There is an increasing demand for greater sustainability in commercial plastics, and while mechanical recycling has advanced, inherent limitations remain whereby material properties diminish with repeated recycling. Thus, depolymerization, or chemical recycling back to monomer to prepare virgin plastic materials with superior properties, presents itself as an alternate route to achieving a circular economy. Herein we report on the ring-opening transesterification polymerization (ROTEP) and ring-closing depolymerization (RCD) of a new alkyl-branched caprolactone monomer, 4-propylcaprolactone (4-PrCL), and the corresponding polymer, poly(4-PrCL) (P4PrCL). Initial kinetic studies were performed using stannous octoate and zinc chloride as baseline catalysts for ROTEP and RCD, respectively. Successful ROTEP to prepare P4PrCL of varying molar masses was confirmed using 1H NMR spectroscopy (Mn = 10–130 kg/mol), SEC (D = 1.3–1.6), and DSC (Tg = –65 °C). Successful RCD (yield &gt; 85% in less than 1 hour) via reactive distillation to recover pure 4-PrCL monomer was confirmed via 1H NMR spectroscopy and GC-MS; the recovered 4-PrCL was also successfully re-polymerized. A series of organo-catalysts were screened for the ROTEP of 4-PrCL to access lower reaction temperatures (reducing energy input) and to enable purification of P4PrCL via devolatilization (eliminating the need for organic solvents). Copolymerization of 4-PrCL with ε-caprolactone (ε-CL) to prepare amorphous polyester diols with tunable entanglement molar mass (Me) was also carried out. These P4PrCL-based diols have the potential to be utilized as soft segments in thermoplastic polyurethanes (TPUs) toward the development of commercially competitive materials capable of recycling via depolymerization.

Preparation and depolymerization of poly(4-propylcaprolactone)

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