Silyl ketene acetals/B(C6F5)3 lewis pair-catalyzed living group transfer polymerization of renewable cyclic acrylic monomers

Article abstract of DOI:10.3390/molecules23030665

This work reveals the silyl ketene acetal (SKA)/B(C₆F₅)₃ Lewis pair-catalyzed room-temperature group transfer polymerization (GTP) of polar acrylic monomers, including methyl linear methacrylate (MMA), and the biorenewable

Full text of DOI:10.3390/molecules23030665

molecules
Article
Silyl Ketene Acetals/B(C6F5)3 Lewis Pair-Catalyzed
Living Group Transfer Polymerization of Renewable
Cyclic Acrylic Monomers
ID
Lu Hu, Wuchao Zhao, Jianghua He * and Yuetao Zhang *
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University,
Changchun 130012, China; hulu14@mails.jlu.edu.cn (L.H.); zhaowc16@mails.jlu.edu.cn (W.Z.)
* Correspondence: hjh2015@jlu.edu.cn (J.H.); ytzhang2009@jlu.edu.cn (Y.Z.); Tel.: +86-157-6432-7755 (Y.Z.)
Received: 30 January 2018; Accepted: 14 March 2018; Published: 15 March 2018
Abstract: This work reveals the silyl ketene acetal (SKA)/B(C6F5)3 Lewis pair-catalyzed
room-temperature group transfer polymerization (GTP) of polar acrylic monomers,
including methyl linear methacrylate (MMA), and the biorenewable cyclic monomers
γ
-methyl-α-methylene-γ-butyrolactone (MMBL) and α-methylene-γ-butyrolactone (MBL) as
well. The in situ NMR monitored reaction of SKA with B(C6F5)3 indicated the formation of
Frustrated Lewis Pairs (FLPs), although it is sluggish for MMA polymerization, such a FLP system
exhibits highly activity and living GTP of MMBL and MBL. Detailed investigations, including the
characterization of key reaction intermediates, polymerization kinetics and polymer structures have
led to a polymerization mechanism, in which the polymerization is initiated with an intermolecular
Michael addition of the ester enolate group of SKA to the vinyl group of B(C6F5)3-activated monomer,
while the silyl group is transferred to the carbonyl group of the B(C6F5)3-activated monomer to
generate the single-monomer-addition species or the active propagating species; the coordinated
B(C6F5)3 is released to the incoming monomer, followed by repeated intermolecular Michael
additions in the subsequent propagation cycle. Such neutral SKA analogues are the real active
species for the polymerization and are retained in the whole process as confirmed by experimental
data and the chain-end analysis by matrix-assisted laser desorption/ionization time of flight mass
spectroscopy (MALDI-TOF MS). Moreover, using this method, we have successfully synthesized
well-defined PMMBL-b-PMBL, PMMBL-b-PMBL-b-PMMBL and random copolymers with the
predicated molecular weights (Mn) and narrow molecular weight distribution (MWD).
Keywords: Frustrated Lewis Pair; group transfer polymerization; silyl ketene acetal; B(C6F5)3
1. Introduction
Lewis pair (LP) polymerization has emerged and attracted intense investigations of the
cooperative (or synergistic) catalytic effects of Lewis acid (LA) and Lewis base (LB) pairs on the
polymerization of conjugated polar alkenes [1–8], since the seminal works on Frustrated Lewis Pairs
(FLPs) by Stephan and Erker in small molecule chemistry [9–12]. For instance, LPs based on strongly
acidic, bulky E(C6F5)3 (E = B, Al) LAs and bulky LBs including phosphines and N-heterocyclic
carbenes (NHCs) have been employed to initiate rapid addition polymerization of conjugated polar
vinyl monomers such as methyl methacrylate (MMA), cyclic and the naturally renewable monomers
α
-methylene-
monomers bearing the C=C–C=N functionality such as 2-vinylpyridine and 2-isopropenyl-2-oxazoline
as well [16 17]. In such polymerizations, the cooperativity of the LA and LB sites of Lewis
γ-butyrolactone (MBL), and γ-methyl-α-methylene-γ-butyrolactone (MMBL) [13–15] and
,
pairs is essential to achieve an effective polymerization system, which was demonstrated by the
borane/phosphine LPs that showed that interacting LPs, and even classical Lewis adducts (CLAs),
Molecules 2018, 23, 665; doi:10.3390/molecules23030665
www.mdpi.com/journal/molecules

Molecules 2018, 23, 665
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can be highly active for the polymerization [18]. This was further nicely demonstrated by Rieger
and co-workers, showing the high activity and high degree of control over the polymerization of
Michael-type and extended Michael monomer systems by the highly interacting organoaluminum
and phosphine LPs [19]. Extending beyond the commonly employed NHC and phosphine LBs,
in 2014, Lu and co-workers first reported the N-heterocyclic olefin-based LPs for the polymerization
of acrylamides, (meth)acrylates and disymmetric divinyl polar monomers as well [20,21]. Although
FLPs or CLAs exhibited high activity for polymerization of conjugated polar alkenes, the application
of such polymerization is hampered by both low initiation efficiencies and chain-termination side
reactions [18,21], evidenced by the much higher observed Mn than the calculated Mn and broad
MWD of the resulting polymers (high Ð values), thus giving rise to low initiation efficiencies (I*) and
rendering the inability to produce well-defined block copolymers. Based on the above facts, it is really
difficult to achieve LP-catalyzed living/controlled polymerization of polar acrylic monomers. More
recently, Taton and co-workers reported the direct employment of organic LPs based on phosphine
and Me3SiNTf2 for living polymerization of MMA, which proceeds through a similar mechanism
compared to the group transfer polymerization (GTP) of MMA [22].
GTP is a successful, commercialized strategy for living polymerization of polar vinyl monomers
proceeding through Mukaiyama-Michael reactions [23
–
25]. −Such GTP process could be catalyzed by
−
[23,26,27], HF2 [23,26– [27– [23,27–29],
28], F− 29], CN−
nucleophilic anionic bases, such as SiMe3F2
N3−
23 28], oxyanions[30 31], hydrogen bioxyanions [30
NHC [33 38], phosphorus-based neculeophiles [39 40] and phosphazene superbases [39, –
[
,
,
–32]. Recently, neutral Lewis bases such as
–
,
41 44] have
been demonstrated be efficient catalysts for living GTP of acrylic monomers. On the other hand,
Brønsted acids have been employed as activators for GTP as well [45], while GTP of acrylates catalyzed
by Lewis acidic catalysts such as zinc halide [46], mercury iodide [47,48], or organoaluminun [46]
requires a much higher catalyst loading (typically 10 mol % based on monomer) to achieve a reasonable
degree of polymerization control. Through the oxidative activation of SKAs with a catalytic amount of
[Ph3C][B(C6F5)4] (TTPB) as low as 0.025 mol % (based on monomer), we have successfully achieved
high-speed living GTP of polar vinyl monomers catalyzed by the silylium ion R3Si+, including
(meth)acrylates [49,50] and renewable MBL and MMBL [51]. Covalently linked, unimolecular silyl
enolate/silylium nucleophile/electrophile bifunctional active species rendered a rate enhancement
by a factor of >40 and high stereoselectivity (at low temperature), as compared to the mononuclear
SKA system [52]. More recently, a tandem (FLP and LA) activation method was developed for GTP
involving the in-situ generation of SKA initiators by 1,4-hydrosilylation of a methacrylate monomer.
In such a process, the catalysts, such as highly electron deficient silylium cations “R3Si+” [53] or
strong LAs E(C6F5)3 (E = B, Al) [5,6,54,55], play dual role in both hydrosilylation (via frustrated
Lewis Pair (FLP)-type activation) and activation of monomer (classical LA activation). However,
such system was shown to be much less effective for E(C6F5)3 (E = B, Al)-catalyzed GTP at high
monomer to initiator ratios [6,53] and ill-controlled for Al(C6F5)3-catalyzed MMA polymerization.
In this context, herein we report the SKA/B(C6F5)3 FLP-catalyzed GTP of polar vinyl monomers at
room temperature, including MMA, renewable MMBL and MBL. More specifically, a highly interacting
iBuSKA/B(C6F5)3 LP system promotes the living GTP of MMBL and copolymerization of MMBL and
MBL to produce well-defined (co)polymers with predicted molecular weights, and narrow molecular
weight distributions. More importantly, this SKA/B(C6F5)3 LP system enabled us to isolate key reaction
intermediates and perform kinetic and mechanistic studies, thereby providing the much-needed
insights into the polymerization mechanism.
2. Results and Discussion
2.1. Slow MMA Polymerization by R3SiH/B(C6F5)3 and SKA/B(C6F5)3
Without B(C6F5)3, all six hydrosilanes, including Et3SiH, Ph3SiH, iBu3SiH, Me2ClSiH,
Me2EtSiH, and Me2PhSiH, showed negligible activity for the polymerization of MMA in

Molecules 2018, 23, 665
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CH2Cl2 at room temperature. In the presence of B(C6F5)3, the in-situ generation of SKA
initiators from the B(C6F5)3-catalyzed hydrosilylation of the monomer MMA with R3SiH exhibited
very low polymerization activity and incomplete monomer conversions for various ratios of
[MMA]0/[R3SiH]0/[B(C6F5)3]0 for up to 24 h (runs 1–18, Table S1). On the other hand, the direct
use of SKA (Scheme 1) as initiator and B(C6F5)3 as catalyst did not lead to obvious enhancement
in MMA polymerization activity compared to R3SiH/B(C6F5)3 systems (runs 19–39, Table S1).
The corresponding PMMA polymers all possessed syndio-biased tacticities with a methyl triad
distribution of around 70% rr, 28% mr and 2% mm, due to the chain-end control nature of this
polymerization system.
Scheme 1. Chemical structures of initiators, catalyst and monomers employed in this study.
2.2. Controlled MMBL Polymerization by R3SiH/B(C6F5)3 and SKA/B(C6F5)3
Next, we investigated the effectiveness of the R3SiH/B(C6F5)3 and SKA/B(C6F5)3 systems for
renewable monomer MMBL, which is a cyclic analogue of MMA and is readily prepared in a two-step
process from the cellulosic biomass-derived levulinic acid. Compared to the MMA polymerization, all
R3SiH/B(C6F5)3 and SKA/B(C6F5)3 systems are more effective and controlled for polymerization of
MMBL, achieving noticeably better initiation efficiency and narrower molecular weight distribution
(Table 1 vs. Table S1). Among the silanes screened, Me2EtSiH and Me2PhSiH achieved quantitative
monomer conversion within 30 min (Mn = 48.1 kg
mol−1, Ð = 1.26, I* = 47% for run 1, Table 1;
·
Mn = 57.4 kg·mol-1, Ð = 1.39, I* = 39% for run 2, Table 1). Replacing the Ph group in the R3SiHstructure
with the electron-withdrawing Cl (i.e., Me2ClSiH) rendered the polymerization reaching completion
in 2 h and producing PMMBL with a Mn of 78.6 kg
mol−1 and a higher Ð value of 1.44 (run 3,
·
Table 1). Interestingly, R3SiH with three Et substituents, Et3SiH, when combined with B(C6F5)3,
exhibited comparable polymerization activity with that for Me2EtSiH, producing PMMBL with a Mn
of 33 kg
mol−1 and a Ð value of 1.27, thus yielding an enhanced initiation efficiency of 68% (run 4,
·
Table 2). With the increase of steric hindrance of substituents, the corresponding R3SiH exhibited
i
drastically decreased polymerization activity (6 h for Ph3SiH and 24 h for Bu3SiH to reach full
monomer conversion) and initiation efficiency (21% for Ph3SiH and 7% for iBu3SiH).

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Table 1. B(C6F5)3-catalyzed polymerization of (M)MBL a.
c
I* e (%)
d
[M]:[I]:[B] b
Mn. (kg/mol)
Run No.
Initiator (I)
Monomer (M)
Time (h)
Conv. (%)
Ð
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Me2EtSiH
Me2PhSiH
Me2ClSiH
Et3SiH
MMBL
MMBL
MMBL
MMBL
MMBL
MMBL
MMBL
MMBL
MMBL
MMBL
MMBL
MMBL
MMBL
MMBL
200:1:1
200:1:1
200:1:1
200:1:1
200:1:1
200:1:1
200:1:1
200:1:1
200:1:1
200:1:1
200:1:1
200:1:1
200:1:1
100:1:1
0.5
0.5
2
0.5
6
24
0.5
0.5
6
2
1
0.5
1
0.17
100
100
100
100
100
100
100
100
100
100
100
100
100
100
48.1
57.4
78.6
33
105
345
45.5
41.5
85.1
68.8
31.9
37
32.2
1.26
1.39
1.44
1.27
1.61
1.51
1.33
1.37
1.82
1.37
1.08
1.21
1.09
1.08
1.20,
1.80
1.12
1.95
1.05
47
39
29
68
21
7
50
54
27
33
71
61
70
86
Ph3SiH
iBu3SiH
Me2PhSKA
Me2(EtO)SKA
Me2ClSKA
PhSKA
MeSKA
EtSKA
iBuSKA
iBuSKA
13.2
14.6 (58%)
98.0 (42%)
12.7
MeSKA
15
MBL
200:1:1
24
100
-
MeSKA
iBuSKA
iBuSKA
16
17
18
MBL
MBL
MBL
100:1:1
200:1:1
100:1:1
0.5
24
0.5
100
100
100
78
84
74
23.4
13.4
a
Carried out in 2.25 mL CH2Cl2 at room temperature, where [MMBL]0 or [MBL]0 = 0.936M. b [M] = [Monomer],
[I] = [Initiator], and [B] = [B(C6F5)3]. Monomer conversions measured by 1H NMR. d Mn and Ð determined by
c
GPC relative to PMMA standards in DMF. e Initiator efficiency I* (%) = Mn(calcd)/Mn(exptl)
= [MW(MMBL)] × ([MMBL]0/[I]0) (conversion) + MW of chain-end groups.
×
100, where Mn(calcd)
Table 2. B(C6F5)3-catalyzed copolymerization of MMBL and MBL by iBuSKA/B(C6F5)3 system a.
c
b
Run No.
M1/M2
Mw (kg/mol)
Đ
Conv. (%)
MBL:100
MMBL:100
1 d
100 MMBL + 100 MBL
28.3
25.7
37.1
1.03
MBL:100
MMBL:100
2
3
100 MMBL/100 MBL
1.07
1.06
MBL:100
MMBL:100
100 MMBL/100 MBL/100 MMBL
a
Carried out in 2.25 mL CH2Cl2 at room temperature, where [MMBL]0 = [MBL]0 = 0.936 M and addition method:
b
catalyst and monomer were premixed, followed by adding initiator to start the polymerization. Monomer
conversions measured by 1H-NMR. c Mn and MWD determined by GPC relative to PMMA standards in DMF. d
MBL and MMBL was added at the same time.
When switching to the SKA/B(C6F5)3 system, both Me2PhSKA/B(C6F5)3 and
Me2(EtO)SKA/B(C6F5)3 system achieved quantitative monomer conversion in 30 min and similar
initiation efficiency (50% vs. 54%) under our current standard polymerization conditions
{[MMBL]0:[SKA]0:[B(C6F5)3]0 = 200:1:1, 0.25 mL MMBL, 2.25 mL CH2Cl2, RT}. Replacing the phenyl
group with a chlorine atom, the rate of MMBL polymerization was drastically decreased and
quantitative monomer conversion was obtained in 6 h for Me2ClSKA/B(C6F5)3 system, producing
PMMBL with a Mn of 85.1 kg
mol−1, a broader Ð value of 1.82 (Table 1, run 9), and thus giving a
·
lower I*% of 27. The combination of PhSKA with B(C6F5)3 produced a polymerization system that is
comparable with that of Me2ClSKA/B(C6F5)3 (run 10 vs. 9, Table 1). Interestingly, by replacing the three
Ph groups with three electron-donating alkyl groups in SKA, we observed a significantly enhanced
initiation efficiency I* (%) (71 for MeSKA, run 11; 61 for EtSKA, run 12; 70 for iBuSKA, run 13).
Since both MeSKA/B(C6F5)3 and iBuSKA/B(C6F5)3 system exhibited the highest initiation
efficiency for the polymerization of MMBL (around 70%) under our current condition, therefore,
we employed both systems to examine their efficacies for polymerization of the homologues of MMBL,
MBL or tulipalin A, which is a natural substance found in tulips and the MBL ring is an integral
building block of many natural products. For polymerization with a 200:1:1 [MBL]:[MeSKA]:[B(C6F5)3]
ratio, it took 24 h for both MeSKA/B(C6F5)3 and iBuSKA/B(C6F5)3 system to reach complete monomer

Molecules 2018, 23, 665
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consumption. However, it should be noted that PMBL produced by MeSKA/B(C6F5)3 system exhibited
a bimodal MWD. For polymerization with 100:1:1 [MBL]:[SKA]:[B(C6F5)3] ratio, only 30 min is needed
to achieve quantitative monomer conversion for both systems, affording PMBL with predicted Mn and
small Ð values (MeSKA: Mn = 12.7 kg
·
mol−1, Ð = 1.12, I* = 78%, Run 16; iBuSKA: Mn = 13.4 kg mol−1
,
·
Ð = 1.05, I* = 74%, Run 18, Table 1). These results indicated iBuSKA/B(C6F5)3 system exhibited better
control on the polymerization than that for MeSKA/B(C6F5)3 system.
In fact, the MMBL polymerization by iBuSKA/B(C6F5)3 system is living and controlled, and near
quantitative monomer conversion was achieved for polymerization with varied [MMBL]/[iBuSKA]
ratio from 100 to 800. GPC traces of PMMBL produced by iBuSKA/B(C6F5)3 system also exhibited
the gradual shift to the high-molar-mass region with an increase in the [M]/[I] ratio from 100 to 800
and maintained a narrow and unimodal MWD (Figure 1). Although increasing the catalyst loading of
B(C6F5)3 would enhance the polymerization rate, the polymer MW is dependent on the concentration
of initiator [iBuSKA] and independent on the concentration of catalyst B(C6F5)3 (vide infra), which is
evidenced by the linear increase of the Mn values of PMMBL produced by iBuSKA/B(C6F5)3 system
with an increase in the [MMBL]/[iBuSKA] ratio from 100 to 800 (Figure 2, R2 = 0.994), while Ð values
remained in the very narrow range of 1.08 to 1.13 (Figure 2). It is noted that Mn value of PMMBL
obtained for polymerization with different [MMBL]/[B(C6F5)3] ratio maintained around 60 kg
mol−1
·
(Figure S21), which revealed that the concentration of [B(C6F5)3] has an effect the polymerization rate,
but not on both polymer MW and MWD. Therefore, B(C6F5)3 is not the initiator but the catalyst for the
activation of monomers. In addition to the aforementioned linearly increased polymer MW with the
increase of varied monomer to initiator ratio, the living characteristics of MMBL polymerization by
iBuSKA/B(C6F5)3 was also confirmed by a plot of the PMMBL Mn vs monomer conversion at a fixed
[MMBL]/[iBuSKA]/[B(C6F5)3] ratio of 400:1:1, which clearly show a straight line (R2 = 0.999) with very
narrow Ð value in the range of 1.08–1.2 (Figure 3).
Figure 1.
GPC traces of PMMBL samples produced by iBuSKA/B(C6F5)3 with various
[MMBL]0/[iBuSKA]0 ratios at RT. Conditions: [MMBL]0/[iBuSKA]0/[B(C6F5)3]0 = 400:4:1, 400:2:1,
400:1:1, 400:0.5:1, [MMBL]0 = 0.936 M.

Molecules 2018, 23, 665
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3
140
120
100
80
2.8
2.6
2.4
2.2
2
■ Mn □ Ð
R² = 0.9943
60
1.8
1.6
1.4
1.2
1
40
20
0
0
200
400
600
800
Monomer/iBuSKA ratio
Figure 2. Plots of Mn and Ð values of PMMBL samples vs [MMBL]0/[iBuSKA]0 ratio at RT. Conditions:
[MMBL]0/[iBuSKA]0/[B(C6F5)3]0 = 400:4:1, 400:2:1, 400:1:1, 400:0.5:1, [MMBL]0 = 0.936 M.
70
60
50
40
30
20
10
0
3
2.8
2.6
2.4
2.2
2
R² = 0.9999
■ Mn □ Ð
1.8
1.6
1.4
1.2
1
0
0.2
0.4
0.6
0.8
1
1.2
Conv(%)
Figure 3. Plots of Mn and Ð values of PMMBL samples vs MMBL conversion by iBuSKA/B(C6F5)3
system with MMBL/iBuSKA/B(C6F5)3 = 400/1/1 ratio at RT.
2.3. Characterization of Key Intermediates
To gain more insights into the polymerization, we studied the in situ NMR reactions of MeSKA
with B(C6F5)3 in 1:1 ratio and observed the formation of FLP, which is evidenced by the fact that
there is no obvious interaction observed in the reaction of MeSKA with B(C6F5)3 (Figures S11 and
S12). Similar FLP was generated in the reaction of iBuSKA with B(C6F5)3 in 1:1 ratio (Figures S13
and S14), too. We also prepared B(C6F5)3
and S10) adduct for the study of their reaction with SKA, respectively. The reaction of MeSKA with
B(C6F5)3 MMA at room temperature in C6D6 generated major products of dimeric SKA analogue
·
MMA (Figures S7 and S8) and B(C6F5)3
·
MMBL (Figures S9
·
Me3SiO(OMe)C=C(Me)CH2CMe2C(OMe)=O· · ·B(C6F5)3 as two isomers (Z/E) in a 3:2 ratio due to
the cis-trans isomerism of MMA (Scheme 2), which could readily derived from the analysis of 1H-
and 13C-NMR spectra (Figures S15 and S17).The characteristic proton signals at 0.18 ppm for major
isomer and 0.16 ppm for minor isomer attributed to the trimethyl substituents of silyl group in the
dimeric SKA analogue is the result of the high field shift from the original 0.18 ppm for neutral MeSKA,

Molecules 2018, 23, 665
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which is different from that for Me3Si+ generated by vinylogous hydride abstraction of MeSKA with
Ph3C+, exhibiting signal at 0.65 ppm [49]. This result suggests that the silyl group in the dimeric
SKA analogue was neutral rather than Me3Si+. In combination with the fact that 19F NMR spectrum
exhibiting similar signals [δ −129.03 (br, 6F, o-F),
with that for B(C6F5)3 MMA adduct (Figure S8), we proposed the structure for corresponding dimeric
SKA analogues as shown in Scheme 2. Similarly, the reaction of iBuSKA with B(C6F5)3
MMA cleanly
−
142.24 (br, 3F, p-F),
−
160.09 (s, 6F, m-F))] (Figure S16)
·
·
afforded the single-monomer-addition product (Scheme 2), as revealed by the 1H-, 19F- and 13C-NMR
spectra (Figure 4a,b, and Figure S20). However, we did not observe the formation of such dimeric SKA
analogue for reaction of SKA with B(C6F5)3·MMBL probably due to high reactivity of MMBL.
Scheme 2. Generation of first-monomer-addition intermediate or active propagating species (
A or B
denotes Z/E isomers).
a
b
Figure 4.
(a
) 1H- and (
b
)
19F-NMR spectra of iBu3SiO(OMe)C=C(Me)CH2CMe2C(OMe)=O···B(C6F5)3.

Molecules 2018, 23, 665
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2.4. Kinetics and Mechanism of Polymerization
To continue the investigation of the mechanistic aspects of the polymerization, we next examined
the kinetics of the MMBL polymerization by the [iBuSKA]/[B(C6F5)3]. The kinetic experiments
employed [MMBL]0/[iBuSKA]0/[B(C6F5)3]0 with varied ratio of 400:1:0.5, 400:1:1, 400:1:2, 400:1:4
and a fixed [MMBL]0/[iBuSKA]0 ratio of 400. As can be seen from the representative kinetic plots
of Ln([MMBL]0/[MMBL]t) vs. time, the polymerization clearly showed a first-order dependence
on [MMBL] for all ratios investigated (Figure 5). Furthermore, a double logarithm plot (insert)
of the apparent rate constants (Kapp) obtained from the slopes of the best-fit lines to the plots of
Ln([MMBL]0/[MMBL]t) vs. time as a function of ln[B(C6F5)3] was fit to a straight line (R2 = 0.960)
with a slop of 2.07. Therefore, the kinetic order with respect to the [B(C6F5)3], given by the slopes of
~2, reveals that the polymerization is second-order in B(C6F5)3 catalyst concentration; these kinetics
results suggest a bimolecular propagation.
Figure 5. First-order kinetic plots for MMBL polymerization by iBuSKA/B(C6F5)3 in CH2Cl2 at RT:
[MMBL]0 = 0.936 M; [SKA]0 = 2.34 mM; [B(C6F5)3]0 = 9.36 mM (∆), 4.68 mM (N), 2.34 mM (ꢀ), 1.17
mM (ꢁ). Inset: plot of ln(kapp) vs. ln[B(C6F5)3].
In the second set of kinetic experiments, with a fixed [MMBL]0/[B(C6F5)3]0 ratio of 400, the kinetic
experiments was carried out with varied [MMBL]0/[B(C6F5)3]0/[iBuSKA]0 ratio of 400:1:0.5, 400:1:1,
400:1:2, 400:1:4. The same first-order dependence was observed for all ratios investigated in this study
(Figure 6). A double logarithm plot (insert) of the apparent rate constants (kapp), obtained from the
slopes of the best-fit lines to the plots of Ln([MMBL]0/[MMBL]t) vs. time, as function of ln[iBuSKA]
was fit to a straight line (R2 = 0.960) with a slop of 1.132, revealing that the propagation is first-order in
the concentration of [iBuSKA]. Overall, the polymerization by the [iBuSKA]/[B(C6F5)3] systems follows
a bimolecular, activated monomer propagation mechanism.
On the basis of the above kinetic results, coupled with mechanistic insights obtained through
monitoring the polymerization and characterization of the reaction intermediates (vide supra),
we propose the following initiation and propagation mechanism for polymerization of polar vinyl
monomers by the SKA/B(C6F5)3 LP system taking MMBL as example (Scheme 3). In this mechanism,
the reaction was initiated with the intermolecular Michael addition of the ester enolate groups
of the SKA to the vinyl group of B(C6F5)3-activated monomer, meanwhile the Si-O bond of SKA
was cleaved and the silyl group was transferred to the carbonyl group of the B(C6F5)3-activated

Molecules 2018, 23, 665
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monomer to generate the single-monomer-addition species or the active propagating species. In the
propagation cycle, the B(C6F5)3 catalyst is released from the propagating chain to the incoming
monomer, followed by the subsequent intermolecular Michael addition to generate the polymeric SKA
intermediate (Scheme 3). Our kinetic studies indicated that the polymerization is first-order dependent
on both monomer and iBuSKA initiator concentration, but second-order dependent on B(C6F5)3 catalyst
concentrations, which revealed that the rate (k2) of the intermolecular Michael addition of the SKA or
its homologues to the B(C6F5)3-activated monomer is comparable with that (k1) for the release of the
catalyst B(C6F5)3 from the ester group of the growing polymer chain to the incoming monomer for
monomer activation (k1 ≈ k2).
Figure 6. First-order kinetic plots for MMBL polymerization by iBuSKA/B(C6F5)3 in CH2Cl2 at RT:
[MMBL]0 = 0.936 M; [B(C6F5)3]0 = 2.34 mM; [SKA]0 = 9.36 mM (∆), 4.68 mM (N), 2.34 mM (ꢀ), 1.17
mM (ꢁ). Inset: plot of ln(kapp) vs. ln[iBuSKA].
The low MW PMMBL produced by the iBuSKA/B(C6F5)3 system was analyzed by matrix-assisted
laser desorption/ionization time of flight mass spectroscopy (MALDI-TOF MS). As can be seen
from Figure 7, The MS spectrum consisted of two series of molecular mass ions. A plot (Figure 8a)
of m/z values for the major series vs the number of MMBL repeat units (n) yielded a straight
line with a slope of 112 (mass of MMBL) and an intercept of 323 corresponding to the sum of
end groups: MeOC(=O)C(Me)2/Si(iBu)3 + Na+. This analysis yielded a polymer chain structure
of MeOC(=O)C(Me)2-(MMBL)n-Si(iBu)3, with both the initiating (the ester enolate group) and
i
termination (the silyl group) chain ends being derived from BuSKA. Furthermore, a linear plot
of m/z values for the minor series in the MS spectrum vs the MMBL repeat units (n) gave the same
slope but a different intercept of 125 (Figure 8b), which corresponds to the sum of end groups:
MeOC(=O)C(Me)2-(MMBL)n-H + Na+. Formation of such enolate/H chain end is presumably a result
of desilylation during the preparation of the MALDI-TOF sample.

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Figure 7. MALDI-TOF mass spectrum of the low-MW PMMBL sample produced by iBuSKA/B(C6F5)3
in CH2Cl2 at RT.

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Figure 8. Plot of m/z values from Figure 7 vs. the number of MMBL repeat units (n) for major series (
and minor series (b).
a)
Scheme 3. Proposed mechanism for MMBL polymerization by SKA/B(C6F5)3.
2.5. Random and Block Copolymerizations of MMBL with MBL
The living features of polymerization of MMBL and MBL by iBuSKA/B(C6F5)3 system, as
demonstrated by the above described experiments, also enabled the synthesis of well-defined copolymers.
As shown in Table 2, when both monomers were added simultaneously, the copolymerization of MBL
and MMBL produced a randomly sequenced copolymer with Mn = 28.3 kg
mol−1 and Ð = 1.03 (run
·
1, Table 2, Figure S23). On the other hand, sequential block copolymerization by polymerizing MMBL
first with [MMBL]/[iBuSKA]/[B(C6F5)3]=100:1:1 without quenching, followed by addition of another
100 equiv. of MBL, successfully afforded linear diblock copolymer PMMBL-b-PMBL. As can be seen
from Figure 9, the GPC traces for the PMMBL produced during the initial MMBL polymerization
shifted to a higher molecular weight region with low MWD value of 1.07, while the Mn increase from
13.2 kg
·
mol−1 (black trace) for the homopolymer PMMBL to 25.7 kg mol−1 (red trace) for the diblock
·
copolymer PMMBL-b-PMBL (run 2, Table 2), which provided further evidence for the formation of
well-defined block copolymer by iBuSKA/B(C6F5)3 system. Through the same sequential monomer
addition method, well-defined triblock copolymer PMMBL-b-PMBL-b-PMMBL was also successfully
prepared with a Mn value of 37.1 kg/mol and a Đ value of 1.06 (Figure 9, blue trace, run 3, Table 2).
We also characterized both block copolymers and random copolymers by 13C-NMR analysis (Figure
S21). In contrast to diblock copolymer PMMBL-b-PMBL showing two signals corresponding to the
PMBL and PMMBL, the yielded random copolymer PMMBL-r-PMBL only exhibited one broad peak

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in the carbonyl signals, which is ranging between the signals for homopolymers PMBL and PMMBL.
The combination of such conclusive 13C NMR results and GPC analyses should provide sufficient
evidence for the formation of random and block copolymer from statistical and sequential block
polymerization, respectively.
Figure 9. GPC traces of homopolymer PMMBL (black), diblock copolymer PMMBL-b-PMBL (red), and
triblock copolymer PMMBL-b-PMBL-b-PMMBL (blue) produced by iBuSKA/B(C6F5)3 in CH2Cl2 at RT.
3. Metarials and Methods
3.1. 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, or an inert gas-filled glovebox.
Toluene, benzene, THF and hexane were refluxed over sodium/potassium alloy distilled under
nitrogen atmosphere, then stored over molecular sieves 4 Å. CH2Cl2 was refluxed over CaH2 distilled
under nitrogen atmosphere, then stored over molecular sieves 4 Å. Benzene-d6 was dried over
molecular sieves 4 Å. NMR spectra were recorded using an Avance II 500 (500 MHz, 1H; 126 MHz,
13C; 471 MHz, 19F) instrument (Bruker, Karlsruhe, Baden-Württemberg, Germany) in appropriate
deuterated solvents at room temperature. Chemical shifts for 1H 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 CFCl3. Air sensitive NMR samples were conducted in Teflon-valve
sealed J. Young-type NMR tubes.
Methyl methacrylate (MMA) was purchased from J&K (Beijing, China), while
α
-methylene-γ-butyrolactone (MBL) and γ-methyl-α-methylene-γ-butyrolactone (MMBL) were
purchased from TCI (Tokyo, Japan). These monomers were first degassed by three freeze-pump-thaw
cycles and dried over CaH2 for 12 h, followed by vacuum distillation. Further purification of MMA
involved titration with tri(n-octyl)aluminum (Strem Chemicals, Newburyport, MA, USA) to a
yellow end point [56], followed by distillation under reduced pressure. All purified monomers were
stored in brown bottles inside a glovebox freezer at
−
30 ◦C. Dimethylphenylsilane (Me2PhSiH), and
triphenylsilane (Ph3SiH) were purchased from TCI. Triisobutylsilane (iBu3SiH), dimethylethylsilane
(Me2EtSiH), dimethylketene methyl trimethylsilyl acetal (MeSKA) were purchased from Sigma-Aldrich
(St. Louis, MO, USA). Butylated hydroxytoluene (BHT-H, 2,6-di-tert-butyl-4-methylphenol),
triethylsilane (Et3SiH), chlorodimethyl-silane (Me2ClSiH), bromopentafluorobenzene were purchased
from J&K. Boron trichloride (1.0 M solution in hexanes) and n-BuLi (2.5 M solution in hexanes)

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were purchased from Energy Chemical (Shanghai, China). Tris(pentafluorophenyl)borane, B(C6F5)3,
was prepared according to literature procedures.[57 58] Literature procedures were also employed
for the preparation of the following compounds: dimethylketene methyl triethylsilyl acetal
,
(EtSKA) [49], dimethylketene methyl triphenylsilyl acetal Me2C=C(OMe)OSiPh3 PhSKA) [50],
(
dimethylketene nethyl triisobutylsilyl acetal Me2C=C(OMe)OSi(iBu)3 (iBuSKA) [50],dimethylketene
methyl dimethyl-phenylsilyl acetal Me2C=C(OMe)OSiMe3 (Me2PhSKA) [54].
3.2. Synthesis of Dimethylketene Methyl Chlorodimethylsilyl Acetal Me2C=C(OMe)OSiMe2Cl (Me2ClSKA)
In an inert gas-filled glovebox, solution containing B(C6F5)3 (128 mg, 0.250 mmol) and MMA
(2.65 mL, 25.0 mmol) in dry CH2Cl2 (50 mL) was charged in a 100 mL Schlenk flask equipped with
a stir bar. This flask was sealed with a rubber septum, removed from the glovebox, interfaced to a
Schlenk line, and brought to
−
78 ◦C for at least 15 min. Chlorodimethylsilane (2.78 mL, 25.0 mmol)
was added dropwise to the above flask under nitrogen atmosphere. The resulting reaction mixture
was allowed to warm up to room temperature slowly and then subjected to vacuum. The residue was
purified by vacuum distillation to afford the final product as colorless oil. Yield: 3.84 g (79%).1H-NMR
(500 MHz, benzene-d6)
δ 3.35 (s, 3H, OMe), 1.64 (s, 3H, =CMe), 1.61 (s, 3H, =CMe), 0.36 (s, 6H, SiMe2Cl).
13C-NMR (126 MHz, benzene-d6) δ 149.2, 92.8, 57.2, 17.1, 16.4, 2.3.
3.3. Synthesis of Chloro(ethoxy)dimethylsilane Me2(EtO)SiCl
Literature procedures [59] were modified for the preparation of Me2(EtO)SiCl. In an inert gas-filled
glovebox, solution of B(C6F5)3 (461 mg, 0.90 mmol) and EtOH (5.08 mL, 90.0 mmol) in dry CH2Cl2
(100 mL) was charged in a 200 mL Schlenk flask equipped with a stir bar. This flask was sealed with a
rubber septum, removed from the glovebox, interfaced to a Schlenk line, and brought to
−
78 ◦C for
at least 20 min. Chlorodimethylsilane(10.0 mL, 90.0 mmol) was added dropwise to the flask under
nitrogen atmosphere. The resulting mixture was allowed to warm up to room temperature slowly and
then subjected to vacuum. The residue was purified by distillation to afford the product as colorless oil.
Yield: 12.48 g (91%).1H-NMR (500 MHz, benzene-d6)
δ 3.61 (q, J = 7.0 Hz, 2H, OCH2), 1.06 (t, J = 7.0 Hz,
3H, OCH2CH3), 0.26 (s, 6H, SiMe2). 13C-NMR (126 MHz, benzene-d6) δ 59.1, 18.1, 2.0.
3.4. Synthesis of Dimethylketene Methyl Ethoxydimethylsilyl Acetal Me2C=C-(OMe)OSiMe2(EtO)
(
Me2(EtO)SKA)
Literature procedures for the general synthesis of ketene trialkylsilyl acetals [60,61] were modified
for the preparation of Me2(EtO)SKA. In an inert gas-filled glovebox, a solution of diisopropylamine
(7.05 mL, 5.06 g, 50.0 mmol) in dry THF (100 mL) was charged in a 200 mL Schlenk flask equipped
with a stir bar. This flask was sealed with a rubber septum, removed from the glovebox, interfaced to a
Schlenk line, and placed in a 0 ◦C ice-water bath for at least 15 min. n-Butyllithium (32.0 mL, 1.6 M in
hexane, 51.2 mmol) was added dropwise to the above flask under nitrogen atmosphere. After stirring at
◦
0 C for 30 min, methyl isobutyrate (5.74 mL, 5.11 g, 50.0 mmol) was added slowly. The reaction mixture
was stirred at this temperature for 30 min, after which Chloro(ethoxy)dimethylsilane (7.61 g, 50.0 mmol)
was added. The mixture was allowed to warm slowly to room temperature and stirred overnight,
after which all volatiles were removed in vacuum and hexanes (50 mL) was added. The resulting
precipitates were filtered off under nitrogen atmosphere, the solvent of the filtrate was removed under
reduced pressure. The residue was purified by distillation under vacuum to afford the product as
1
colorless oil. Yield: 8.79 g (86%). H-NMR (500 MHz, benzene-d6)
δ 3.72 (q, J = 7.0 Hz, 2H, OCH2CH3),
3.39 (s, 3H, OMe), 1.72 (s, 3H, =CMe), 1.68 (s, 3H, =CMe), 1.13 (t, J = 7.0 Hz, 3H, OCH2CH3), 0.21 (s, 6H,
SiMe2). 13C-NMR (126 MHz, benzene-d6) δ 149.8, 90.7, 58.6, 56.6, 18.5, 17.1, 16.5, −2.5.

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3.5. Isolation of Adduct B(C6F5)3·MMA
To a solution of B(C6F5)3 (512 mg, 1.0 mmol) in hexane (15 mL) was added MMA (110 mg,
◦
1.1 mmol) at room temperature with reacting for 30 min. The solution was brought to
glovebox freezer for 1 h. After filtration and removal of organic solvents in vacuo, the B(C6F5)3
was obtained as a white powder (546 mg, 89%).1H-NMR (benzene-d6)
−
30 C in
·
MMA
δ 5.91 (s, 1H, =CH), 5.05–5.03
(s, 1H, =CH), 3.29 (s, 3H, OMe), 1.63 (s, 3H, Me).19F-NMR (benzene-d6) δ −130.18 (d, J = 20.2 Hz, 6F,
o-F), −144.90 (t, J = 20.8 Hz, 3F, p-F), −160.70 (m, 6F, m-F).
3.6. Isolation of Adduct B(C6F5)3·MMBL
The B(C6F5)3
·
MMBL adduct was isolated as a white powder in 91% yield using the same
procedure as described for the isolate of the adduct B(C6F5)3
·
MMA. 1H-NMR (500 MHz, benzene-d6)
δ
6.06 (t, J = 3.0 Hz, 1H, =CH), 4.86 (t, J = 2.7 Hz, 1H, =CH), 3.67 (ddq, J = 8.0, 6.3, 5.7 Hz, 1H, OCH), 1.56
(ddt, J = 17.3, 8.0, 2.7 Hz, 1H, CH2), 1.10 (ddt, J = 17.3, 5.7, 2.9 Hz, 1H, CH2), 0.26 (d, J = 6.3 Hz, 3H, Me).
19F- NMR (471 MHz, benzene-d6) δ −134.81 (dd, J = 23.2, 7.3 Hz, 6F, o-F),
p-F), −163.93 (m, 6F, m-F).
−
156.72 (t, J = 20.7 Hz, 3F,
3.7. NMR Reaction of Me2C=C(OMe)OSiMe3 (MeSKA) with B(C6F5)3
In an inert gas-filled glovebox, solution of MeSKA (1.74 mg, 0.01 mmol) in 0.3 mL of C6D6 was
charged in a Teflon-valve-sealed J. Young-type NMR tube. A 0.3 mL C6D6 solution of B(C6F5)3
(5.12 mg, 0.01 mmol) was added to this tube via pipet at room temperature and allowed to react for
~15 min before the NMR spectra were recorded, which showed there were no reaction between MeSKA
1
and B(C6F5)3. H-NMR (500 MHz, benzene-d6)
δ 3.33 (s, 3H, OMe), 1.73 (s, 3H, =CMe), 0.65 (s, 3H,
=CMe), 0.18 (s, 9H, SiMe3). 19F-NMR (471 MHz, benzene-d6) δ −128.82 (d, 21.7 Hz, 6F, o-F),
J = 21.3 Hz, 3F, p-F), −160.06 (m, 6F, m-F).
−
141.81 (t,
3.8. NMR Reaction of MeSKA with B(C6F5)3·MMA
In an inert gas-filled glovebox, solution of MeSKA (1.74 mg, 0.01 mmol) in 0.3 mL of C6D6 was
charged in a Teflon-valve-sealed J. Young-type NMR tube. A 0.3 mL C6D6 solution of B(C6F5)3 MMA
·
(6.12 mg, 0.01 mmol) was slowly added to this tube via pipet at room temperature and allowed to react
for ~15 min before the NMR spectra were recorded. The resulting mixture showed the clean formation
of the species Me3SiO(OMe)C=C(Me)CH2CMe2C(OMe)=O···B(C6F5)3 (
ratio: major isomer 1A and minor isomer 1B, plus a small amount of unreacted starting materials. 1A
1H-NMR (500 MHz, benzene-d6)
3.395 (s, 3H, OMe), 3.25 (s, 3H, COOMe), 2.53(s, 2H, CH2), 1.65 (s,
1) as two isomers (Z/E) in 3:2
:
δ
3H, Me), 1.31 (s, 6H, Me2), 0.18 (s, 9H, SiMe3); 1B: δ 3.392 (s, 3H, OMe), 3.30 (s, 3H, COOMe), 2.46 (s, 2H,
CH2), 1.72 (s, 3H, Me), 1.32 (s, 6H, Me2), 0.16 (s, 9H, SiMe3); 19F-NMR (471 MHz, benzene-d6) δ −129.03
(br, 6F, o-F),
−
142.24 (br, 3F, p-F),
−
160.09 (s, 6F, m-F). 13C-NMR (126 MHz, benzene-d6)
δ 178.30, 178.2,
153.3, 152.2, 149.4, 147.4, 138.7, 136.7, 92.1, 91.6, 57.2, 55.6, 51.4, 51.3, 43.14, 43.05, 42.5, 42.4, 25.8, 16.5,
15.4, 0.3, 0.1.
3.9. NMR Reaction of Me2C=C(OMe)OSi(iBu)3 (iBuSKA) with B(C6F5)3
This reaction was carried out in the same manner as the reaction of MeSKA with B(C6F5)3, which
1
shown there were no reaction between iBuSKA and B(C6F5)3. H-NMR (500 MHz, benzene-d6)
δ
3.38 (s,
3H, OMe), 1.98 (sept, J = 6.6 Hz, 3H, CHMe2), 1.70 (s, 3H, =CMe), 1.69 (s, 3H, =CMe), 1.05 (d, J = 6.6 Hz,
18H, CHMe2), 0.84 (d, J = 6.8 Hz, 6H, SiCH2). 19F-NMR (471 MHz, benzene-d6) δ −128.82 (d, J = 21.7,
6F, o-F), −141.83 (t, J = 20.9 Hz, 3F, p-F), −160.07 (m, 6F, m-F).
3.10. NMR Reaction of iBuSKA with B(C6F5)3·MMA
This reaction was carried out in the same manner as the reaction of MeSKA with B(C6F5)3
·
MMA,
forming cleanly iBu3SiO(OMe)C=C(Me)CH2CMe2C(OMe)=O···B(C6F5)3 (
3) as two isomers (Z/E) in

Molecules 2018, 23, 665
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4:3 ratio: major isomer 2A and minor isomer 2B
.
2A: 1H-NMR (500 MHz, benzene-d6)
δ
3.401 (s, 3H,
OMe), 3.30 (s, 3H, COOMe), 2.52 (s, 2H, CH2), 1.98 (m, 3H, CHMe2), 1.71 (s, 3H, Me), 1.32 (s, 6H, CMe2),
1.05 (d, J = 6.5 Hz, 18H, CHMe2), 0.85 (d, J = 6.9 Hz, 6H, SiCH2); 2B 3.397 (s, 3H, OMe), 3.37 (s,
:
δ
3H, COOMe), 2.56 (s, 2H, CH2), 1.98 (m, 3H, CHMe2), 1.68 (s, 3H, Me), 1.36 (s, 6H, CMe2), 1.04 (d,
J = 6.8 Hz, 18H, CHMe2), 0.82 (d, J = 6.9 Hz, 6H, SiCH2); 19F-NMR (471 MHz, benzene-d6) δ −128.84
(br, 6F, o-F),
−
141.82 (br,3F,p-F),
−
159.99 (m, 6F, m-F). 13C-NMR (126 MHz, benzene-d6)
δ 149.4, 147.4,
138.7, 136.7, 92.5, 58.6, 56.9, 51.3, 51.3, 43.0, 42.4, 42.3, 26.8, 26.7, 26.5, 26.4, 25.9, 25.7, 24.5, 16.6, 15.1.
3.11. General Polymerization Procedures
Polymerizations were performed in 20 mL glass reactors inside the inert gas-filled glovebox for
◦
ambient temperature (ca. 25 C) runs. In a typical procedure, a predetermined amount of B(C6F5)3 and
monomer (1 mL for MMA, 250 µL for MMBL or 205 µL for MBL, 200 equiv. relative to the SKA) were
dissolved in dry CH2Cl2. A solution of a SKA (1 equiv. of a LA) in 1.0 mL of solvent were added rapidly
via a gastight syringe to the solution of B(C6F5)3-monomer. The amount of the monomer was fixed for
all polymerizations. After stirring for the measured time, a 0.1 mL aliquot was withdrawn from the
reaction mixture and quickly quenched into 0.6 mL of undried “wet” CDCl3 stabilized by 250 ppm of
BHT-H; the quenched aliquots were later analyzed by 1H-NMR to obtain the monomer conversion.
After the polymerization was stirred for the stated reaction time then the reactor was taken out of the
glovebox, and quenched by addition of 10 mL of 5% HCl-acidified methanol. The quenched mixture
was isolated by filtration and dried in vacuo overnight at room temperature to a constant weight.
3.12. Polymerization Kinetics
Kinetic experiments were carried out in a stirred glass reactor at ambient temperature (ca. 25 ◦C)
inside an inert gas-filled glovebox using the polymerization procedure already described above,
with the [B(C6F5)3]/[iBuSKA] ratio was 0.5/1, 1/1, 2/1, 4/1, [MMBL]0 was fixed at 0.936 M and
[
iBuSKA]0 was fixed at 2.34 mM, where [B(C6F5)3] = 1.17, 2.34, 4.68, 9.36 mM in 2.5 mL mixture
solutions. At appropriate time intervals, 0.1 mL aliquots were taken from the reaction mixture and
quickly quenched into 0.6 mL of undried “wet” CDCl3 mixed with 250 ppm BHT-H. The quenched
aliquots were analyzed by 1H-NMR for determining the [MMBL]t at a given time t and Ln([MMBL]0:
[MMBL]t). Apparent rate constants (kapp) were extracted from the slopes of the best fit lines to the plots
of Ln([MMBL]0: [MMBL]t) vs. time. Another set of kinetic experiments were carried out to determine
the kinetic order with respect to [iBuSKA]. In these experiments, with the [B(C6F5)3]/[iBuSKA] ratio
was 1/0.5, 1/1, 1/2, 1/4, [MMBL]0 was fixed at 0.936 M and [B(C6F5)3]0 was fixed at 2.34 mM, where
[
iBuSKA] = 1.17, 2.34, 4.68, 9.36 mM in 2.5 mL mixture solutions. The rest of the procedure was same as
the described above.
3.13. Polymer Characterizations
Number-average molecular weight (Mn) and molecular weight distributions (Ð = Mw/Mn) of
◦
polymers were measured by gel permeation chromatography (GPC) at 35 C and a flow rate of
1 mL/min, with DMF (HPLC grade, containing 50 mmol/L LiBr) as an eluent on a Waters 1515
instrument (Milford, MA, USA) equipped with a Waters 4.6
WAT054466, WAT044226, WAT044223 columns (Polymer Laboratories, Milford, MA, USA; linear range
of molecular weight = 500 to 4
106). The instrument was calibrated with 10 PMMA standards,
×
30 mm guard column and three Waters
×
and chromatograms were processed with Waters Breeze 2 software (version: 6.01.2154.026; Milford,
MA, USA). Tacticities of PMMA was measured by 1H-NMR in CDCl3, while 13C-NMR of P(M)MBL
(co)polymers were recorded in DMSO-d6.
The isolated low-MW polymer samples were analyzed by matrix-assissted laser
desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS); the experiment was
performed on a BrukerAutoflex speed TOF/TOF mass spectrometer (Karlsruhe, Baden-Württemberg,
Germany) in linear, positive ion, reflector mode using a 25 KV accelerating voltage. A thin layer of

Molecules 2018, 23, 665
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1% CF3COONa solution was first deposited on the target plate, followed by 0.6
µL of both sample
and matrix (trans-2-[3-(4-tertbutylphenyl)-2-methyl-2-propylidene]malonitrile (DCTB), 20 mg/mL
in THF). The spectra of the sample was externally calibrated using a peptide calibration mixture
(4–6 peptides). The raw data were processed in the Bruker Daltonics FlexAnalysis 3.3.80.0 software
(Karlsruhe, Baden-Württemberg, Germany).
4. Conclusions
In summary, FLPs based on bulky organo-LA, B(C6F5)3, and SKAs either preformed or in-situ
generated from the hydrosilylation with R3SiH, were employed for GTP of polar vinyl monomers,
including linear MMA as well as the cyclic renewable monomers MMBL and MBL. Although
the polymerizations of MMA by both R3SiH/B(C6F5)3 and SKA/B(C6F5)3 systems are sluggish,
SKA/B(C6F5)3 system promotes the living/controlled GTP of MMBL and MBL, achieving polymers
with predicted Mn values and narrow Đ values.
The formation of FLP was confirmed by in situ NMR reactions of SKA with B(C6F5)3. Detailed
investigations into the characterization of key reaction intermediates, polymerization kinetics and
polymer structures have led to a GTP polymerization mechanism, in which the reaction was
initiated with the intermolecular Michael addition of the SKA enolate group to vinyl group of
B(C6F5)3-activated monomer, while the silyl group is transferred to the carbonyl group of the
monomer to generate the SKA analogues and release B(C6F5)3 which reenters the next catalytic
cycle for the activation of incoming monomer. The living features of such iBuSKA/B(C6F5)3FLP system
enabled us to successfully synthesized well-defined diblock, triblock copolymer PMMBL-b-PMBL and
PMMBL-b-PMBL-b-PMMBL with predicted Mn by using such iBuSKA/B(C6F5)3 FLP system.
Supplementary Materials: Supplementary materials are available online.
Acknowledgments: This work was supported by the National Natural Science Foundation of China (Grant No.
21422401, 21374040, 21774042), 1000 Young Talent Plan of China funds, Talents Fund of Jilin Province, and the
startup funds from Jilin University.
Author Contributions: Lu Hu carried out the synthesis of all silyl ketene acetals (SKAs), and SKA/B(C6F5)3 Lewis
Pair-catalyzed (co)polymerization of all monomers, investigation of polymerization kinetics, characterization
through in situ NMR reaction and MALDI-TOF MS analyses and draft the manuscript. Wuchao Zhao measured
both the number average molecular weight (Mn) and molecular weight distributions (Ð = Mw/Mn) of yielded
polymers by GPC. Jianghua He analyzed and summarized all data. Yuetao Zhang supervised the whole research
work, and finalized the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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