Entropy-Driven Design of Depolymerizable Polyolefins from Strained Bridged Bicyclic Monomers

Abstract

Polymers produced by ring-opening metathesis polymerization (ROMP) of strained cyclic olefin monomers, such as norbornene and cyclobutene, are challenging to depolymerize back to their constituent monomers due to their favorable polymerization thermodynamics. Current strategies for creating depolymerizable ROMP polymers focus on designing low-strain monomers with small enthalpic driving forces, which facilitate depolymerization by reducing the monomer polymerizability. Because polymerization thermodynamics is governed by both enthalpic and entropic contributions, we reason that depolymerizable polymers could be achieved from highly strained cyclic olefin monomers if the entropic penalty of polymerization is sufficiently large. Here, we present a depolymerizable polymer system based on a series of strained bicyclo[3.2.1] monomers, which combine a substantial enthalpic driving force (−6 to −11 kcal/mol) with a significant entropic penalty of polymerization (−15 to −24 cal/mol/K). The large entropic penalty, arising from the rigid polymer backbone, lowers the ceiling temperature and imparts depolymerizability to the polymer system, leading to monomer recovery (74–99%) under standard ring-closing metathesis conditions. Moreover, the enthalpic driving force remains sufficient to enable efficient ring-opening metathesis polymerization and block copolymer synthesis. This entropy-driven strategy thus unlocks access to depolymerizable polymers from strained cyclic olefin monomers that are not traditionally considered building blocks for such materials, offering a new direction for the design of chemically recyclable polymers with an expanded monomer scope.