1.1 Introduction

The polyurethanes are a special group of heterochain polymers, characterised by the following structural unit [1-33]:

The urethane groups -NH-COO- are esters of carbamic acid, an hypothetically unstable (and impossible to obtain under normal conditions) acid [R-NH-COOH]. It is possible to synthesise the urethane groups by various methods [22], but the most important one is the reaction between an isocyanate and an alcohol [1-33]:

The first urethane was synthesised, by this route, as early as 1849 by Wurtz [6, 16, 22, 30]. In 1937, following very systematic and intensive research works at IG Farbenindustrie, in Germany, Dr. Otto Bayer synthesised the first polyurethane, by the reaction of a diisocyanate with a polyester having two terminal hydroxyl groups (called polyester diol, in fact an ?, ? -telechelic polymer with terminal hydroxyl groups) [1, 2]:

In fact, Bayer invented a new method for the synthesis of macromolecular compounds: the polyaddition reaction, which is a special case of polycondensation, with the difference that the reaction product is exclusively the polymer. In the classical polycondensation reactions, the products are: the polycondensation polymer and a low molecular weight (MW) compound (water, alcohols, and so on). The fact that in the polyaddition reactions the product is only the polymer is of great technological importance, especially for the purity and the morphology of the resulting macromolecular compound.

In the slightly more than 65 years of the existence of polyurethanes, the growth of the polyurethanes was constant and the prediction for...