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.