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

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 degrees 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 degrees 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.

Automated summary

This study reveals that fusing four- and five-membered rings into cyclooctene can reduce the ring strain energies of the monomers and affect the enthalpy changes of polymerization. Despite the narrow range of enthalpy changes, different monomers exhibit varying ceiling temperatures at different concentrations. Furthermore, the presence of remote geminal substituents can influence the depolymerization temperature, even when not directly attached to the cyclooctene.