Silyl Ketene Acetals/B(C6F5)3 Lewis Pair-Catalyzed Living Group Transfer Polymerization of Renewable Cyclic Acrylic Monomers

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).


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][2][3][4][5][6][7][8], since the seminal works on Frustrated Lewis Pairs (FLPs) by Stephan and Erker in small molecule chemistry [9][10][11][12].For instance, LPs based on strongly acidic, bulky E(C 6 F 5 ) 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-γ-butyrolactone (MBL), and γ-methyl-α-methylene-γ-butyrolactone (MMBL) [13][14][15] and 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 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), 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 M n than the calculated M n 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 Me 3 SiNTf 2 for living polymerization of MMA, which proceeds through a similar mechanism compared to the group transfer polymerization (GTP) of MMA [22].
In this context, herein we report the SKA/B(C 6 F 5 ) 3 FLP-catalyzed GTP of polar vinyl monomers at room temperature, including MMA, renewable MMBL and MBL.More specifically, a highly interacting iBu SKA/B(C 6 F 5 ) 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(C 6 F 5 ) 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.S1).On the other hand, the direct use of SKA (Scheme 1) as initiator and B(C 6 F 5 ) 3 as catalyst did not lead to obvious enhancement in MMA polymerization activity compared to R 3 SiH/B(C 6 F 5 ) 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.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.

Results and Discussion
Scheme 1.Chemical structures of initiators, catalyst and monomers employed in this study.

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 twostep 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 = 57.4kg•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 drastically decreased polymerization activity (6 h for Ph3SiH and 24 h for i Bu3SiH to reach full monomer conversion) and initiation efficiency (21% for Ph3SiH and 7% for i Bu3SiH).
When switching to the SKA/B(C6F5)3 system, both Me2Ph SKA/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 Me2Cl SKA/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 Ph SKA with B(C6F5)3 produced a polymerization system that is comparable with that of Me2Cl SKA/B(C6F5)3 (run 10  Next, we investigated the effectiveness of the R 3 SiH/B(C 6 F 5 ) 3 and SKA/B(C 6 F 5 ) 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 R 3 SiH/B(C 6 F 5 ) 3 and SKA/B(C 6 F 5 ) 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, Me 2 EtSiH and Me 2 PhSiH achieved quantitative monomer conversion within 30 min (M n = 48.1 kg•mol −1 , Ð = 1.26,I* = 47% for run 1, Table 1; M n = 57.4kg•mol -1 , Ð = 1.39,I* = 39% for run 2, Table 1).Replacing the Ph group in the R 3 SiHstructure with the electron-withdrawing Cl (i.e., Me 2 ClSiH) rendered the polymerization reaching completion in 2 h and producing PMMBL with a M n of 78.6 kg•mol −1 and a higher Ð value of 1.44 (run 3, Table 1).Interestingly, R 3 SiH with three Et substituents, Et 3 SiH, when combined with B(C 6 F 5 ) 3 , exhibited comparable polymerization activity with that for Me 2 EtSiH, producing PMMBL with a M n 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 R 3 SiH exhibited drastically decreased polymerization activity (6 h for Ph 3 SiH and 24 h for i Bu 3 SiH to reach full monomer conversion) and initiation efficiency (21% for Ph 3 SiH and 7% for i Bu 3 SiH).When switching to the SKA/B(C 6 F 5 ) 3 system, both Me2Ph SKA/B(C 6 F 5 ) 3 and Me2(EtO) SKA/B(C 6 F 5 ) 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(C 6 F 5 ) 3 ] 0 = 200:1:1, 0.25 mL MMBL, 2.25 mL CH 2 Cl 2 , 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 Me2Cl SKA/B(C 6 F 5 ) 3 system, producing PMMBL with a M n 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 Ph SKA with B(C 6 F 5 ) 3 produced a polymerization system that is comparable with that of Me2Cl SKA/B(C 6 F 5 ) 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 Me SKA, run 11; 61 for Et SKA, run 12; 70 for iBu SKA, run 13).
Since both Me SKA/B(C 6 F 5 ) 3 and iBu SKA/B(C 6 F 5 ) 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]:[ Me SKA]:[B(C 6 F 5 ) 3 ] ratio, it took 24 h for both Me SKA/B(C 6 F 5 ) 3 and iBu SKA/B(C 6 F 5 ) 3 system to reach complete monomer consumption.However, it should be noted that PMBL produced by Me SKA/B(C 6 F 5 ) 3 system exhibited a bimodal MWD.For polymerization with 100:1:1 [MBL]:[SKA]:[B(C 6 F 5 ) 3 ] ratio, only 30 min is needed to achieve quantitative monomer conversion for both systems, affording PMBL with predicted M n and small Ð values ( Me SKA: M n = 12.7 kg•mol −1 , Ð = 1.12,I* = 78%, Run 16; iBu SKA: M n = 13.4 kg•mol −1 , Ð = 1.05,I* = 74%, Run 18, Table 1).These results indicated iBu SKA/B(C 6 F 5 ) 3 system exhibited better control on the polymerization than that for Me SKA/B(C 6 F 5 ) 3 system.
In fact, the MMBL polymerization by iBu SKA/B(C 6 F 5 ) 3 system is living and controlled, and near quantitative monomer conversion was achieved for polymerization with varied [MMBL]/[ iBu SKA] ratio from 100 to 800.GPC traces of PMMBL produced by iBu SKA/B(C 6 F 5 ) 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(C 6 F 5 ) 3 would enhance the polymerization rate, the polymer MW is dependent on the concentration of initiator [ iBu SKA] and independent on the concentration of catalyst B(C 6 F 5 ) 3 (vide infra), which is evidenced by the linear increase of the M n values of PMMBL produced by iBu SKA/B(C 6 F 5 ) 3 system with an increase in the [MMBL]/[ iBu SKA] ratio from 100 to 800 (Figure 2, R 2 = 0.994), while Ð values remained in the very narrow range of 1.08 to 1.13 (Figure 2).It is noted that M n value of PMMBL obtained for polymerization with different [MMBL]/[B(C 6 F 5 ) 3 ] ratio maintained around 60 kg•mol −1 (Figure S21), which revealed that the concentration of [B(C 6 F 5 ) 3 ] has an effect the polymerization rate, but not on both polymer MW and MWD.Therefore, B(C 6 F 5 ) 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 iBu SKA/B(C 6 F 5 ) 3 was also confirmed by a plot of the PMMBL M n vs monomer conversion at a fixed

Characterization of Key Intermediates
To gain more insights into the polymerization, we studied the in situ NMR reactions of Me SKA 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 Me SKA with B(C6F5)3 (Figures S11 and S12).Similar FLP was generated in the reaction of iBu SKA with B(C6F5)3 in 1:1 ratio (Figures S13 and S14), too.We also prepared B(C6F5)3•MMA (Figures S7 and S8) and B(C6F5)3•MMBL (Figures S9 and S10) adduct for the study of their reaction with SKA, respectively.The reaction of Me SKA with B(C6F5)3•MMA at room temperature in C6D6 generated major products of dimeric SKA analogue 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 1 H-and 13 C-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 Me SKA, which is different from that for Me3Si + generated by vinylogous hydride abstraction of Me SKA 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 19

Characterization of Key Intermediates
To gain more insights into the polymerization, we studied the in situ NMR reactions of Me SKA with B(C 6 F 5 ) 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 Me SKA with B(C 6 F 5 ) 3 (Figures S11 and  S12).Similar FLP was generated in the reaction of iBu SKA with B(C 6 F 5 ) 3 in 1:1 ratio (Figures S13  and S14), too.We also prepared B(C 6 F 5 ) 3 •MMA (Figures S7 and S8) and B(C 6 F 5 ) 3 •MMBL (Figures S9  and S10) adduct for the study of their reaction with SKA, respectively.The reaction of Me SKA with B(C 6 F 5 ) 3 •MMA at room temperature in C 6 D 6 generated major products of dimeric SKA analogue Me 3 SiO(OMe)C=C(Me)CH 2 CMe 2 C(OMe)=O• • •B(C 6 F 5 ) 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 1 Hand 13 C-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 Me SKA, which is different from that for Me 3 Si + generated by vinylogous hydride abstraction of Me SKA with Ph 3 C + , exhibiting signal at 0.65 ppm [49].This result suggests that the silyl group in the dimeric SKA analogue was neutral rather than Me 3 Si + .In combination with the fact that 19 F NMR spectrum exhibiting similar signals [δ −129.03(br, 6F, o-F), −142.24(br, 3F, p-F), −160.09(s, 6F, m-F))] (Figure S16) with that for B(C 6 F 5 ) 3 •MMA adduct (Figure S8), we proposed the structure for corresponding dimeric SKA analogues as shown in Scheme 2. Similarly, the reaction of iBu SKA with B(C 6 F 5 ) 3 •MMA cleanly afforded the single-monomer-addition product (Scheme 2), as revealed by the 1 H-, 19 F-and 13 C-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(C 6 F 5 ) 3 •MMBL probably due to high reactivity of MMBL.
B(C6F5)3•MMA at room temperature in C6D6 generated major products of dimeric SKA analogue 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 1 H-and 13 C-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 Me SKA, which is different from that for Me3Si + generated by vinylogous hydride abstraction of Me SKA 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 19 F NMR spectrum exhibiting similar signals [δ −129.03(br, 6F, o-F), −142.24(br, 3F, p-F), −160.09(s, 6F, m-F))] (Figure S16) 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 iBu SKA with B(C6F5)3•MMA cleanly afforded the single-monomer-addition product (Scheme 2), as revealed by the 1 H-, 19 F-and 13 C-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.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(C 6 F 5 ) 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(C 6 F 5 ) 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(C 6 F 5 ) 3 -activated monomer to generate the single-monomer-addition species or the active propagating species.In the propagation cycle, the B(C 6 F 5 ) 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 iBu SKA initiator concentration, but second-order dependent on B(C 6 F 5 ) 3 catalyst concentrations, which revealed that the rate (k 2 ) of the intermolecular Michael addition of the SKA or its homologues to the B(C 6 F 5 ) 3 -activated monomer is comparable with that (k 1 ) for the release of the catalyst B(C 6 F 5 ) 3 from the ester group of the growing polymer chain to the incoming monomer for monomer activation (k 1 ≈ k 2 ).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 The low MW PMMBL produced by the iBu SKA/B(C 6 F 5 ) 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 termination (the silyl group) chain ends being derived from i 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.

Random and Block Copolymerizations of MMBL with MBL
The living features of polymerization of MMBL and MBL by iBu SKA/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 = 28.3kg•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]/[ iBu SKA]/[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 iBu SKA/B(C6F5)3 system.Through the same sequential monomer addition method, well-defined triblock copolymer PMMBLb-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 13 C-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 in the carbonyl signals, which is ranging between the signals for homopolymers PMBL and PMMBL.The combination of such conclusive 13 C NMR results and GPC analyses should provide sufficient evidence for the formation of random and block copolymer from statistical and sequential block polymerization, respectively.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 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 iBu SKA 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).The low MW PMMBL produced by the iBu SKA/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 termination (the silyl group) chain ends being derived from i 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.Scheme 3. Proposed mechanism for MMBL polymerization by SKA/B(C 6 F 5 ) 3 .

Random and Block Copolymerizations of MMBL with MBL
The living features of polymerization of MMBL and MBL by iBu SKA/B(C 6 F 5 ) 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 M n = 28.3kg•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]/[ iBu SKA]/[B(C 6 F 5 ) 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 M n 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 iBu SKA/B(C 6 F 5 ) 3 system.Through the same sequential monomer addition method, well-defined triblock copolymer PMMBL-b-PMBL-b-PMMBL was also successfully prepared with a M n 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 13 C-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 in the carbonyl signals, which is ranging between the signals for homopolymers PMBL and PMMBL.The combination of such conclusive 13 C NMR results and GPC analyses should provide sufficient evidence for the formation of random and block copolymer from statistical and sequential block polymerization, respectively.

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, 1 H; 126 MHz, 13 C; 471 MHz, 19 F) instrument (Bruker, Karlsruhe, Baden-Württemberg, Germany) in appropriate deuterated solvents at room temperature.Chemical shifts for 1 H and 13 C spectra were referenced to internal solvent resonances and are reported as parts per million relative to SiMe4, whereas 19 F-NMR spectra were referenced to external CFCl3.Air sensitive NMR samples were conducted in Teflonvalve sealed J. Young-type NMR tubes.

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.CH 2 Cl 2 was refluxed over CaH 2 distilled under nitrogen atmosphere, then stored over molecular sieves 4 Å.Benzene-d 6 was dried over molecular sieves 4 Å.NMR spectra were recorded using an Avance II 500 (500 MHz, 1 H; 126 MHz, 13 C; 471 MHz, 19 F) instrument (Bruker, Karlsruhe, Baden-Württemberg, Germany) in appropriate deuterated solvents at room temperature.Chemical shifts for 1 H and 13 C spectra were referenced to internal solvent resonances and are reported as parts per million relative to SiMe 4 , whereas 19 F-NMR spectra were referenced to external CFCl 3 .Air sensitive NMR samples were conducted in Teflon-valve sealed J. Young-type NMR tubes.

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(C 6 F 5 ) 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 CH 2 Cl 2 .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(C 6 F 5 ) 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" CDCl 3 stabilized by 250 ppm of BHT-H; the quenched aliquots were later analyzed by 1 H-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.

Polymer Characterizations
Number-average molecular weight (M n ) and molecular weight distributions (Ð = M w /M n ) 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 × 30 mm guard column and three Waters WAT054466, WAT044226, WAT044223 columns (Polymer Laboratories, Milford, MA, USA; linear range of molecular weight = 500 to 4 × 10 6 ).The instrument was calibrated with 10 PMMA standards, and chromatograms were processed with Waters Breeze 2 software (version: 6.01.2154.026;Milford, MA, USA).Tacticities of PMMA was measured by 1 H-NMR in CDCl 3 , while 13 C-NMR of P(M)MBL (co)polymers were recorded in DMSO-d 6 .

CH 2
Cl 2 at room temperature.In the presence of B(C 6 F 5 ) 3 , the in-situ generation of SKA initiators from the B(C 6 F 5 ) 3 -catalyzed hydrosilylation of the monomer MMA with R 3 SiH exhibited very low polymerization activity and incomplete monomer conversions for various ratios of [MMA] 0 /[R 3 SiH] 0 /[B(C 6 F 5 ) 3 ] 0 for up to 24 h (runs 1-18, Table

Scheme 1 .
Scheme 1.Chemical structures of initiators, catalyst and monomers employed in this study.
Molecules 2018, 23, x 5 of 19 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 iBu SKA/B(C6F5)3 was also confirmed by a plot of the PMMBL Mn vs monomer conversion at a fixed [MMBL]/[ iBu SKA]/[B(C6F5)3] ratio of 400:1:1, which clearly show a straight line (R 2 = 0.999) with very narrow Ð value in the range of 1.08-1.2(Figure 3).

Scheme 2 .
Scheme 2. Generation of first-monomer-addition intermediate or active propagating species (A or B denotes Z/E isomers).

Figure 7 .
Figure 7. MALDI-TOF mass spectrum of the low-MW PMMBL sample produced by iBu SKA/B(C6F5)3 in CH2Cl2 at RT.Figure 7. MALDI-TOF mass spectrum of the low-MW PMMBL sample produced by iBu SKA/B(C 6 F 5 ) 3 in CH 2 Cl 2 at RT.

Figure 7 .
Figure 7. MALDI-TOF mass spectrum of the low-MW PMMBL sample produced by iBu SKA/B(C6F5)3 in CH2Cl2 at RT.Figure 7. MALDI-TOF mass spectrum of the low-MW PMMBL sample produced by iBu SKA/B(C 6 F 5 ) 3 in CH 2 Cl 2 at RT.

Figure 8 .
Figure 8. Plot of m/z values from Figure 7 vs. the number of MMBL repeat units (n) for major series (a) and minor series (b).

Figure 8 .
Figure 8. Plot of m/z values from Figure 7 vs. the number of MMBL repeat units (n) for major series (a) and minor series (b).
a Carried out in 2.25 mL CH2Cl2 at room temperature, where [MMBL]0 = [MBL]0 = 0.936 M and addition method: catalyst and monomer were premixed, followed by adding initiator to start the polymerization.b Monomer conversions measured by 1 H-NMR.c Mn and MWD determined by GPC relative to PMMA standards in DMF.d MBL and MMBL was added at the same time.