TABLE OF CONTENTS Radical polymerisation is used in the synthesis of oligo-polyols for polyurethanes (PU). One example is the synthesis of graft polyether polyols, but during radical graft polymerisation new hydroxyl groups are not created. Other examples of radical polymerisation used in the fabrication of oligo-polyols for PU is the synthesis of acrylic polyols (see Chapter 10). The hydroxyl groups of acrylic polyols are generated by using hydroxyalkyl acrylates or hydroxyalkyl methacrylates as comonomers, the hydroxyl groups being lateral substituents of the macromolecular chains and not terminal hydroxyl groups. One typical example of radical polymerisation with generation of terminal hydroxyl groups is the synthesis of hydroxy-telechelic polybutadiene. It is important that, for utilisation in elastic PU, the resulting carbocatenary chains with terminal hydroxyl groups, have a low glass transition temperature (T g) of around 50 to 70 C, such as: polybutadiene, polyisoprene, polybutylacrylate, etc.
It is well known that radical polymerisation is a chain reaction characterised by initiation, propagation and termination reactions [1]:
Initiation:
Propagation:
Termination
By recombination:
By disproportionation:
It is very clear that if the initiator has hydroxyl groups, and if the termination takes place exclusively by recombination then a polymeric diol is obtained [2, 3], which is ideal for polyurethane. If the termination takes place by disproportionation, only monofunctional compounds are obtained, which cannot be used in PU. The vinylic and dienic monomers used in practice have various termination mechanisms. Some monomers give only recombination reactions, such as...
