Thermotropic and chiroptical properties of poly{(+)-2,5-bis[4-((S)-2- methylbutoxy)phenyl]styrene} and its random copolymer with polystyrene

Poly{(+)-2,5-bis[4-((S)-2-methylbutoxy)phenyl]styrene} (PMBPS) and its random copolymers with polystyrene were synthesized via atom transfer radical polymerization and conventional radical copolymerization, respectively. The dependences of thermotropic and chiroptical properties on the degrees of polymerization (DPs) of homopolymers and on the compositions of copolymers were investigated to correlate the liquid crystallinity and helical chain structure of PMBPS. When DP < 53, PMBPS revealed no mesophase; when 53 < DP < 80, the polymer formed columnar nematic (φ_N) phase; whereas, when DP ≥ 80, hexatic columnar nematic (φ_HN) phase was generated. The monomer MBPS displayed positive optical rotation, while the oligomers and polymers showed negative optical rotations. The strength intensified with DP and leveled off at DP = 53, suggesting a stable helix started. The random nature of copolymers was indicated by the reactivity ratios of two monomers (r _MBPS = 0.67, r_st = 0.98), and further supported by smooth variation of single glass transition temperature. When the feed molar fraction of MBPS (x) was less than 59%, the optical rotations of copolymers were positive and scaled linearly with the proportion of chiral monomer, implying no new chiral structure developed. When x > 59%, the optical rotations decreased with increasing MBPS content and finally became negative when x reached above 90%, suggesting the MBPS segment was long enough to develop a dominating helical structure with negative optical rotation. Among all the copolymers, only one with x value of as high as 99% could achieve mesophase (φ_N). It was considered that a long enough helical structure, which acted as cylindrical building blocks, was the origin of PMBPS to form a liquid crystalline phase.