Thermal stability and physical properties of poly(alkyl fumarate)s with Bornyl and Isobornyl groups as the geometrically isomeric esters

Abstract

Poly(dialkyl fumarate)s (PDRFs) with a poly(substituteda methylene) structure are highly transparent amorphous polymers with excellent heat resistance, mechanical, and optical properties. When optical polymer materials are used for electronic devices, both chemical and physical stability are required for the thermal properties of polymers. In this study, we conducted radical polymerization and copolymerization of dibornyl fumarate (DBoF) and diisobornyl fumarate (DIBF) with a symmetric ester structure as well as bornyl isopropyl fumarate (BoiPF) and isobornyl isopropyl fumarate (IBiPF) with an asymmetric structure to investigate the thermal and optical properties of the resulting PDRFs as the poly(substituted methylene)s including bicycloalkyl ester groups with a fixed molecular conformation in the side chain. It was revealed that the polymerization reactivity of the fumarates containing a bornyl group was higher than the fumarates including an isobornyl group. Monomer reactivity ratios were determined to be r₁ = 0.83 and r₂ = 0.92 for the copolymerization of DBoF (M₁) and diisopropyl fumarate (DiPF, M₂) while r₁ = 0.54 and r₂ = 1.60 for the system of DIBF (M₁) and DiPF (M₂). The thermal decomposition behavior of the resulting PDRFs depended on the geometric structure of the bornyl and isobornyl ester groups; for example, the onset temperatures of thermal decomposition were 306–320 °C and 240–269 °C for the Bo- and IB-containing PDRFs, respectively. The β-transition temperature also significantly increased from 64 °C for the diisopropyl ester of PDRF to 132 °C for the diborny ester. On the other hand, refractive index kept constant in a range of 1.46–1.48 for the Bo- and IB-containing PDRFs independent of the kind of a geometric structure and the contents. In order to clarify the thermal stability of the polymers, we investigated a mechanism for olefin elimination from the ester alkyl groups upon heating in the absence and presence of an acid, and discussed the effect of the geometrical structures, i.e., endo and exo configuration of the ester alkyl groups on the thermal decomposition mechanism. The aggregation structure of the polymer chains was also analyzed based on the results of wide-angle X-ray scattering (WAXS) analysis in the solid state to examine the relationship between the molecular packing and the physical properties of the PDRFs.