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

A series of silyl methacrylates [CH2=C(CH3)CO2SiR3] with varying sly] group bulkiness [R3Si: Me3Si, Et3Si, Me(2)tBuSi, iPr(3)Si, Ph(2)tBuSi, Ph3Si, and (Me3Si)(3)Si] 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 trimethylsilyl-diazomethane 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%; Me(2)tBuSi) to atactic (mr = 50%; iPr(3)Si) and highly isotactic [mm = 93%; (Me3Si)(3)Si] 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.