Copolymerization composition drift

Vinyhdene chloride copolymerizes randomly with methyl acrylate and nearly so with other acrylates. Very severe composition drift occurs, however, in copolymerizations with vinyl chloride or methacrylates. Several methods have been developed to produce homogeneous copolymers regardless of the reactivity ratio (43). These methods are appHcable mainly to emulsion and suspension processes where adequate stirring can be maintained. Copolymerization rates of VDC with small amounts of a second monomer are normally lower than its rate of homopolymerization. The kinetics of the copolymerization of VDC and VC have been studied (45—48). 

The batch-suspension process does not compensate for composition drift, whereas constant-composition processes have been designed for emulsion or suspension reactions. It is more difficult to design controUed-composition processes by suspension methods. In one approach (155), the less reactive component is removed continuously from the reaction to keep the unreacted monomer composition constant. This method has been used effectively in VT)C-VC copolymerization, where the slower reacting component is a volatile and can be released during the reaction to maintain constant pressure. In many other cases, no practical way is known for removing the slower reacting component. 

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. 

The alternating tendency of the copolymers is advantageous in that the polymerizations can be carried out to high conversions with little or no compositional drift. For random copolymerizations in which there is preferential incorporation of one monomer due to a mismatch in reactivity ratios, the compositional variations with conversion can be substantial. Such compositional heterogeneities in resist materials can lead to severe problems during image development. 

In the early 1930s, Nobel laureate Staundinger analyzed the product obtained from the copolymerization of equimolar quantities of VC and VAc. He found that the first product produced was high in VC, but as the composition of the reactant mixture changed because of a preferential depletion of VC, the product was becoming higher in VAc. This phenomenon is called the composition drift. 

Except in very special cases (azeotropic copolymerizations), copolymerization via radical mechanism shows a drift in the composition of the copolymers produced through the polymerization process. Emulsion copolymerization obeys this rule too, although the special features of its mechanism can change the drift process. The most common way to obviate that composition drift is to use the semi-continuous process where, after polymerization has been initiated with a small percent of the total charge (say 10 to 20 %) like in the batch process, most of the charge is added continuously at a much smaller rate (Ra) than the rate (Rp) at the end of the batch period, so that the added charge is polymerized quite instantaneously (J, 2). Then,the composition drift is limited to the initial period and most of the product does possess actually a constant composition. 

One way to achieve this result relies on the change in the relative monomer reactivity following composition drifts. Thus, in a combination ofhigh and low reactivity monomers, the former will preferentially react first, leaving a considerable proportion of the latter for copolymerization when the supply of the high reactive monomer is depleted. This has been confirmed in the terpolymerization of methyl methacrylate/butyl acrylate/vinyl acetate in the presence of the maleate Surfmer reported in Figure 6.49. 

Under the copolymerization of more than two monomers Eqs. (5.3) cannot be integrated explicitly, and in order to determine the system trajectories one should need the numerical calculations. Examples of such calculations of the conversional change of composition and structure characteristics of the terpolymers have been reported in Refs. [195-200]. One should pay special attention to Ref. [200] where the programs for the computer realization of such calculations are presented. Under the copolymerization of four or more monomers, the composition drift with the conversion was calculated [7,8] only within the framework of the simplified terminal model described above in Sect. 4.6. 

Unfortunately, as far as the author knows, there are only a few publications where the problem of the validity of this or that model over a wide range of conversions and initial monomer feed compositions was discussed carefully enough. Here one might mention the works listed as Refs. [310,201] on the bulk copolymerization of styrene and heptyl acrylate, where the adequacy of the terminal model was undoubtedly proved, and its parameters rj = 0.87 and r2 = 0.27 were estimated. Really, the calculated copolymer composition and monomer feed composition drift with conversion are in full agreement with both NMR (Table 6.9) and UV (Fig. 18) data. 

Now, if you recall the copolymer equation relating the composition of the copolymer formed at any instant of time (FA, FB), to the monomer feed composition (fA, fB) in a batch copolymerization (Equation 6-5), it should be clear that unless you have rA = rB 1, so that Fa - fA, then one of the monomers is going to be used up faster than the other (unless rA < 1, rB < 1 and you start with a monomer composition corresponding to the azeotrope condition). That means copolymer composition varies with conversion—we say there is 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. 

Dispersancy Solution copolymers are comparatively easy to produce in dispersant form as copolymerization with an appropriate polar monomer is relatively straightforward. If the polar monomer is also a methacrylate, reactivity ratios are essentially the same and no special procedures are required to produce random copolymers. Commercial examples have included dimethyl (or diethyl)aminoethyl methacrylate [11], hydroxyethyl methacrylate [12] and dimethylamino-ethyl methacrylamide [13]. 2-Methyl-5-vinyl pyridine [14] has also been used commercially, reactivity ratios are such that it copolymerizes slightly faster than alkyl methacrylates. Although composition drift is not severe, it should be added in a programmed fashion if a uniform distribution is desired. V-vinyl pyrrolidinone, in contrast, copolymerizes very sluggishly with methacrylates and is best incorporated via a graft reaction [15], sometimes also grafted in combination with V-vinyl imidazole [16]. Since solution chemistry is used to produce dispersant polymethacrylates, like preparation of the base polymer, only relatively simple process modifications are necessary to produce dispersants commercially. 

The reactivity ratios for the free-radical copolymerization of styrene (rj = 0.4) and acrylonitrile (r2 = 0.04) result in uneven incorporation of each monomer into the copolymer as seen in Figure 3. Thus, most SAN and ABS polymers are made at the crossover point (A in Figure 3) to avoid composition drift. 

During batch copolymerization, composition may drift with conversion because of differences in comonomer reactivity and can result in less valuable... 

Figure 13. Experimental monomer composition (o) for an AAM-DMAEM inverse microsuspension copolymerization at 50 C. The reaction conditions are the same as in Figure 12. The dashed line is the predicted compositional drift based on the reactivity ratios measured in solution polymerization. The solid lines are the 95% confidence limits.

Some copolymerization systems are not strictly alternating, but still they show a tendency toward alternation. This occurs when both and r2 < 1. The alternating trend increases as the reactivity ratios approach zero. An interesting feature of these systems is that they present the so-called azeotropic composition, at which Fj = /j. At this composition, the copolymer formed has the same composition as the monomers in the feed and, therefore, systems copolymerizing at this condition do not show compositional drift. It can be shown that a necessary condition that the reactivity ratios have to satisfy in order for a copolymerization system to show an azeotropic point is that either both and r2 < 1 or both and T2 > 1. 

More often than not, reactivity differs from monomer to monomer. This is evident when the reactivity ratios differ from a value of one. Thus, if one is operating at concentrations other than the azeotropic composition, batch copolymerization will result in a changing copolymer composition throughout the reaction. For example, a copolymerization with rj > 1 and r2 < 1 would result in the instantaneous copolymer composition decreasing in monomer 1 as monomer conversion increases. The degree of compositional drift that leads to a heterogeneous copolymer composition depends on the ratio of reactivity ratios where heterogeneity increases with the... 

Two basic monomer feed policies employed in a semibatch copolymerization can be used to minimize composition drift [163]. Many highly effective commercial processes are based on one or a combination of these policies. Additional promising derivations of these policies have also been presented [167-173]. Henceforth, we refer to the two basic feed policies as Policy I and Policy II, as described in following sections. 

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. 

Hint 1. Plot Fj versus /i in a batch copolymerization for different combinations of r and r2 and observe the composition drift. Is the direction of composition drift always the same Are azeotropic points stable or unstable to small perturbations in monomer concentration ... 

Our discussion thus far has indicated that during copolymerization, the composition of both the feed and the polymer vary with conversion. To follow this composition drift, it is necessary to int ate the copolymer equation — a problem that is complex. Consider a system that is composed initially of M total moles of the two monomers (M = M, + Mj) and in which the resulting copolymer is richer in M, than the feed (Fi > fj). When dM moles have been polymerized, the polymer will contain F, dM moles of Ml while the feed content of M, wUl be reduced to (M - dM) (f, - dfi) moles. Writing a material balance for Mji... 

This phenomenon, known as composition drift, is a feature of many copolymerizations and has been attributed to the greater reactivity of one of the monomers in the mixture. Consequently, in a copolymetization, it is necessary to distinguish between the composition of a copolymer being formed at any one time in the reaction and the overall composition of the polymer formed at a given degree of conversion. 

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