomer is the rate-
Overall, the above study supported the hypothesis that the
silylium-catalyzed 1,4-hydrosilylation of MMA with R3SiH
generates in situ the RSKA initiator. This hydrosilylation cata-
lyzed by the strongly Lewis acidic silylium ion “Et3Si1”
resembles that catalyzed by the strong Lewis acid
B(C6F5)3,27–30 and the similar mechanism invoking the FLP
FIGURE 5 1H NMR spectrum for in situ generation of Me2PhSKA from the 1:1:0.1 ratio reaction of Me2PhSiH 1 MMA 1 B(C6F5)3 at 20
ꢀC in CDCl3. Peaks marked with an * were due to the NMR solvent and silicon grease.
1900
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 1895–1903

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
nisms proposed in Schemes 1 and 2, respectively, the result-
ing PMMA should have linear chain structure of
a
HA[MMA]nAH, where the initiation end (H) is derived from
the hydrosilylation of MMA by R3SiAH and the termination
end (H) is derived from hydrolysis of the SKA chain end [–
CH2C(Me)@C(OMe)OSiEt3] through quenching and/or matrix
preparation procedures (see Experimental). To confirm this
hypothesis, we used matrix-assisted laser desorption/ioniza-
tion time-of-flight mass spectroscopy (MALDI-TOF MS) to
analyze the chain-end groups of a low-MW oligomer sample
-
with Mn 5 2.80 kg/mol and D 5 1.08 (measured by GPC),
which was prepared by Et3SiH/[Et3Si(L)]1[B(C6F5)4]–
(L 5 toluene) at 20 ꢀC in C6H5F with MMA/Et3SiH/
[Et3Si(L)]1 [B(C6F5)4]– 5 25/1/0.1 and quenched with wet
hexanes. As can be seen from Figure 7, the MALDI-TOF MS
spectrum of the sample consisted of two series of molecular
ion peaks, with the spacing between the two neighboring
molecular ion peaks within each series being that of the
exact molar mass of the oligomer repeat unit, MMA (m/
z 5 100.03). Further analysis of the m/z values of the same
molecular ion series clearly indicated that series (a) corre-
sponds to the structure of [HA[MMA]nAH 1 Li]1, whereas
SCHEME 2 Proposed “R3Si1”-catalyzed chain propagation
cycle via repetitive iterations of Mukaiyama-Michael addition of
the growing SKA chain to the activated monomer. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
series
(b)
corresponds
to
the
structure
of
limiting step and the release of the Lewis acid catalyst from
its coordinated last inserted monomer unit in the growing
polymer chain to the incoming monomer is relatively fast.34
[HA[MMA]nAH 1 Na]1 (Fig. 7); for example, the observed
and the calculated (in parenthesis) m/z values of 1509.77
(1509.82) and 1525.68 (1525.79) agreed well with each
other for the 15-mer structures of [HA[MMA]15AH 1 Li]1
and [HA[MMA]15AH 1 Na]1, respectively, (ions from the
MALDI matrix). Overall, this result provides an additional
piece of critical evidence to support not only the above pro-
posed initiation and propagation mechanisms, but also the
living characteristics of this silylium-catalyzed methacrylate
polymerization that produces the well-defined and controlled
PMMA.
A survey of kinetics of the current “R3Si1”-catalyzed living poly-
merization system with the in situ generated RSKA initiator
through the “R3Si1”-catalyzed hydrosilylation of MMA with
R3SiH also showed clearly the zero-order dependence on mono-
mer concentration (Fig. 6). Hence, the chain propagation mecha-
nism proposed for this polymerization system, as outlined in
Scheme 2, follows that already established for other silylium-
catalyzed polymerization processes.9,10,12 Consistent with this
mechanistic picture, the PMMA produced by the current cata-
lyst/initiator system is a syndiotactic-biased material (71% rr).
CONCLUSIONS
Lastly, if the current “Et3Si1”-catalyzed living methacrylate
polymerization follows the initiation and propagation mecha-
To the best of our knowledge, this work represents the first
successful silylium-catalyzed living polymerization of
FIGURE 7 MALDI-TOF Mass Spectrum of a low-MW oligomer sample prepared by Et3SiH/[Et3Si(L)]1[B(C6F5)4]– (L 5 toluene) at 20
ꢀC in C6H5F with MMA/Et3SiH/[Et3Si(L)]1[B(C6F5)4]– 5 25/1/0.1 and quenched with wet hexanes: H–(MMA)n–H 1 Li1 (a) and H–
(MMA)n–H 1 Na1 (b).
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 1895–1903
1901

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
methacrylates (represented by MMA in this study) by the in
situ generated SKA initiator through the “Et3Si1”-catalyzed
1,4-hydrosilylation of monomer with R3SiH. Although the liv-
mined by analysis of oligomeric MMA samples with MALDI-
TOF MS, are also consistent with the above proposed chain
initiation and propagation mechanisms.
n
ing/controlled GTP of acrylates (represented by BA) by the
in situ generated SKA through B(C6F5)3-catalyzed hydrosily-
lation of monomer with R3SiH became known while this
study was still progress,31 in our hands the R3SiH/B(C6F5)3
system exhibited no activity for polymerization of less reac-
tive, sterically bulkier methacrylate monomers such as MMA.
In contrast, the present system consisting of the stronger
silylium Lewis acid, R3SiH/[Et3Si(L)]1[B(C6F5)4]–, catalyzes
high-speed and living methacrylate polymerization. The liv-
ing characteristics of the MMA polymerization by [Et3SiH]/
[Et3Si(L)]1[B(C6F5)4]– were revealed by four sets of experi-
ments. First, the monomer conversion increased gradually
with time and the Mn of the polymer increased linearly with
monomer conversion, while the MWD remained narrow
ACKNOWLEDGMENTS
This work was supported by the National Science Foundation
of China (NSFC- No. 21274015) and the Fundamental Research
Funds for the Central Universities (No. DUT14LK28) for the
study carried out at DLUT, as well as by the US National Science
Foundation (NSF-1150792) for the study carried out at CSU.
REFERENCES AND NOTES
1 Selected recent examples and reviews: (a) R. J.
Wehmschulte, K. K. Laali, G. L. Borosky, D. R. Powell, Organo-
metallics 2014, 33, 214622149; (b) K. Muther, P. Hrobarik, V.
Hrobarikova, M. Kaupp, M. Oestreich, Chem. Eur. J. 2013, 19,
16579–16594; (c) S. J. Connelly, W. Kaminsky, D. M. Heinekey,
€
-
throughout the polymerization (D 5 1.05–1.09). Second, the
measured Mn values were identical or nearly identical to
those calculated ones, which yielded essentially quantitative
initiation efficiencies for all the conversion points and varied
monomer-to-initiator ratio runs, thereby further highlighting
the high degree of control and essentially no chain termina-
tion or transfer events in this silylium-catalyzed polymeriza-
tion system. Third, the Mn of the polymer was found to
increase linearly with increasing the monomer-to-initiator
ratio without broadening the polymer MWD, and sequential
additions of monomer feeds successfully extended the poly-
mer chain. Fourth, the well-defined chain structure of
HA[MMA]nAH was confirmed by MALDI-TOF MS analysis of
the MMA oligomers.
€
Organometallics 2013, 32, 747827481; (d) A. Schafer, M.
€
€
Reißmann, S. Jung, A. Schafer, W. Saak, E. Brendler, T. Muller,
€
Organometallics 2013, 32, 4713–4722; (e) A. Schafer, M.
€
€
Reißmann, A. Schafer, W. Saak, D. Haase, T. Muller, Angew.
Chem. Int. Ed. 2011, 50, 12636–12638; (f) M. Nava, C. A. Reed,
€
Organometallics 2011, 30, 479824800; (g) K. Muther, R.
€
€
Frohlich, C. Muck-Lichtenfeld, S. Grimme, M. Oestreich, J. Am.
Chem. Soc. 2011, 133, 12442212444; (h) C. A. Reed, Acc.
Chem. Res. 2010, 43, 1212128; (i) H. F. T. Klare, K. Bergander,
M. Oestreich, Angew. Chem. Int. Ed. 2009, 48, 9077–9079; (j) S.
Duttwyler, Q.-Q. Do, A. Linden, K. K. Baldridge, J. S. Siegel,
€
Angew. Chem. Int. Ed. 2008, 47, 1719–1722; (k) T. Kuppers, E.
Bernhardt, R. Eujen, H. Willner, C. W. Lehmann, Angew. Chem.
Int. Ed. 2007, 46, 634626349; (l) T. A. Kochina, D. V. Vrazhnov,
E. N. Sinotova, M. G. Voronkov, Russ. Chem. Rev. 2006, 75,
€
952110; (m) T. Muller, Adv. Organomet. Chem. 2005, 53,
The real initiator of the polymerization system, RSKA, is gen-
erated in situ through the “Et3Si1”-catalyzed 1,4-hydrosilyla-
tion of monomer with R3SiH, as evidenced by monitoring the
1552215; (n) K.-C. Kim, C. A. Reed, D. W. Elliott, L. J. Mueller,
F. Tham, L. Lin, J. B. Lambert, Science 2002, 297, 8252827; (o)
J. B. Lambert, Y. Zhao, S. M. Zhang, J. Phys. Org. Chem. 2001,
14, 3702379; (p) C. A. Reed, Acc. Chem. Res. 1998, 31,
3252332; (q) Z. Xie, A. Benesi, C. A. Reed, Organometallics
1995, 14, 393323941; (r) J. B. Lambert, S. Zhang, S. M. Ciro,
Organometallics 1994, 13, 243022443; (s) Z. Xie, D. J. Liston, T.
1
reaction with H NMR. This silylium-catalyzed hydrosilylation
was proposed to proceed via FLP-type activation of the
hydrosilane in the form of the isolable silylium-silane com-
plex, [Et3SiAHASiR3]1[B(C6F5)4]–, a mechanism similar to
the well-established hydrosilylation of carbonyl compounds
catalyzed by the strong Lewis acid B(C6F5)3.30 However,
there is a key difference between these two Lewis acid-
catalyzed 1,4-hydrosilylation systems: while 1,4-hydrosilyla-
tion of MMA catalyzed by B(C6F5)3 generates cleanly the cor-
responding RSKA, the hydrosilylation catalyzed by “R3Si1”
produces RSKA predominantly, together with a small amount
of the unconsumed silane and/or the oligomeric species, due
to the ability of “R3Si1” to catalyze the subsequent, relatively
fast oligomerization/polymerization of MMA once the real
ꢀ
Jelınek, V. Mitro, R. Bau, C. A. Reed, J. Chem. Soc., Chem.
Commun. 1993, 3842386; (t) J. B. Lambert, S. Zhang, J. Chem.
Soc., Chem. Commun. 1993, 3832384; (u) J. B. Lambert, S.
Zhang, C. L. Stern, J. C. Huffman, Science 1993, 260,
191721918.
2 Selected recent examples: (a) L. Ratjen, M. van Gemmeren,
F. Pesciaioli, B. List, Angew. Chem. Int. Ed. 2014, 53,
876528769; (b) M. Mahlau, P. Garcıa-Garcıa, B. List, Chem. Eur.
J. 2012, 18, 16283216287; (c) O. Allemann, S. Duttwyler, P.
Romanto, K. K. Baldridge, J. S. Siegel, Science 2011, 332,
5742577; (d) P. Garcıa-Garcıa, F. Lay, P. Garcıa-Garcıa, C.
Rabalakos, B. List, Angew. Chem. Int. Ed. 2009, 48, 436324366;
(e) C. H. Cheon, H. Yamamoto, J. Am. Chem. Soc. 2008, 130,
924629247; (f) K. Hara, R. Akiyama, M. Sawamura, Org. Lett.
2005, 7, 562125623.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
R
polymerization initiator SKA is generated.
Polymerization kinetics of the present system are consistent
with the previously established “R3Si1”-catalyzed propaga-
tion mechanism in that the CAC bond forming step via inter-
molecular Mukaiyama-Michael addition of the propagating
species to the R3Si1-activated monomer is the rate-limiting
step and the release of the “R3Si1” catalyst to the incoming
monomer is relatively fast. The polymer end groups, deter-
€
3 Selected recent examples: (a) K. Muther, J. Mohr, M.
Oestreich, Organometallics 2013, 32, 664326646; (b) A. Schulz,
A. Villinger, Angew. Chem. Int. Ed. 2012, 51, 4526–4528; (c) R.
K. Schmidt, K. Muther, C. Muck-Lichtenfeld, S. Grimme, M.
Oestreich, J. Am. Chem. Soc. 2012, 134, 442124428; (d) K.
Leszczynska, A. Mix, R. J. F. Berger, B. Rummel, B. Neumann,
H.-G. Stammler, P. Jutzi, Angew. Chem. Int. Ed. 2011, 50, 6843–
€
€
1902
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 1895–1903

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
€
6846; (e) K. Muther, M. Oestreich, Chem. Commun. 2011, 47,
3342336; (f) F. M. Ibad, P. Langer, A. Schulz, A. Villinger, J.
Am. Chem. Soc. 2011, 133, 21016221027; (g) H. F. T. Klare, M.
Oestreich, Dalton Trans. 2010, 39, 917629184; (h) C. Douvris, C.
M. Nagaraja, C.-H. Chen, B. M. Foxman, O. V. Ozerov, J. Am.
Chem. Soc. 2010, 132, 494624953; (i) S. Duttwyler, C. Douvris,
N. L. P. Fackler, F. S. Tham, C. A. Reed, K. K. Baldridge, J. S.
Siegel, Angew. Chem. Int. Ed. 2010, 49, 7519–7522; (j) C.
Douvris, O. V. Ozerov, Science 2008, 321, 118821190; (k) R.
20 E. Y.-X. Chen, T. J. Marks, Chem. Rev. 2000, 100, 1391–1434.
€
21 P. Jutzi, C. Muller, A. Stammler, H. Stammler, Organometal-
lics 2000, 19, 1442–1444.
22 (a) E. B. Tjaden, D. C. Swenson, R. F. Jordan, Organometal-
lics 1995, 14, 371–386; (b) H. W. Turner, G. G. Hlatky, R. R.
Eckman, U.S. Patent 5,198,401, 1993.
23 K. Ute, H. Ohnuma, T. Kitayama, Polym. J. 2000, 32, 1060–
1062.
€
24 R. Zhuang, A. H. E. Muller, Macromolecules 1995, 28, 8035–
€
Panisch, M. Boldte, T. Muller, J. Am. Chem. Soc. 2006, 128,
8042; 8043–8050.
9676–9682.
25 K. Ute, H. Ohnuma, I. Shimizu, T. Kitayama, Polym. J. 2006,
38, 999–1003.
4 (a) Q. Wang, H. Zhang, G. K. S. Prakash, T. E. Hogen-Esch, G.
A. Olah, Macromolecules 1996, 29, 669126694; (b) G. A. Olah,
X.-Y. Li, Q. Wang, G. Rasul, G. K. S. Prakash, J. Am. Chem.
Soc. 1995, 117, 896228966.
26 For examples of SKA synthesis by metal-catalyzed hydrosi-
lylation of a,b-unsaturated esters, see: (a) Hydrosilylation, A
Comprehensive Review on Recent Advances, B. Marciniec,;
Springer, 2009; Chapter 9, pp 289–339; (b) G. Z. Zheng, T. H.
Chan, Organometallics 1995, 14, 70–79; (c) A. Revis, T. K. Hilty,
J. Org. Chem. 1990, 55, 2972–2973; (d) I. Ojima, M. Kumagai, J.
Orgmet. Chem. 1976, 111, 43–60.
5 G. Toskas, M. Moreau, M. Masure, P. Sigwalt, Macromole-
cules 2001, 34, 473024736.
6 M. Cypryk, J. Chojnowski, J. Kurjata, ACS Symp. Ser. 2007,
964, 10226.
7 G. A. Olah, Q. Wang, X.-Y. Li, G. Rasul, G. K. S. Prakash,
Macromolecules 1996, 29, 185721861.
27 D. J. Parks, W. E. Piers, J. Am. Chem. Soc. 1996, 118, 9440–
9441.
8 Y. Zhang, K. Huynh, I. Manners, C. A. Reed, Chem. Commun.
2008, 4942496.
28 J. M. Blackwell, D. J. Morrison, W. E. Piers, Tetrahedron
2002, 58, 8247–8254.
9 Y. Zhang, E. Y.-X. Chen, Macromolecules 2008, 41, 36242.
29 Selected reviews: (a) D. W. Stephan, Acc. Chem. Res. 2015,
48, 306–316; (b) D. W. Stephan, G. Erker, Chem. Sci. 2014, 5,
262522641; (c) Frustrated Lewis Pairs I & II; D. W. Stephan, G.
Erker, Eds.; Topics in Current Chemistry; Springer: New York,
2013; Vols. 332 & 334; (d) G. Erker, Pure Appl. Chem. 2012, 84,
220322217; (e) D. W. Stephan, Org. Biomol. Chem. 2012, 10,
574025746; (f) G. Erker, Dalton Trans. 2011, 40, 747527483; (g)
D. W. Stephan, G. Erker, Angew. Chem. Int. Ed. 2010, 49,
46276; (h) D. W. Stephan, Dalton Trans. 2009, 312923136; (i)
D. W. Stephan, Org. Biomol. Chem. 2008, 6, 153521539.
10 Y. Zhang, E. Y.-X. Chen, Macromolecules 2008, 41,
635326360.
11 (a) K. Fuchise, Y. Chen, T. Satoh, T. Kakuchi, Polym. Chem.
2013, 4, 427824291; (b) M. Rikkou-Kalourkoti, O. W. Webster, C.
S. Patrikios, Encyclopedia of Polymer Science and Technology;
Wiley: New York, 2013; Vol. 99, pp 1277; (c) O. W. Webster, Adv.
Polym. Sci. 2004, 167, 1–34; (d) D. Y. Sogah, W. R. Hertler, O. W.
Webster, G. M. Cohen, Macromolecules 1987, 20, 1473–1488; (e)
O. W. Webster, W. R. Hertler, D. Y. Sogah, W. B. Farnham, T. V.
RajanBabu, J. Am. Chem. Soc. 1983, 105, 5706–5708.
€
30 (a) A. Y. Houghton, J. Hurmalainen, A. Mansikkamaki, W. E.
12 G. M. Miyake, Y. Zhang, E. Y.-X. Chen, Macromolecules
2010, 43, 4902–4908.
Piers, H. M. Tuononen, Nat. Chem. 2014, 6, 983–988; (b) S.
Rendler, M. Oestreich, Angew. Chem. Int. Ed. 2008, 47,
599726000; (c) D. J. Parks, J. M. Blackwell, W. E. Piers, J. Org.
Chem. 2000, 65, 3090–3098.
13 Y. Zhang, L. O. Gustafson, E. Y.-X. Chen, J. Am. Chem. Soc.
2011, 133, 13674–13684.
14 R. Kakuchi, K. Chiba, K. Fuchise, R. Sakai, T. Satoh, T.
Kakuchi, Macromolecules 2009, 42, 874728750.
31 K. Fuchise, S. Tsuchida, K. Takada, Y. Chen, T. Satoh, T.
Kakuchi, ACS Macro Lett. 2014, 3, 1015–1019.
15 (a) H. Yamamoto, Tetrahedron 2007, 63, 8377–8412; (b) M. B.
Boxer, H. Yamamoto, J. Am. Chem. Soc. 2006, 128, 48–49; (c) K.
Ishihara, Y. Hiraiwa, H. Yamamoto, Syn Lett. 2001, 1851–1854.
32 MMA polymerizations by Et3SiH/B(C6F5)3 in a MMA/Et3SiH/
B(C6F5)3 ratio of 400/1/1 in C6H5F (i.e., the standard monomer-
to-initiator-to-catalyst ratio and conditions employed in this
work) or 30/1/0.01 in CH2Cl2 (i.e., the standard conditions
employed in ref. 31), yielded no polymer formation at RT for
up to 24 h.
16 B. Mathieu, L. Ghosez, Tetrahedron Lett. 1997, 38,
549725500.
17 K. Fuchise, R. Sakai, T. Satoh, S.-I. Sato, A. Narumi, S.
Kawaguchi, T. Kakuchi, Macromolecules 2010, 43, 558925594.
33 (a) G. Stojcevic, H. Kim, N. J. Taylor, T. B. Marder, S.
Collins, Angew. Chem. Int. Ed. 2004, 43, 552325526; (b) Y. Li,
D. G. Ward, S. S. Reddy, S. Collins, Macromolecules 1997, 30,
187521883.
18 K. Takada, K. Fuchise, T. Kubota, T. Ito, Y. Chen, T. Satoh,
T. Kakuchi, Macromolecules 2014, 47, 551425525.
ꢀ
ꢀ
19 Y. Zhang, F. Lay, P. Garcıa-Garcıa, B. List, E. Y.-X. Chen,
Chem. Eur. J. 2010, 16, 10462–10473.
34 E. Y.-X. Chen, Chem. Rev. 2009, 109, 5157–5214.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 1895–1903
1903