Poly(3-hydroxyoxetane)—an analog of poly(vinyl alcohol): Synthesis,characterization, and properties

The spontaneous polymer formed from 3-hydroxyoxetane (HO), as first reported by Wojtowicz and Polak, is linear, low molecular weight, water-soluble, atactic, poly(3-hydroxyoxetane) (PHO) of high crystallinity with ? OCH₂CH(OH)CH₂OH end units. The highly crystalline nature of this atactic polymer may be related to the crystalline nature of atactic poly(vinyl alcohol) since PHO can be considered a copolymer of vinyl alcohol and formaldehyde. Spontaneous PHO apparently is formed in a cationic polymerization by the carboxylic acids produced by the air oxidation of HO on standing at room temperature for several months. The polymerization can be duplicated by the addition of 2% hydroxyacetic acid to HO. The rate of this unusual cationic polymerization increases greatly with acid strength, e.g., trifluoromethanesulfonic acid reacts explosively with pure HO. A mechanism is proposed for this cationic polymerization. High molecular weight, water-soluble, linear atactic, and highly crystalline PHO (mp = 155°C) was made by polymerizing the trimethylsilyl ether of HO with the i-Bu₃Al–0.7 H₂O cationic catalyst followed by hydrolysis. Two ¹H-NMR methods for measuring the tacticity of PHO were developed based on finding two different types of methylene units at 400 MHz with the methine protons decoupled. Also, an ¹H-NMR method was developed for measuring branching in HO polymers. High molecular weight, linear PHO with enhanced isotacticity (80%) has been obtained in low yield as a water-insoluble fraction with T_m = 223°C. The low molecular weight PHO prepared previously by the base-catalyzed, rearrangement polymerization of glycidol is highly branched.