Synthesis of dihydroxy telechelic oligomers of β-butyrolactone catalyzed by titanium(IV)-alkoxides and their use as macrodiols in polyurethane chemistry

We report on a solvent-free approach for the synthesis of low molecular weight, α,ω-dihydroxy telechelic poly(β-butyrolactone). In the presence of Ti(IV) alkoxides, mixtures of β-butyrolactone and diols, like di- or triethylene glycol, were reacted in ratios between 4:1 and 10:1. The oligomerization proceeds at elevated temperatures (80–100°C). Different alkoxide substituents (R = Me, iPr, tBu) of the Ti(IV)(OR)₄ catalyst were investigated. The resulting oligomers were characterized by nuclear magnetic resonance (NMR), infra-red (IR), gel-permeation chromatography (GPC), titration, and matrix-assisted laser desorption-time-of-flight mass spectrometry (MALDI-ToF-MS) analysis. Aside from low molecular weight products, special effort was devoted to achieve high O-acyl cleavage selectivity and to circumvent the formation of unsaturated end-groups in order to form exclusively dihydroxy-telechelic oligomers. Optimized results in terms of selectivity and reaction rates were achieved at 100°C using catalyst loadings of 0.2 mol% with respect to the monomer. The molecular weights determined by GPC were in good accordance with the ratio of monomer to diol used, confirming successful oligomer formation. Polyurethanes prepared from crude macrodiols without any additional catalyst feature molecular weights up to 50,000 g/mol. The reported work serves as concept to utilize β-lactones for tailored polyol synthesis; the resulting products are suitable for polyurethane chemistry.