Tailored Chain Sequences of Brominated Syndiotactic Polystyrene Copolymers via Post-Polymerization Functionalization in the Heterogeneous Gel State

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Virginia Tech


This dissertation demonstrates the preparation of blocky brominated syndiotactic polystyrene (sPS-co-sPS-Br) copolymers with tailored chain sequences using a simple, post-polymerization functionalization method conducted in the heterogeneous gel state, and investigates the effect of sPS reaction state and sPS/solvent gel morphology on the copolymer microstructure and thermal properties. Gel-state (Blocky) brominated copolymers were prepared from a 10 w/v% sPS/carbon tetrachloride (CCl4) gel and a 10 w/v% sPS/chloroform (CHCl3) gel in a matched set containing 6−32 mol% p-bromostyrene (Br-Sty) units. For comparison, a matched set of randomly brominated copolymers was prepared using a homogeneous solution-state (Random) reaction method and a set of brominated copolymers was prepared using a heterogenous powder-state (Powder) reaction method. The degree of bromination was evaluated using 1H nuclear magnetic resonance (NMR) spectroscopy. Powder-state bromination produced copolymers with a limited degree of functionalization of up to 12 mol% Br and required a threefold longer reaction time than the gel-state method conducted on the sPS/CHCl3 gel, demonstrating that the powder-state method is time-consuming and the dense sPS powder is incapable of producing copolymers with high Br-content. Microstructural characterization provided by 13C NMR spectroscopy, showed that bromination of sPS produces multiple peaks in the quaternary carbon region of the NMR spectrum, signifying through-bond communication between neighboring styrene and Br-Sty monomers. This work provides the first high-resolution comonomer sequencing of brominated sPS copolymers. Characterization of the quaternary carbon spectrum, assisted by band selective gradient heteronuclear multiple bond correlation (bsgHMBC) spectroscopy, electronic structure calculations, and simulated statistically random copolymer chains, revealed that each resonance peak could be assigned to a styrene or Br-Sty unit that exists in the center of a unique sequence of five monomers (i.e., a pentad) along the copolymer chain (e.g., ssssb where s = styrene and b = brominated styrene). Our comonomer sequencing method demonstrated that the Blocky and Powder copolymers have block-like character. Remarkably, the Blocky copolymers exhibit notably higher degrees of blockiness and larger fractions of sssss and bbbbb pentads at low Br contents (i.e., 32 mol% Br), relative to the Powder copolymers, confirming their blocky microstructure. Quenched films of the Blocky copolymers, analyzed using ultra-small-angle (USAXS) and small-angle X ray scattering (SAXS), show micro-phase separated morphologies that are reminiscent of conventional block copolymer phase behavior, supporting that the Blocky copolymers contain distinct segments of pure sPS and segments of randomly brominated sPS. 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. Simulations of blocky copolymers were developed based on the semicrystalline gel morphology to rationalize the effect of gel-state functionalization on copolymer microstructure and crystallization behavior. The simulations 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 runs of pure sPS that is advantageous for preserving desired crystallizability of the resulting blocky copolymers. To investigate the effect of sPS/solvent gel morphology on copolymer microstructure and crystallization behavior, the sPS/CCl4 and sPS/CHCl3 copolymers were compared directly. Characterization of the sPS/solvent gels using USAXS/SAXS, revealed that the gels exhibit different morphologies and average lamella thicknesses. Microstructural analysis showed that the sPS/CHCl3 copolymers contain larger fractions of sssss pentad and a greater degree of blockiness. The sPS/CHCl3 copolymers contain larger phase domains, supporting that these copolymers contain longer distinct segments of pure sPS and randomly brominated sPS in a multiblock-like microstructure. In addition, the sPS/CHCl3 copolymers are more crystallizable during conditions of rapid cooling and crystallize faster at low supercooling relative to their sPS/CCl4 analogs. Simulated average chains of the Blocky copolymers, generated from the empirical pentad sequence distributions, provide strong evidence that the runs of pure sPS in the Blocky copolymers originate from the crystalline stems within the crystalline lamellae. Thus, the simulations support that semicrystalline blocky brominated copolymers with tailored chain sequences, phase behavior, and crystallization properties and can be prepared simply by changing the gelation solvent.



gel-state, post-polymerization functionalization, bromination, heterogeneous, blocky, microstructure, comonomer sequence, degree of blockiness, syndiotactic polystyrene