Blocky bromination of syndiotactic polystyrene via post-polymerization functionalization in the heterogeneous gel state

Abstract

This work demonstrates the successful blocky bromination of syndiotactic polystyrene (sPS-co-sPS-Br) copolymers containing 6–30 mol% p-bromostyrene units, using a post-polymerization functionalization method conducted in the heterogeneous gel state. For comparison, a matched set of randomly brominated sPS-co-sPS-Br copolymers was prepared using homogeneous (solution-state) reaction conditions. The degree of bromination and copolymer microstructure were evaluated using ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy. The NMR spectra of gel-state (Blocky) and solution-state (Random) copolymers exhibit strikingly different resonance frequencies and peak intensities above 6 mol% Br and provide direct evidence that functionalization in the gel state produces copolymers with non-random “blocky” microstructures. Quenched films of the Blocky copolymers, analyzed using ultra-small-angle X-ray scattering (USAXS) and small-angle X-ray scattering (SAXS), show micro-phase separated morphologies, which further supports that the Blocky copolymers contain distinct segments of pure sPS and segments of randomly brominated sPS unlike their completely Random analogs. Crystallization behavior of the copolymers, examined using differential scanning calorimetry (DSC), demonstrates that the Blocky copolymers are more crystallizable and crystallize faster at lower supercooling compared to their Random analogs. Computer simulations of the blocky copolymers were developed based on the semicrystalline morphology of a 10 w/v% sPS/CCl₄ gel, to rationalize the effect of heterogeneous functionalization on copolymer microstructure and crystallization behavior. The simulations were found to agree with the microstructural analysis based on the NMR results and confirm that restricting the accessibility of the brominating reagent to monomers well removed from the crystalline fraction of the gel network produces copolymers with a greater prevalence of long, uninterrupted sPS homopolymer sequences. Thus, the blocky microstructure is advantageous for preserving desired crystallizability of the resulting blocky copolymers.