Comonomer feed composition

Figure 6-1 shows the variation in the copolymer composition as a function of the comonomer feed composition for different values of r Mayo and Walling, 1950]. The term ideal... 

Fig. 6-1 Dependence of the instantaneous copolymer composition F on the initial comonomer feed composition/i for the indicated values of ri, where rir2 — 1. After Walling [1957] (by permission of Wiley, New York) from plot in Mayo and Walling [1950] (by permission of American Chemical Society, Washington, DC).

The copolymer has the alternating structure I irrespective of the comonomer feed composition. Moderate alternating behavior occurs when either (1) both r and r2 are small (>] ri — very small, close to 0) or (2) one r value is small and the other r is zero (>] ri — 0). The copolymer composition tends toward alternation but is not the perfectly... 

One can show the drift of copolymer composition with conversion for various comonomer feed compositions by a three-dimensional plot such as that in Fig. 6-4 for the radical copolymerization of styrene (M, )-2-vinylthiophene (M2). This is an ideal copolymerization with r — 0.35 and r-L — 3.10. The greater reactivity of the 2-vinylthiophene results in its being incorporated preferentially into the first-formed copolymer. As the reaction proceeds, the feed and therefore the copolymer become progressivley enriched in styrene. This is shown by Fig. 6-5, which describes the distribution of copolymer compositions at 100% conversion for several different initial feeds. 

Even with the Kelen Tudos refinement there are statistical limitations inherent in the linearization method. The independent variable in any form of the linear equation is not really independent, while the dependent variable does not have a constant variance [O Driscoll and Reilly, 1987]. The most statistically sound method of analyzing composition data is the nonlinear method, which involves plotting the instantaneous copolymer composition versus comonomer feed composition for various feeds and then determining which theoretical plot best fits the data by trial-and-error selection of r and values. The pros and cons of the two methods have been discussed in detail, along with approaches for the best choice of feed compositions to maximize the accuracy of the r and r% values [Bataille and Bourassa, 1989 Habibi et al., 2003 Hautus et al., 1984 Kelen and Tudos, 1990 Leicht and Fuhrmann, 1983 Monett et al., 2002 Tudos and Kelen, 1981]. 

Deviations are also observed in some copolymerizations where the copolymer formed is poorly soluble in the reaction medium [Pichot and Pham, 1979 Pichot et al., 1979 Suggate, 1978, 1979]. Under these conditions, altered copolymer compositions are observed if one of the monomers is preferentially adsorbed by the copolymer. Thus for methyl methacrylate (M1 )-/V-vinylcarbazole (M2) copolymerization, r — 1.80, r2 = 0.06 in benzene but r — 0.57, > 2 0.75 in methanol [Ledwith et al., 1979]. The propagating copolymer chains are completely soluble in benzene but are microheterogeneous in methanol. /V-vinylcarba-zole (NVC) is preferentially adsorbed by the copolymer compared to methyl methacrylate. The comonomer composition in the domain of the propagating radical sites (trapped in the precipitating copolymer) is richer in NVC than the comonomer feed composition in the bulk solution. NVC enters the copolymer to a greater extent than expected on the basis of feed composition. Similar results occur in template copolymerization (Sec. 3-10d-2), where two monomers undergo copolymerization in the presence of a polymer. Thus, acrylic acid-2-hydroxyethylmethacrylate copolymerization in the presence of poly(V-vinylpyrrolidone) results in increased incorporation of acrylic acid [Rainaldi et al., 2000]. 

Thus, the terminal model for copolymerization gives us expressions for copolymer composition (Eqs. 6-12 and 6-15), propagation rate constant (Eq. 6-71), and polymerization rate (Eq. 6-70). The terminal model is tested by noting how well the various equations describe the experimental variation of F, kp, and Rp with comonomer feed composition. 

Figures 6-12 and 6-13 shows plots of copolymer composition and propagation rate constant, respectively, versus comonomer feed composition for styrene-diethyl fumarate copolymerization at 40°C with AIBN [Ma et al., 2001]. The system follows well the implicit penultimate model. The copolymer composition data follow the terminal model within experimental error, which is less than 2% in this system. The propagation rate constant shows a penultimate effect, and the results conform well to the implicit penultimate model with si = 0.055, S2 — 0.32.

Given the preceding mechanistic discussion, one would not expect the reactivity ratios to represent true kinetic parameters. Indeed, the reactivity ratios are sensitive to monomer and simple electrolyte concentrations, comonomer feed compositions, temperature and whether the reaction was carried out in aqueous solutions or a heterophase system. Clearly the data from the various groups are partly inconsistent, but still some general conclusions regard DADMAC/AAM copolymerization, listed in Table 6, are reasonable. 

The extent of reaction at which gelation tx curs in the copolymerization of a vinyl monomer and a divinyl monomer can be increased by increasing the concentration of free-radical initiator, at fixed polymerization temperature and comonomer feed composition. Explain why this should be so. 

Sawamoto et aL used the RuCl2(PPh3)3/Al(OzPr)3 catalyst to prepare St/MMA copolymers [126]. They found that the polymerization proceeded well using 1-phenylethyl bromide as the initiator and that the composition of the copolymer matched the comonomer feed composition, or behaved azeotropically [126]. The polymers were well-defined, with predictable molecular weights and relatively low polydispersities (Mw/Mn<1.5). The reactivity ratios were similar to those determined from conventional free radical processes. Later work used a NiBr2(n-Bu3P)2 catalyst system for the ATRP of a 50/50 mixture of MMA/MA and MMA/nBA [127]. The results indicated that the copolymerization was controlled with copolymer Mn=ll,800 (Mw/Mn=1.47) and 12,500 (Mw/Mn=1.47),respectively. 

Figure 6.6 Dependence of instantaneous copolymer composition F[ on initial comonomer feed composition /j in an ideal copolymer. The reactivity ratios satisfy = 1. (5ee insert for the color representation of the figure.)...

Figure 6.8 Dependence of instantaneons copolymer composition Fj on initial comonomer feed composition/j for different values...

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