Several Considerations on the Polyaddition Reaction

As mentioned previously, the synthesis of polyurethanes, by the reaction of a diisocyanate (or polyisocyanate) with oligo-diols (or oligo-polyols), is a polyaddition reaction (or step-addition polymerisation), a particular type of polycondensation reaction. There is a great difference between the polycondensation and the polyaddition reactions and the classical radical polymerisation or ionic (living) polymerisation reactions. In radical polymerisations (typical chain reactions), the high MW polymer is formed at the beginning of polymerisation. The reaction system is constituted from monomer and high 

MW polymer. The radical polymerisations are characterised by strong transfer reactions, simultaneous with the polymerisation reaction. 

Living ionic polymerisations are characterised by a linear increase of the MW in the resulting polymer, with the conversion. In the reaction system there are the monomer and the polymer. In living ionic polymerisations, the termination reactions are absent. 

In our particular type of step-addition polymerisation, monomers, dimers, trimers, oligomers and polymers are the reactive species which participate in the chain growth. Initially, the monomers react with monomers and give dimers, dimers react with monomers and dimers and give trimers and tetramers, respectively. The high MW polymer is formed only in the last stages of the poly addition reaction, at high conversion rates. Chain transfer and termination reactions are absent. 

Szycher, Szy chefs Handbook of Polyurethanes, CRC Press, Boca Raton, FL, USA, 1999. 

---