Evaluating Trans‐Benzocyclobutene‐Fused Cyclooctene as a Monomer for Chemically Recyclable Polymer

Abstract

Chemically recyclable polymers offer a promising solution to address the issues associated with the unsustainable use of plastics by converting the traditional linear plastic economy into a circular one. Central to developing chemically recyclable polymers is to identify the appropriate monomers that enable practical conditions for polymerization and depolymerization and ensure useful stability and material properties. Our group has recently demonstrated that transcyclobutane-fused cyclooctene (tCBCO) meets the abovementioned requirements and is a promising candidate for developing chemically recyclable polymers. Herein, encour-aged by the success with tCBCO, we investigate the thermodynamics of polymerization of a relevant system, trans-benzocyclobutene-fused-cyclooctene, which can be viewed as tCBCO with an additional benzene ring. The study shows that introducing an additional benzene ring favors polymerization and disfavors depolymerization, and the effect is predominantly entropic. The benzo-effect can be leveraged to fine-tune the thermodynamics of polymerization and depolymerization to facilitate the chemical recycling of polymers.

Deconstruction of Polymers through Olefin Metathesis

“…This exploration of substituent effects was further expanded with a study of the polymerization/depolymerization behavior of a system based on a cyclooctene monomer with a transfused benzocyclobutene ring (M41). 78 Interestingly, the thermodynamics study of this monomer (with initial monomer concentration of ∼0.1 M, Table 2, entry 10) revealed that while the ΔH p was only slightly larger than that of the unsubstituted tCBCO monomer M31 (−2.4 vs −2.1 kcal mol −1 ; Table 2, entry 3), its entropy of polymerization, ΔS p , was positive (4.6 cal mol −1 K −1 , calculated for [M] 0 = 1.0 M) as opposed to negative for the unsubstituted tCBCO monomer (−3.4 cal mol −1 K −1 ). This behavior was also reflected in its depolymerization, with no depolymerization seen at the initial olefin concentration of 200 mM (ΔS p = 1.4 cal mol −1 K −1 at [M] = 200 mM).…”