B(C₆F₅)₃-Catalyzed Group Transfer Polymerization of n-Butyl Acrylate with Hydrosilane through In Situ Formation of Initiator by 1,4-Hydrosilylation of n-Butyl Acrylate

Abstract

The group transfer polymerization (GTP) of nbutyl acrylate (nBA) using hydrosilane (R 3 SiH) and tris-(pentafluorophenyl)borane (B(C 6 F 5 ) 3 ) has been studied, which did not need to use the initiator of a silyl ketene acetal (SKA) as the starting polymerization component. B(C 6 F 5 ) 3 catalyzed the in situ 1,4-hydrosilylation of nBA by R 3 SiH to generate the corresponding SKA prior to the polymerization of nBA, which was confirmed by the 1 H NMR measurement of the model reaction. The formed SKA performed as the initiator for the B(C 6 F 5 ) 3 -catalyzed GTP of nBA leading to well-defined polymers with targeted molar masses and low dispersities. S ilyl ketene acetal (SKA) is widely used as one of the versatile reagents in many organic reactions, such as the Mannich reaction, 1 the Mukaiyama aldol reaction, 2 and the Mukaiyama-Michael reaction, 3 which are important carbon− carbon bond forming reactions. In general, SKAs are synthesized by the reaction of lithium enolates with triorganosilyl chlorides or triflates. 4 Besides, the 1,4-hydrosilylation of an α,β-unsaturated ester using hydrosilane is an alternative method for the synthesis of SKAs. 5 In polymer chemistry, SKA is mainly utilized as an initiator for the group transfer polymerization (GTP) of acrylic monomers, which proceeds through repetitive iterations of Mukaiyama-Michael reaction. 6 GTP is one of the living anionic polymerizations, in which the initial molar ratio of the monomer-to-SKA is decisive to control the molar mass of the obtained polymer. However, SKA is relatively unstable toward moisture and impurities, which causes the difficulty in producing the targeted molar mass, particularly, a high molar mass. Therefore, it is important to improve the GTP method in terms of the initiating and propagating processes.Organocatalysts have been found to be more effective for the controlled/living GTPs through fixing the SKA structures in comparison to a conventional Lewis acid and base. For instance, Taton et al. and Waymouth et al. reported that one of the organic Lewis bases, N-heterocyclic carbene, was versatile for the GTP of acrylates, methacrylates, N,N-dimethylacrylamide, and methacrylonitrile. 7 In addition, Chen et al. reported that the Lewis acid of triphenylmethyl salts, such as triphenylmethyl tetrakis(pentafluorophenyl)borate, realized the living GTPs of methacrylates, acrylates, and butyrolactone-based vinylidene monomers. 8 We also reported the Brønsted acid-catalyzed controlled/living GTPs of acrylate and acrylamide using the triisopropylsilyl ketene acetal and silyl ketene amide, respectively. 9,10 However, these well-controlled GTPs still required the use of conventional or originally designed SKAs. Accordingly, of great interest is to develop a GTP method that does not require the use of SKA as the starting polymerization component. Thus, we aimed to design a new GTP method that (1) SKA is in situ generated by the catalytic 1,4-hydrosilylation of an α,β-unsaturated ester as the monomer with a hydrosilane prior to the polymeriza...

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

“…To the best of our knowledge, this work represents the first successful silylium‐catalyzed living polymerization of methacrylates (represented by MMA in this study) by the in situ generated SKA initiator through the “Et 3 Si +” ‐catalyzed 1,4‐hydrosilylation of monomer with R 3 SiH. Although the living/controlled GTP of acrylates (represented by n BA) by the in situ generated SKA through B(C 6 F 5 ) 3 ‐catalyzed hydrosilylation of monomer with R 3 SiH became known while this study was still progress, in our hands the R 3 SiH/B(C 6 F 5 ) 3 system exhibited no activity for polymerization of less reactive, sterically bulkier methacrylate monomers such as MMA. In contrast, the present system consisting of the stronger silylium Lewis acid, R 3 SiH/[Et 3 Si(L)] + [B(C 6 F 5 ) 4 ] – , catalyzes high‐speed and living methacrylate polymerization.…”

“…Accordingly, we hypothesized that this facile B(C 6 F 5 ) 3 ‐catalyzed hydrosilylation of carbonyl substrates could provide a new, convenient GTP process for polymerization of acrylic monomers directly using R 3 SiH, instead of the sensitive SKA. In fact, while our work on this front was still in progress, Kakuchi and coworkers reported the living/controlled GTP of acrylate n BA by the in situ generated SKA initiator through B(C 6 F 5 ) 3 ‐catalyzed hydrosilylation of n BA with R 3 SiH . However, in our hands this R 3 SiH/B(C 6 F 5 ) 3 system exhibited no activity for polymerization of less reactive, sterically bulkier methacrylate monomers such as MMA .…”

Organocatalyzed Group Transfer Polymerization

“…The B(C 6 F 5 ) 3 ‐catalyzed polymerization of n ‐butyl acrylate ( n BA) was first carried out using hydrosilane in CH 2 Cl 2 at room temperature under the conditions of [ n BA] 0 /[hydrosilane] 0 /[B(C 6 F 5 ) 3 ] 0 = 30/1/0.01 and [ n BA] 0 = 1.0 mol L −1 104. Alkylsilanes, such as triethylsilane (Et 3 SiH), tri‐ n ‐butylsilane ( n ‐Bu 3 SiH), and triisopropylsilane ( i ‐Pr 3 SiH), and arylsilanes, such as dimethylphenylsilane (Me 2 PhSiH), methyldiphenylsilane (MePh 2 SiH), and triphenylsilane (Ph 3 SiH), were used to clarify the effect of the hydrosilane structure on the GTP characteristics.…”

Elusive Silane–Alane Complex [SiH⋅⋅⋅Al]: Isolation, Characterization, and Multifaceted Frustrated Lewis Pair Type Catalysis

“…In fact, this finding provides further evidence,d erived from the varied Lewis acidity and oxophilicity of the LA, to support the aforementioned FLPtype silane activation mechanism proposed by Piers and coworkers. [4][5][6][7] Polymerization of the reactive n-butyl acrylate by in situ hydrosilylation of monomer catalyzed by B(C 6 F 5 ) 3 was reported recently by Kakuchi and co-workers, [26] but in our hands the borane/silane system exhibited no polymerization activity towards the less reactive and more sterically hindered MMA (in aM MA/Et 3 SiH/B(C 6 F 5 ) 3 ratio of 400/1/1 in C 6 H 5 For3 0/1/0.01 in CH 2 Cl 2 at RT for 24 h). Ther eason is that, although B(C 6 F 5 ) 3 can effectively catalyze 1,4-hydrosilylation of the monomer,i tp rovides insufficient activation of the monomer for the polymerization step.I nc ontrast, the alane/silane system effectively polymerizes MMA (see Table S4 in the Supporting Information), thanks to its capacity to perform required tandem catalysis for this polymerization.…”