Azeotropic copolymerisation

An interesting case occurs if ri < 1 and t2 < 1. The F-f curve intersects the diagonal (Figure 2.6) and we speak of azeotropic copolymerisation. In the intersection, the copolymer composition does not change as conversion proceeds, since fi = F = (1 — r2)/(2 — n — T2). However, this is an instable equilibrium. In general, there will be no crystallisation. 

A very simple method to obtain polymeric dispersions in liquid polyethers is to make a mixture between a polyether polyol and a polymeric latex, such as the azeotropic copolymer styrene - ACN (StACN copolymer), obtained by emulsion copolymerisation, having around 20-40% solid content. The water is eliminated step-by-step by vacuum... 

To estimate compositional inhomogeneity of synthesised copolymers, integral curves of the distribution by composition estimated by the Laury-Mayer method are calculated based on values of copolymerisation constants obtained, which confirmed that the compositional inhomogeneity decreased on approaching the azeotropic composition of the initial monomeric mixture [52, 65]. Based upon the previously mentioned data it is found that MMA-4FMA copolymers are the most homogeneous as defined by both structure of the macromolecules and type of units distribution in the chains and create compositionally homogeneous polymeric materials with the best optical characteristics [3]. 

Butadiene and acrylonitrile have different copolymerisation parameters. The azeotropic composition of the monomer mixture is about 38% acrylonitrile (at 25 °C). Where the mixture of monomers contains less than 38 % acrylonitrile the copolymers have acrylonitrile contents that are larger than those which would correspond to the acrylonitrile inputs. This results in continuous alteration of the monomer composition and hence in a lack of chemical uniformity. In industrial production additional acrylonitrile is therefore incorporated in a series of stages when definite degrees of polymerisation have been reached. 

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