Two-step polymerization technique

In contrast in the two-step polymerization technique (Fig. 1.6), in the first step of the reaction a prepolymer is produced through the reaction of a SS oligomer with an excess of diisocyanate, followed by chain extension with a short diol(urethane or ester) or a diamine (urea, urethaneurea, amide, or ester-amide) to form the HS and also to increase the overall molecular weight of the polymer. 

This book introduces a modified two-step polymerization technique. Partial urethane chains are employed as the soft segment phase. Such modified two-step polymerization can improve the molecule fiexibility of the SMPU. After the modified two-step polymerization, short polyols and rigid 4,4 -diphenylmethane diisocyanate (MDI) form the soft segment phase. To further improve the phase separation, rigid chain extenders can be used. 

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