Ring-opening polymerization of β-thiobutyrolactone catalyzed by phosphazenes

Abstract

Thiolactones that give access to polythioesters (PTEs) upon ring-opening polymerization (ROP) have recently attracted increasing attention especially due to their biocompatibility and improved degradability among other properties. We report herein on the ROP of the simplest four-membered ring thiolactone, namely racemic β-thiobutyrolactone (rac-TBL), mediated by commercially available phosphazene bases, affording the corresponding poly(3-thiobutyrolactone) (P3TB). The reactions proceeded at room temperature, in toluene or in THF, with the apparent reactivity increasing monotonously with the basicity and steric hindrance of the phosphazene (BEMP ∼ tBu-P₁ < tBu-P₂ < tBu-P₄). This trend suggests that initiation of the ROP proceeds, at least to some extent, by abstraction of one acidic methylene hydrogen from the monomer, to generate eventually a phosphazenium–thiocarboxylate ion-pair species that next acts as the initiating/chain-propagating species. ROP performed in THF was kinetically controlled affording P3TBs with defined molar mass and fair dispersity, some features of a living ROP. Detailed ¹H, ¹³C, ³¹P and DOSY NMR spectroscopic, MALDI-ToF/ESI mass spectrometric and TGA studies conducted on the polymers produced using tBu-P₄ supported the concomitant formation of {[tBu-P₄]H}⁺ and essentially α-thiocrotonate,ω-[thiocarboxylate]⁻ end-capped linear P3TBs; yet, the presence of cyclic P3TBs could not be excluded. The molar mass values, as determined by NMR analysis from this thiocrotonate end-group (M_n,NMR), increased linearly with the monomer conversion, yet with a nonzero intercept, suggesting slow initiation vs. propagation. The molar mass of the recovered P3TBs was moderately controlled (M_n,SEC = 7.5–18.5 kg mol⁻¹, M_n,NMR = 5.2–22.4 kg mol⁻¹), displaying relatively narrow dispersities (Đ_M = 1.39–1.52 in THF). This work provides the second example of chemical synthesis of P3TB.