Non-isocyanate routes to urethane-polymers
The conventional synthetic route to urethane polymers involves the use of hazardous dflsocyanates which are synthesised using phosgene, the use of which produces a significant chlorine-waste stream. The processing of high molecular mass specialty polyurethanes regularly requires the use of organic solvents. This research aims to address these issues by separately employing enzyme-catalysis in order to synthesise high molecular mass polyurethanes and by investigating the use of high pressure CO2 as an alternative to the use of organic solvents in the synthesis of urethane polymers. The main building blocks in the enzyme-catalysed synthesis of urethane polymers are based on di-functional carbamate-blocks, synthesised by reaction of diamines with ethylenecarbonate or dimethylcarbonate. In addition to the more conventional di-functional carbamate-blocks, the synthesis of a novel mono-carbamate based on an amino acid has been investigated. The mechanism of the polymerization of adipic acid and hexane diol was investigated. The reaction was optimised for the high vacuum reaction in bulk and solution, as well as for the polymerisation under Dean-Stark distillation conditions. Although the dimethylcarbonate-based carbamate-compounds proved to be too resistant against enzymatic cleavage, the enzyme-catalysed co-polymerisation of ethylene-based carbamate-blocks with adipic acid and hexane diol eventually led to the synthesis of high molecular mass segmented urethane polymers in excess of 30000 g moi1. The crystallisation behaviour of both enzyme-catalysed polyesters and polyurethanes has been investigated and the differences in crystallisation for the polyesters have been shown to be caused by the presence of small quantities of residual enzyme. The enzyme appears to act as a nucleating agent for the crystallisation and increases the uniformity of crystallisation and the rate at which a crystalline structure was obtained upon cooling. The thermal properties of the enzyme-catalysed polyurethanes have been determined and compared to conventional polymers. Although the molecular mass of the enzyme-catalysed urethane polymers are comparable to those obtained via the conventional route, it has not been possible to reproduce their physical p rope rties. The use of high pressure CO2 to eliminate the need of a solvent in the synthesis of a urethane polymer was studied. The use of a pressure of 4.5 bar 002 has been shown to enhance the reaction rate for this reaction procedure. GPC analysis confirmed that polyurethanes with a molecular mass of approximately 53000 g moi1 were synthesised in 200 minutes, whereas the same reaction under atmospheric conditions required a longer reaction time (260 minutes) before the same molecular mass polyurethane was obtained.
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