Stereospecific Free Radical and RAFT Polymerization of Bulky Silyl Methacrylates for Tacticity and Molecular Weight Controlled Poly(methacrylic acid)

Abstract

A series of silyl methacrylates [CH2C(CH3)CO2SiR3] with varying silyl group bulkiness [R3Si: Me3Si, Et3Si, Me2 tBuSi, iPr3Si, Ph2 tBuSi, Ph3Si, and (Me3Si)3Si] were synthesized and radically polymerized to efficiently give soluble polymers with the exception of the highly bulky tris(trimethylsilyl)silyl methacrylate (TTMSSMA), which resulted in insoluble polymers. All the polymers can easily be converted into poly(methacrylic acid) (PMAA) via acid- or fluoride-induced deprotection of the silyl groups and further into poly(methyl methacrylate) (PMMA) via methylation with trimethylsilyldiazomethane for the analysis of molecular weight and tacticity. The tacticity was dependent on the bulkiness of the silyl substituents; the isotacticity increased with increasing bulkiness. Thus, a series of PMAAs and PMMAs with various tacticities ranging from syndiotactic-rich (rr = 74%; Me2 tBuSi) to atactic (mr = 50%; iPr3Si) and highly isotactic [mm = 93%; (Me3Si)3Si] enchainment were obtained by conventional radical polymerization of silyl methacrylates followed by simple postreactions. The high isotacticity and insolubility of poly(TTMSSMA) suggested the formation of helical polymers as in the polymerization of similarly bulky triarylmethyl methacrylate. Reversible addition–fragmentation chain-transfer (RAFT) polymerization also worked for these silyl methacrylates, which resulted in well-defined polymers with controlled molecular weights and various tacticities. RAFT polymerization was further applied to the synthesis of novel stereoblock polymers, such as stereo-triblock PMAA and PMMA that consisted of syndiotactic-rich, atactic, and isotactic stereogradient segments.

Asymmetric anionic polymerization of tris(trimethylsilyl)silyl methacrylate: a highly isotactic helical chiral polymer

“…The monomer was polymerized faster in THF than in toluene (Figure 1a), a result that is consistent with these solvents' effects on the anionic polymerization of MMA 41 and TrMA, 42 in which the propagating anion is more reactive in THF because of the coordination of THF to the countercation, Li þ . All of the obtained polymers were insoluble, similar to those obtained in the radical polymerization, 31 most likely because of the rigid helical structure.…”

“…The obtained poly(TTMSSMA)s were then converted into PMMAs to measure the molecular weight and tacticity; this conversion was performed via the deprotection of the silyl groups, followed by methylation. 31 The molecular weights based on PMMA calibration by size-exclusion chromatography were higher than the calculated values assuming that one molecule of Fl-Li generates one polymer chain. The initiator efficiency obtained in toluene (1.5%) was much lower than that in THF (45%) because of the more pronounced aggregation of the initiator in the nonpolar toluene.…”

“…30 We also achieved the facile transformation of the resulting polymers into isotactic poly(methacrylic acid) (PMAA) by removal of the silyl group. 31 These results suggest that the TTMSS group in the methacrylate is sufficiently bulky to dictate the isotactic radical propagation, as is done by the triaryl group. The anionic polymerization of this novel bulky silyl methacrylate has not been studied previously.…”

Stereospecific reversible deactivation radical polymerization of biomass‐based acrylates for precise control of tacticity and molecular weight

“…59,010) and Đ < 1.3, and isotacticity ( mm ) of resulting polymer increased from 4% to 12% by adding Sc(OTf) 3 . Stereospecific RAFT polymerization of bulky silyl methacrylates also gave polymers with Đ = 1.29–1.61 and tacticities of, for instance, mm = 93% and rr = 74% depending on the bulkiness of the silyl groups . The addition of Lewis acid AlCl 3 to atom transfer radical polymerization (ATRP) of acrylamide also afforded moderate control in both aspects of tacticity and polydispersity, that is, Đ ranging from 1.03 to 1.42 and improved tacticity with meso content ranging from 57% to 76% at the expense of controllability of the molecular weight .…”

Selective Michael Reaction Controlled by Supersilyl Protecting Group

“…[5] The anionic polymerization of MMA is known to be one of the most popular anionic polymerization processes and herein we will report the successful termination of the process using the supersilyl protecting group [6,7] to generate chemoselective single and double Michael addition products with high diastereoselectivities.T he tris(trimethylsilyl)silyl methacrylate (TTMSSMA) was used by Okamoto et al for the tacticity-and molecular-weight-controlled radical polymerization to generate poly(TTMSSMA). [8] Supersilyl methacrylate could be prepared directly from methacrylic acid and tris(triethylsilyl)silyl (TTESS) triflate by our reported method. [6a] We first examined the Michael reaction of supersilyl methacrylate 1a with t BuLi in THF at À78 8 8Cf or 2h,a fter which the reaction was quenched with saturated aqueous NH 4 Cl solution, and observed the formation of the 1:1M ichael adduct 2a in 78 %y ield [Scheme 2, Eq.…”