Understanding the Structure–Polymerization Thermodynamics Relationships of Fused-Ring Cyclooctenes for Developing Chemically Recyclable Polymers

Abstract

Polymers that can be chemically recycled to their constituent monomers offer a promising solution to address the challenges in plastics sustainability through a circular use of materials. The design and development of monomers for next-generation chemically recyclable polymers require an understanding of the relationships between the structure of the monomers/polymers and the thermodynamics of polymerization/depolymerization. Here we investigate the structure–polymerization thermodynamics relationships of a series of cyclooctene monomers that contain an additional ring fused at the 5,6-positions, including trans-cyclobutane, trans-cyclopentane, and trans-five-membered cyclic acetals. The four- and five-membered rings trans-fused to cyclooctene reduce the ring strain energies of the monomer, and the enthalpy changes of polymerizations are found to be in the range of ?2.1 to ?3.3 kcal mol^–1. Despite the narrow range of enthalpy changes, the ceiling temperatures at 1.0 M span from 330 to 680 °C, due to the low entropy changes, ranging from ?2.7 to ?5.0 cal mol^–1 K^–1. Importantly, geminal substituents on the trans-five-membered cyclic acetal fused cyclooctenes are found to reduce the ceiling temperature by ～300 °C, although they are not directly attached to the cyclooctene. The remote gem-disubstituent effect demonstrated here can be leveraged to promote depolymerization of the corresponding polymers and to tune their thermomechanical properties.