Chain polymerization copolymer composition drift

In most copolymeiization, the monomers are consumed at different rates dictated by the steric and electronic properties of the reactants. Consequendy, both the monomer feed and copolymer composition will drift with conversion. Thus conventional copolymers are generally not homogeneous in composition at the molecular level. In RAFT polymerization processes, where all chains grow throughout the polymerization, the compositional drift is captured within the chain stmcture (Scheme 32). All chains will have similar composition and the copolymers formed have a gradient or tapered structure - poly(monomer A-grad-monomer B). 

In a batch reactor, the relative monomer concentrations will change with time because the two monomers react at different rates. For polymerizations with a short chain life, the change in monomer concentration results in a copolymer composition distribution where polymer molecules formed early in the batch will have a different composition from molecules formed late in the batch. For living polymers, the drift in monomer composition causes a corresponding change down the growing chain. This phenomenon can be used advantageously to produce tapered block copolymers. 

Block copolymer synthesis from living polymerization is typically carried out in batch or semi-batch processes. In the simplest case, one monomer is added, and polymerization is carried out to complete conversion, then the process is repeated with a second monomer. In batch copolymerizations, simultaneous polymerization of two or more monomers is often complicated by the different reactivities of the two monomers. This preferential monomer consumption can create a composition drift during chain growth and therefore a tapered copolymer composition. 

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. 

Simultaneous polymerization of two monomers by chain initiation usually results in a copolymer whose composition is different from that of the feed. This shows that different monomers have different tendencies to undergo copolymerization. These tendencies often have little or no resemblance to their behavior in homopolymerization. For example, vinyl acetate polymerizes about twenty times as fast as styrene in a free-radical reaction, but the product obtained by free-radical polymerization of a mixture of vinyl acetate and styrene is found to be almost pure polystyrene with hardly any content of vinyl acetate. By contrast, maleic anhydride, which has very little or no tendency to undergo homopolymerization with radical initiation, readily copolymerizes with styrene forming one-to-one copolymers. The composition of a copolymeir thus cannot be predicted simply from a knowledge of the polymerization rates of the different monomers individually. The simple copolymer model described below accounts for the copolymerization behavior of monomer pairs. It enables one to calculate the distribution of sequences of each monomer in the macromolecule and the drift of copolymer composition with the extent of conversion of monomers to polymer. 

Tip 13 (related to Tip 12) Copolymerization, copolymer composition, composition drift, azeotropy, semibatch reactor, and copolymer composition control. Most batch copolymerizations exhibit considerable drift in monomer composition because of different reactivities (reactivity ratios) of the two monomers (same ideas apply to ter-polymerizations and multicomponent cases). This leads to copolymers with broad chemical composition distribution. The magnirnde of the composition drift can be appreciated by the vertical distance between two items on the plot of the instantaneous copolymer composition (ICC) or Mayo-Lewis (model) equation item 1, the ICC curve (ICC or mole fraction of Mj incorporated in the copolymer chains, F, vs mole fraction of unreacted Mi,/j) and item 2, the 45° line in the plot of versus/j. 

Copolymer composition can be predicted for copolymerizations with two or more components, such as those employing acrylonitrile plus a neutral monomer and an ionic dye receptor. These equations are derived by assuming that the component reactions involve only the terminal monomer unit of the chain radical. This leads to a collection of N x N component reactions and x 1) binary reactivity ratios, where N is the number of components used. The equation for copolymer composition for a specific monomer composition was derived by Mayo and Lewis [74], using the set of binary reactions, rate constants, and reactivity ratios described in Equation 12.13 through Equation 12.18. The drift in monomer composition, for bicomponent systems was described by Skeist [75] and Meyer and coworkers [76,77]. The theory of multicomponent polymerization kinetics has been treated by Ham [78] and Valvassori and Sartori [79]. Comprehensive reviews of copolymerization kinetics have been published by Alfrey et al. [80] and Ham [81,82], while the more specific subject of acrylonitrile copolymerization has been reviewed by Peebles [83]. The general subject of the reactivity of polymer radicals has been treated in depth by Jenkins and Ledwith [84]. 

Nearly everything discussed so far is based on conventional radical copolymerization. This means that initiation of new chains, and termination of growing chains, takes place continuously throughout the duration of the polymerization. The effect of composition drift as described above is a broad CCD, that is, chains of varying composition coexist in a sample after a nonazeotropic batch polymerization. In the present section, the focus will be on copolymers that are made via controlled/LRP. 

Composition Drift and Sequence Length Distributions Automatic continuous online monitoring of polymerization continuous measurements of comonomer concentrations can be used to compute the average instantaneous molar fraction of a comonomer A in the copolymer chain formed at any given moment F based on ... 

If r = r, the structure is strictly alternating since each monomer wants only to add the other. If r r =l, the structure is random, since P and Q chains have an equal probability of adding either monomer. If and are both greater than unity, a block copolymer results, since cross polymerization is unlikely. Equation 16.151 describes the instantaneous copolymer composition. If the monomers are not consumed at the same rate, there will be significant compositional drift over the course of the polymerization. 

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. 

It is known that the gradient (tapered) nature of copolymers, which can be synthesized in free-radical polymerization processes, is due to a drift in the free monomer composition during solution polymerization [17]. Such copolymers can be considered as a special type of block copolymers in which the composition of one component varies along the chain. With a decreasing difference in the monomer reactivity rations, the formation of gradient sta-... 

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