Functional Group Transformation Approach to Chemically Recyclable Polymers from Ultra-Low to Moderate Strain Monomers

Ring-opening metathesis polymerization (ROMP) has been
widely used
for the synthesis of functional polymers. However, most ROMP-derived
polymers are nondepolymerizable, limiting their sustainability and
eco-friendiness. While recent advances in designing low-strain cyclic
olefin monomers have enabled the ROMP synthesis of depolymerizable
polyolefins, the scope of these monomers remains limited due to the
narrow range of ring strain energies (RSEs = 4.7–5.4 kcal/mol)
required to allow both polymerization and depolymerization in a closed-loop
recycling process. Herein, we present a new class of chemically recyclable
polyolefins based on cycloheptene derivatives with RSEs ranging from
3.8 to 7.2 kcal/mol. The wide range of RSEs enabled the establishment
of a structure–polymerizability–depolymerizability relationship,
shedding light on the role of RSE in both polymerization and depolymerization.
A functional group transformation (FGT) strategy, harnessing reversible
ketone-to-acetal chemistry, was developed to overcome the low polymerizability
of low-strain monomers and the moderate depolymerizability of polymers
made from moderate-strain monomers. This FGT approach not only enhanced
the chemical recycling of moderately depolymerizable polyolefins but
also provided access to highly depolymerizable polyolefins that are
challenging to synthesize through direct ROMP of ultralow strain monomers.
Moreover, the thermal properties of the chemically recyclable polyolefins
developed in this study are highly tunable, with a broad range of
glass transition temperatures (−7 to 104 °C), highlighting
their potential for various applications.