Poly(alkylene Phosphates): Synthetic Strategies

Poly(alkylene phosphate)s are an interesting class of polymers because both the polymer chain and the polymer substituents can be readily varied. Several synthetic methods are available for the preparation of these polymers. The most versatile involve poly(alkylene phosphonate) s as intermediates. Two of the most promising methods for the preparation of poly(alkylene phosphonate)s are the ring opening polymerizations of cyclic phosphonates and the condensation polymerizations of dimethyl phosphite and dialcohols. The ring opening polymerizations of the cyclic phosphonates catalyzed by triisobutylaluminum are very sentitive to the nature of the alkylene group. With R = −(CH2)3−, the polymerization gives a monomer: polymer mole ratio of 1:1. The polymer can be readily separated from the monomer using fractional precipitation, and the polymer only very slowly reeuilibrates to form monomer. In contrast, with R = −(CH2)2− the polymerization occurs without catalyst to give a monomer: polymer mole ratio of 1:9. The polymer is of very low molecular weight and cannot be separated from the monomer by fractional crystallization due to rapid reequilibration. With R = −(CH2CH(CH2OMe))−, no polymerization occurs in the absence of a catalyst, and the polymerization gives a monomer: polymer mole ratio of 1:1. Attempts to purify the polymer by fractional crystallization gave poor yields of the polymer. Reaction of the −(CH2)2− or −(CH2CH(CH2OMe))− monomer-polymer mixtures with chlorine and then with either imidazole and methanol or with excess diethylamine gives poly(alkylene phosphate)s that are readily separated from the monomeric impurities by fractional precipitation. The condensation polymerizations of dimethyl phosphite with triethylene glycol and 1,12-dodecanediol have been carried out and followed by 1H, 13C and 31P NMR spectroscopy. These studies indicate that, under the reaction conditions used in these polymerizations, the polymers form at a much lower temperatures than are reported in the literature, and that the first step in the polymerization appears to be displacement of one of the methoxy groups from the phosphite by one mole of the diol. Attempts to generate higher molecular weight polymers by heating the reaction mixture to 180 – 200 °C for long periods of time caused decomposition of the polymers.