Average copolymer composition

Figure 7.8 Mole fractions styrene (Mj) and methyl methacrylate (M2) in feedstock (f) and copolymers (F) as a function of the extent of polymerization. Average copolymer compositions are also shown. [From V. E. Meyer and R. K. S. Chan, Polym. Prepr. 8 209(1967), used with permission.]...

Very similar variations in average copolymer composition with conversion have recently been observed in the styrene methyl methacrylate system by both Johnson et al ( and by Dionisio and O Driscoll (. The reason for the variation may be due to a viscosity effect on propagation rate constants QO). 

When calculating the average copolymer composition and the probabilities P Uk] of the sequences of monomeric units it is possible to set Ta=0 in the expressions in (7), that is to neglect the finiteness of the size of the macromolecules. In this case the absorbing Markov chain (7) is replaced by the ergodic Markov chain with transition matrix Q whose elements ... 

In these formulas the letter X stands for the average copolymer composition, while of denotes the dispersion of the SCD quantitatively characterizing its width. The second of these statistical characteristics is extremely significant for the thermodynamics of the melt of a heteropolymer specimen, being in a simple way AHmix = RT jof connected with the specific enthalpy of mixing Affmix per mole of monomeric units. Here T is the absolute temperature, R represents the gas constant, whereas / denotes the Flory /-parameter whose values are available from the literature for many pairs of monomeric units (see, for example, [7]). 

Average copolymer compositions of SAN samples were determined by elemental analysis, yielding weight percent acrylonitrile in the polymer. Compositions of S/MA and S/MA/MM were determined by sequential hydrolysis and pyridine titration to obtain maleic anhydride content and by infrared analysis for methyl methacrylate content. 

The average copolymer composition is determined from H-NMR spectra of the copolymer according to H-NMR spectrum (200 MHz) ofpoly(NAM-co-NAS) in CDC (21). 

Average copolymer composition and molecular weight characteristics of poly(NAM-co-NAS)... 

Table IV - Instantaneous and average copolymer composition versus conversion (run Mj)...

Several works [311-313, 200] are devoted to a circumstantial research of systems where terpolymerization of styrene and acrylonitrile with a third monomer (brominated acrylate or methacrylate) was studied. It was shown that the terminal model can be employed to describe such systems. One can see this from Table 6.10 and Figs. 20 and 21, where the conversion drifts of the average copolymer composition are presented. 

Step-growth polymerizations at high temperatures produce nearly random copolymers because of end-group interchange reactions like (5-14) between macromolecules. Interfacial and low-temperature solution polycondensations are conducted under essentially irreversible conditions, by contrast. In these cases the average copolymer composition and blocklike character of the product may depend on the reaction conditions and relative reactivity of the functional groups involved in the polymerization. 

Figure 3. Average copolymer composition with conversion in a batch reactor. Key -------------,MA -------, ST.

Problem 7.5 A monomer pair with rj = 0.2 and T2 = 5.0 is copolymerized beginning with a molar monomer ratio [Mi]/[M2] = 60/40. Assuming that the copolymer composition within a 10 mol% conversion interval is constant, calculate instantaneous monomer and copolymer compositions and cumulative average copolymer compositions at 10 mol% conversion intervals up to 100% total conversion. Show the results graphically as change in composition of the copolymer and the monomer mixture during copolymerization. 

With r = 0.52 and T2 = 0.46, calculate from Eq. (7.24) the changes in instantaneous monomer and copolymer compositions as a function of conversion and compare the results graphically with the above experimental data. Also calculate the cumulative average copolymer composition at different conversions. 

The essential procedure for calculating the composition drift with conversion is that fi is decreased or increased in suitable increments from (/i), to 0 or to 1.0. For each value of f, the corresponding degree of conversion is obtained from Eq. (7.24) and the corresponding instantaneous copolymer composition from Eq. (7.18). With the monomer mixture composition, fi, and the degree of conversion p = 1 — N/No thus known, it is then easy to also calculate cumulative average copolymer composition Fi from Eq. (P7.6.2). For the given monomer system and feed composition, Eq. (7.18) shows that < /i, i.e., the... 

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