Highly Transparent Benzothiazole-Based Block and Random Copolymers with High Refractive Indices by RAFT Polymerization

The development of thermally stable, highly transparent polymers with superior refractive indices and Abbe’s numbers are important for the further development of next-generation technology such as miniaturized opto-integrated devices and advanced lenses. Highly transparent benzothiazole-based copolymers having either block or random sequences and high refractive indices were synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization. The conventional free-radical and RAFT copolymerizations of 2-benzothiazolyl vinyl sulfide (BTVS) and 2-vinylnaphthalene (VNA) produced random copolymers with adjustable BTVS contents between 8 and 54%. Chain extending the macrochain transfer agent, which was prepared by the RAFT polymerization of BTVS, with VNA was well-controlled, enabling the synthesis of block copolymers with high BTVS contents (up to 70%) and reasonable polymer yields (up to 60%). The benzothiazole-based homopolymer, poly(BTVS), exhibited a high refractive index (1.7432) and a reasonable Abbe’s number (17.0). Excellent transmittances (>93%) of 400 nm light were observed for both copolymers composed of BTVS and VNA, and high refractive indices of 1.7178–1.6672 were achieved. The resulting benzothiazole-based copolymers exhibited high refractive indices (>1.7), high transparencies, tunable low glass-transition temperatures (Tg = 75–150 °C), and high thermal stabilities (Td5 > 240 °C) by adjusting the chemical structure, composition, and sequence of the comonomer. This report presents development of the first high-refractive-index polymers derived from S-vinyl sulfide derivatives with benzothiazole side chains, exhibiting the synergistic effects of combining aromatic naphthalene and heteroaromatic benzothiazole units.