End-Group Analysis of Poly(ε-Caprolactone) Initiated with Water, Ethylene Glycol, and 1,4-Butanediol

Low molecular weight poly(ε-caprolactone) (PC), produced using stannous octoate as catalyst and initiated with water, ethylene glycol, or 1,4-butanediol, was subjected to end-group analysis using gated decoupling (decgate) 13C-NMR spectrometry. Within the main-chain ε-caprolactone repeat unit, the carbon resonances were assigned as follows: carbonyl, 173.9; α, 34.1; β, 25.5; γ, 24.6; δ, 28.3; ε, 64.3 ppm. The spectra of water-initiated polymers were consistent with a linear PC chain carrying one carboxylic acid end group and one hydroxyl end group. The presence of the terminal hydroxyl group yielded different chemical shifts for the ε-, δ-, γ-, α-, and carbonyl, but not the β-carbon of the ultimate repeat unit. The terminal acid group yielded different chemical shifts for the adjacent carbonyl, α-, β-, and γ-Carbons. Spectra of diol-initiated polymers were consistent with a linear PC chain containing the initiator residue and carrying two hydroxyl end groups. The terminal hydroxyl groups caused the same chemical shift patterns within the ultimate repeat units as were observed in the water-initiated PCs.