Life RAFT: A bulky methacrylate monomer, triphenylmethyl methacrylate (TrMA), was polymerized with reversible addition-fragmentation chain transfer (RAFT) agents. Stereogradient polymers in which the isospecificity increased spontaneously as the monomer concentration decreased were formed by a polymerization-depolymerization equilibrium that can convert a less stable growing polymer terminal into a more stable form (see picture).

In this study, the stereocomplexation between a novel stereospecific cyclic vinyl polymer, that is, cyclic syndiotactic poly(methyl methacrylate) (st-PMMA), with the complementary linear isotactic (it-) PMMA was investigated. Surprising new insight into the effects of the topology (i.e., end groups), size, and tacticity of the assembling components on stereocomplex formation was obtained. Characterization of the stereocomplexes revealed that the self-assembly of cyclic st-PMMAs and linear it-PMMAs resulted in the formation of an unprecedented “polypseudorotaxane-type” supramolecular assembly. This stereocomplex exhibited remarkably different physical properties as compared to the conventional PMMA triple-helix stereocomplex as a result of the restricted topology imposed by the cyclic st-PMMA assembling component.

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.

The synthesis of stereogradient polymers with tacticities that vary from predominantly syndiotactic to highly isotactic was investigated by reversible addition-fragmentation chain transfer (RAFT) copolymerization of bulky methacrylates, such as triphenylmethyl methacrylate (TrMA) and 1phenyldibezosuberyl methacrylate (PDBSMA) and methacrylic acid (MAA) in both non-polar and polar solvents. The MAA monomer showed increased reactivity in toluene because of hydrogen bonding and was consumed slightly faster than TrMA or PDBSMA. However, the RAFT copolymerization of TrMA and MAA in 1,4-dioxane resulted in consumption of both monomers at the same rate. The copolymers can be easily converted to homopoly(MAA) by the acid hydrolysis of the bulky group and converted further to poly(methyl methacrylate) by methyl esterification using trimethylsilyldiazomethane to analyze the molecular weights and tacticity. The molecular weights of the polymers obtained in both solvents increased with monomer conversion, which indicates that controlled/living radical copolymerization proceeded irrespective of the solvents. 13 C NMR analyses of the polymers revealed that stereogradient polymers were produced in toluene, in which the tacticity changed from mm ¼ 11% to nearly 100%, whereas the copolymers obtained in 1,4-dioxane resulted in nearly atactic enchainment (rr/mr/mm z 38/49/13), independent of monomer conversion. A similar stereogradient copolymer was also obtained by RAFT copolymerization of PDBSMA and MAA in toluene, where the isotacticity changed more gradually from mm ¼ 14% to nearly 100%.

We report the synthesis of a well-defined molecular bottlebrush polymer with stereoregular side chains (i.e., syndiotactic PMMA). The simultaneous control over the molecular weight, side-chain tacticity, and architecture allows the macromolecule to stereocomplex with the complementary linear stereoregular polymers (i.e., isotactic PMMAs) in controlled manners. By modulating the feed ratio of the complexing materials and chain length of the linear assembling component, a variety of crystalline materials with different sizes and morphologies, including discrete spherical nanoparticle, multiple-particle assembly, and cross-linked network structure, can be produced. Among these, uniformed sized, stable nanocrystals that exhibit temperature-induced solution assembly and disassembly properties can be derived from a combined process of PMMA triple-helix stereocomplex formation and polymer architecture-directed intramolecular crystallization. This work has established a new, facile synthetic protocol toward stimuli-responsive organic nanocrystals, which is applicable to the fabrication of a wide variety of functional crystal nanomaterials with practical applications.