Suspended Alicyclic Skeleton: Synthetic Approach to Zero-Birefringence Polycarbonate

Poly(cyclohexene carbonate) (PCHC) is a recyclable, sustainable, alicyclic polycarbonate that can be synthesized from CO2. However, the optical properties of PCHC, including birefringence, are not well understood. Herein, we designed novel carbonate units via two approaches: "fusion" and "suspension" of a norbornane skeleton to obtain PCHC-based zero-birefringence polycarbonates. Compared to PCHC, the in-plane retardation of the 100% uniaxially stretched norbornane-fused PCHC film was reduced, whereas the photoelastic coefficient unexpectedly increased. In contrast, the photoelastic birefringence of norbornane-suspended PCHC decreased. Furthermore, the copolymer synthesized via the copolymerization of cyclohexene carbonate with a tetracyclododecane-suspended monomer exhibited almost zero photoelastic birefringence. This result indicates that the photoelastic birefringence can be reduced by "suspending" the cyclic skeleton rather than "fusing" it. Computational studies suggested that the chair cyclohexane skeleton suppresses photoelastic birefringence. The results obtained in this study have the potential to guide the design of low-birefringence polycarbonates and other polymers.