Molecular Weight-Controlled Cationic Polymerization of Tetrahydrofuran Using a Squaramidinium Hydrogen-Bond Donor Catalyst

Poly(tetrahydrofuran) (PTHF) has long-standing industrial relevance as the soft block of elastic fibers and thermosets. Despite its commercial importance, the synthesis of PTHF with molecular weight (MW) control beyond 20 kDa has proved challenging. In this work, we disclose a MW-controlled synthesis of PTHF up to 175 kDa by cationic ring-opening polymerization, using a cationic squaramidinium hydrogen-bond donor (HBD) catalyst in combination with an α-phosphonooxymethyl ether initiator at room temperature. Mechanistic studies support a reversible-deactivation polymerization pathway, wherein the HBD catalyst facilitates the anchimeric ionization of the primary alkyl phosphate chain end, generating the propagating oxonium species. The stability of the phosphate chain end was demonstrated by isolating PTHF with high chain-end fidelity and subsequently extending the PTHF macroinitiator to higher molecular weights. This system was further applied to the copolymerization of THF and 1,3-dioxolane (DXL), affording copolymers with tunable thermal properties. We demonstrate near-quantitative chemical recycling of both PTHF and PTHF-<i>co</i>-PDXL back to monomer, highlighting the potential of this strategy for the development of sustainable polymers.