FORMATION OF POLY(ETHYLENE ETHER CARBONATE) DIOLS - PROPOSED MECHANISM AND KINETIC-ANALYSIS

The ring-opening polymerization of ethylene carbonate (EC), using organotin-based catalysts such as dibutyltin diacetate, dibutyltin dilaurate, and dibutyltin dimethoxide as well as the inorganic catalyst sodium stannate trihydrate, was investigated. The soluble organotin catalysts at 2 mol % concentration produced an ultimate conversion which leveled to about 80% after 72 h. The use of dibutyltin diacetate and dibutyltin dilaurate resulted in the formation of an insoluble byproduct which was removed by column chromatography. The heterogenous sodium stannate trihydrate catalyst yielded higher polymerization rates and ultimate conversions >90%. Since alcohols act as initiators in EC polymerization, molecular weights higher than M(n)BAR = 4600 were not attained due to the adventitious presence of ethylene glycol (MEG) in the monomer, at a prevailing concentration, [MEG]in situ. Ethylene carbonate polymerization using glycols as initiators and sodium stannate trihydrate as catalyst was proposed to proceed by a mixed mechanism, consisting of ring-opening steps in which carbonyl and alkylene attack are competitive and transcarbonation steps which are responsible for the steady-state concentrations of oligo(ethylene glycol) condensates. The latter, principally ethylene and bis(ethylene glycol), due to their high boiling points remain in equilibrium with the polymer product throughout the polymerization. A kinetic interpretation of the mechanism was developed and analyzed. The polymerization was found to be first order in [EC], and a pseudo rate constant of polymerization was extracted from the kinetic analysis and found to be 1.8 X 10(-4) M-1 s-1 at 165-degrees-C. The prevailing level of adventitious ethylene glycol in the monomer was found to be 0.12 M.