Poly(thioester) by Organocatalytic Ring-Opening Polymerization

Abstract

Organocatalysts typically used for the ring-opening polymerization (ROP) of cyclic ester monomers are applied to a thiolactone, ε-thiocaprolactone (tCL). In the absence of an H-bond donor, a nucleophilic polymerization mechanism is proposed. Despite the decreased ability of thioesters and thiols (versus esters and alcohols) to H-bond, H-bonding organocatalysts—a thiourea in combination with an H-bond accepting base—are also effective for the ROP of tCL. The increased nucleophilicity of thiols (versus alcohols) is implicated in the increased Mw/Mn of the poly(thiocaprolactone) versus poly(caprolactone), but deleterious transesterification is suppressed in the presence of a thiourea. The thioester monomer, tCL, is shown to be thermodynamically similar to ε-caprolactam but kinetically similar to ε-caprolactone.

Controlled Organocatalytic Ring-Opening Polymerization of ε-Thionocaprolactone

“…The comparable binding between CL and TU was measured to be K eq = 42 ± 5, 7 and a similarly dramatic perturbation from this latter strong binding value was previously measured for tCL, K eq = 2.7 ± 0.5. 9 The remarkable ability of 1 to activate tnCL and tCL toward ROP despite the weak binding exhibited by 1 toward these monomers suggests an incongruity in the approximation of “magnitude of binding” as “extent of activation”. …”

“…In contrast, the thioester, tCL, was observed to exhibit behavior that is both more and less reactive than VL. 9 …”

“…9 The ROP of this monomer was postulated to proceed through a classic transesterification mechanism mediated by H-bond activation of thioester by thiourea and thiol end group by base. 9 The other S-substituted caprolactone, ε-thionocaprolactone (tnCL, Scheme 1), has been the subject of only two published reports. 10,11 Under cationic polymerization conditions, the ROP of tnCL proceeds with quantitative inversion of substitution at the thionoester to generate the same poly(thiocaprolactone) previously reported by Overberger and our group.…”

“…10,11 Under cationic polymerization conditions, the ROP of tnCL proceeds with quantitative inversion of substitution at the thionoester to generate the same poly(thiocaprolactone) previously reported by Overberger and our group. 9,13,14 The anionic ROP of tnCL from alkyllithium reagents retains the S-carbonyl substitution, but reaction control suffers, and this method does not allow for M n control, copolymerization, or end group selection. 10 Partial inversion of substitution occurs with weak nucleophiles, resulting in a mixed polymer backbone.…”

High-performance pan-tactic polythioesters with intrinsic crystallinity and chemical recyclability

“…In addition, to afford these crystalline materials demands either stereocomplexation of the preformed enantiomeric polymers from separate pools of enantiopure monomers or the elaborate stereoselective polymerization of the racemic monomer pool. When compared to the extensively studied ROP of lactones (21,22), the ROP of thiolactones has been examined to a much lesser extent (23)(24)(25)(26)(27)(28). A notable develop-ment on that front is that the ROP of chiral N-substituted cis-4thia-l-proline thiolactones leads to polythioesters that are readily functionalizable (via the N site on the pyrrolidine ring) and show full chemical recyclability (23).…”