ABSTRACT
The complex polyaddition process of diisocyanates and polyols, as presented above by means of the seemingly simple reaction (2), can be shown in more details as a system of sequential-parallel irreversible reactions which yield longer and longer chains of urethane oligomers and polymers. Assuming that the studied process is based solely on the nucleophilic addition of the –OH group to –NCO, one can suggest a relatively simple kinetic scheme for the polymerisation process, based on a theoretically infi nite system (44) of sequential-parallel reactions [23]:
In that scheme, A stands for the difunctional hydroxyl monomer, B stands for diisocyanate, while AnBm stands for the oligourethane structure, which can be diisocyanate-urethane AnBn+1, urethane diol An+1Bn or oligourethane AnBn with the terminal group –NCO on one chain end and with the –OH group on the other. For the needs of kinetic analysis, it is also necessary to differentiate the oligomer structures AnBm from the viewpoint of reactivity of their functional cap groups. One can assume, for the needs of simplifi cation, that all the –OH groups in the monomer (D) and in the urethane-hydroxyl oligomers (d) are primary, secondary or tertiary groups, and hence their reactivity specifi cations are the same. In case, however, of 2,4-TDI and urethane-isocyanate oligomers derived from that monomers, it is necessary to differentiate –NCO groups from the viewpoint of their substitution at the benzene
ring. It will be convenient to designate those groups in the monomer with block letters “E” and “F”, and the analogous groups in oligomers with small letters “e” and “f ”, respectively. The group E at the position 4 within 2,4-TDI is more reactive.
