Elastic polyurethanes

Whilst the crystalline fibres and their thermoplastic counterparts are no longer of importance, elastic polyurethane fibres, commonly known as spandex fibres, are of significance. These will be considered further in Section 27.4.1. 

Oligomer I is used as a regulator and stabiliser in the production of elastic polyurethanes based on polyethers, rigid and semirigid polyurethanes oligomer II is used in the production of elastic polyurethanes based on esters. 

Not all copolymers which are produced by step-growth processes are random in nature. Block copolymers are also of major interest. An example of the synthesis of elastic polyurethane fibers was given in Section 1.5.4. Block and graft copolymers of polysiloxane-poly(alkylenc ethers) with segments like5-9 are used as surfactants in the production of polyurethane foams with uniform cell sizes. 

Gibson et al. reported that the copolymerization of poly(THF) and a diol-pseudorotaxane consisting of 4,4/-bipyridinium salt and bis-p-phenylene crown ether with diisocyanate afforded the corresponding polyurethane with the interlocked structure [48] (Scheme 6). Although this polyurethane has a pseudopolyrotaxane structure, the interlocked structure is stable because the interaction between 4,4/-bipyridinium salt and bis-p-phenylene crown ether is strong enough to keep the inclusion complex. In this elastic polyurethane, the rotaxane unit acted as a hard segment. 

A polyol of low functionality, having around 2-3 hydroxyl groups/mol and with a high MW of 2000-10000 daltons, leads to an elastic polyurethane and on the contrary, a low MW oligo-polyol of 300-1000 daltons, with a high functionality of around 3-8 hydroxyl groups/mol leads to a rigid crosslinked polyurethane. 

As was mentioned previously, for practical reasons, the oligo-polyols are divided in the present book into two important groups oligo-polyols for elastic polyurethanes and oligo-polyols for rigid polyurethanes. 

Oligo-polyols for elastic polyurethanes Oligo-polyols for rigid polyurethanes... 

The polyether polyols for elastic polyurethanes are low MW polymers, with terminal hydroxyl groups, characterised by the following general repeating unit (see Figure 4.1). 

Because the functionality of an oligo-polyol for elastic polyurethane is 2-3 hydroxyl groups/mol, the general formula shown in Figure 4.1 becomes the formula shown in Figure 4.2. 

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