Polyurethane Networks Based on Poly(ethylene Ether Carbonate) Diols

Ethylene carbonate was bulk-polymerized via a ring-opening mechanism using monoethylene glycol (MEG) as initiator and sodium stannate trihydrate as heterogeneous catalyst. Due to the loss of CO2 from a fraction of the repeat units during polymerization, a copolymer comprised of ethylene oxide and ethylene carbonate repeat units, poly (ethylene ether carbonate) (PEEC) resulted. The purified polymers were shown to be hydroxy-terminated by acetylation of the chain end and subsequent titration. Molecular weights higher than M(n)BAR approximately 4600 g mol-1 were not attainable due to a prevailing level of MEG present adventitiously in the system. PEEC-based polyurethane networks resulted from the reaction of four different molecular weight PEEC diols with a trifunctional isocyanate, triphenylmethane triisocyanate. Network characterization included equilibrium swelling, determination of sol content by solvent extraction, glass transition (T(g)) by differential scanning calorimetry, and tensile properties by stress-strain measurements. A network curing schedule of 7 days produced networks with high sol contents (10%). Extension of the curing schedule to 19 days produced networks with low sol contents (2-5%) with the minimum sol content centred about a stoichiometric ratio, OH/NCO, of approximately one. Tensile strength ranged from 1.3 to 12.3 MPa, modulus ranged from 1.5 to 26 MPa, and strain at break ranged from 60 to 240 for networks from PEECs of M(n)BAR in the range of 730-4400 g mol-1. Tensile properties, T(g)s and equilibrium swelling measurements indicated that at increased isocyanate content the crosslink density of the network increased, possibly due to side reactions such as isocyanurate or allophanate formation