Single chain compositional drift

One potential problem with conventional free-radical copolymerization is that the reactivity ratios of the two monomers tend to be different from one another [6]. On one hand this leads to non-random sequences of the monomers on a single chain (usually the product of the reactivity ratios is less than one so that there is a tendency to form alternating sequences) and, on the other, to substantial composition drift if the polymerization is carried out in bulk to high conversions. Random copolymers with a range of compositions as a result of composition drift may however be useful in practice, allowing a compositionally graded interface to be formed. 

This is shown in Figure 16.1. Consider, for example, an initial monomer composition of /,=/j=0.5. For r =0.1, F, 0.1. Since the polymer is very rich in monomer 2, the monomer composition will move in the direction of higher/,. This will continue until the end of the polymerization, when the polymer will be almost completely made up of monomer 1. Thus, the first chains polymerized will have a very low composition of monomer 1 (/j=0.1) while the last chains polymerized will be homopolymer of monomer 1 (( = 1.0). For living polymerization, the same argument can be made for compositional drift within a single chain. Thus, for free radical polymerization, compositional drift will take the form of a wide distribution of copolymer composition among the chains. The compositional drift can be described by the integrated copolymer equation for batch polymerization [ 12]. 

It is clear that copolymerization is a much more complex process than polymerization using a single monomer. For example, in addition copolymerization using two monomers the tendency of each type of monomer to add to the growing chain may be different. This can lead to a variation of copolymer composition during the reaction even when equimolar amounts of the two types of monomer are used initially. This phenomenon is known as composition drift and is a common feature in copolymerization reactions. 

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