Copolymerization with depropagation

Propagation reactions in radical polymerization and copolymerization arc generally highly exothermic and can be assumed to be irreversible. Exceptions to this general rule arc those involving monomers with low ceiling temperatures (Section 4.5.1). The thermodynamics of copolymerization has been reviewed by Sawada.85 

Some of the most important systems known to involve reversible propagation steps are  

Copolymerizations of other monomers may also be subject to similar effects given sufficiently high reaction temperatures (at or near their ceiling temperatures - Section 4.5.1). The depropagation of methacrylate esters becomes measurable at temperatures 100 °C (Section 4.5.1).96 O Driseoll and Gasparro86 have reported on the copolymerization of MMA with S at 250 °C. 

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Copolymerization. See also Copolymers of acrylonitrile, 11 202-204 anionic, 7 624-626 catalytic, 7 627-632 cationic, 7 626-627 chloroprene-sulfur, 19 833-834 of cyclic olefins, 16 112-113 with depropagation, 7 617-619 free-radical, 7 611-624 heterogeneous, 11 203-204 homogenous, 11 202-203 with metallocene catalysts, 16 111... 

The process based on cationic polymerization of 1,3,5-trioxane employs a different principle for stabilization of polymer. Trioxane is copolymerized with a few percent of 1,3-dioxolane (or ethylene oxide). The sequence of —OCH2— units is then separated from time to time by —OCH2CH2— units. The product of copolymerization is subsequently heated to eliminate the terminal units (unstable fraction). Depropagation proceeds until the stable —CH2CH2OH group is reached ... 

Copolymerization with a properly chosen comonomer, however, may be a way to introduce into the polymer chain monomers that are otherwise unable to polymerize, or to enhance the conversion of monomers showing limited polymerizability. The previous section describes one of the possibilities related to retardation of depropagation. An analogous system is the radical copolymerization of S02 with olefins. 

Olefins with electron withdrawing substituents (—CN, —COOH, —COOR) cannot enter into copolymerization with sulfur dioxide. The reactivities of the olefins with sulfur dioxide is obtained by using cyclohexene as a standard at — 20°C, far below the ceiling temperature where depropagation is negligible [18]. 

With most common monomers, the rate of the reverse reaction (depropagation) is negligible at typical polymerization temperatures. However, monomers with alkyl groups in the a-position have lower ceiling temperatures than monosubstituted monomers (Table 4.10). For MMA at temperatures <100 °C, the value of is <0.01 (Figure 4.4). AMS has a ceiling temperature of <30 °C and is not readily polymerizable by radical methods. This monomer can, however, be copolymerized successfully (Section 7.3.1.4). 

In contrast to the kinetic approach, deviations from the terminal model have also been treated from a thermodynamic viewpoint [Kruger et al., 1987 Lowry, 1960 Palmer et al., 2000, 2001]. Altered copolymer compositions in certain copolymerizations are accounted for in this treatment in terms of the tendency of one of the monomers (M2) to depropagate. An essential difference between the kinetic and thermodynamic treatments is that the latter implies that the copolymer composition can vary with the concentrations of the monomers. If the concentration of monomer M2 falls below its equilibrium value [M]c at the particular reaction temperature, terminal M2 units will be prone to depropagate. The result would be a... 

Fig. 6-14 Effect of depropagation on copolymer comosition in the anionic copolymerization of vinylmesitylene (MJ-a-methylstyrene (M2) at 0°C for/2 constant at 0.91. The dashed-line plots are the calculated curves for Lowry s cases I and II (with r = 0.20 and r2 = 0.72) the experimental data follow the solid-line curve. After Ivin and Spensley [1967] (by permission of Marcel Dekker, New York).

The ability to determine which copolymerization model best describes the behavior of a particular comonomer pair depends on the quality of the experimental data. There are many reports in the literature where different workers conclude that a different model describes the same comonomer pair. This occurs when the accuracy and precision of the composition data are insufficient to easily discriminate between the different models or composition data are not obtained over a wide range of experimental conditions (feed composition, monomer concentration, temperature). There are comonomer pairs where the behavior is not sufficiently extreme in terms of depropagation or complex participation or penultimate effect such that even with the best composition data it may not be possible to conclude that only one model fits the composition data [Hill et al., 1985 Moad et al., 1989]. 

Some monomers with no tendency toward homopolymerization are found to have some (not high) activity in copolymerization. This behavior is found in cationic copolymerizations of tetrahydropyran, 1,3-dioxane, and 1,4-dioxane with 3,3-bis(chloromethyl)oxetane [Dreyfuss and Dreyfuss, 1969]. These monomers are formally similar in their unusual copolymerization behavior to the radical copolymerization behavior of sterically hindered monomers such as maleic anhydride, stilbene, and diethyl fumarate (Sec. 6-3b-3), but not for the same reason. The copolymerizability of these otherwise unreactive monomers is probably a consequence of the unstable nature of their propagating centers. Consider the copolymerization in which M2 is the cyclic monomer with no tendency to homopolymerize. In homopolymerization, the propagation-depropagation equilibrium for M2 is completely toward... 

Ivin and Spensley (10) tested the Lowry Case II model and equations for the anionic copolymerization of vinyl mesitylene (Mi) with a-methyl-styrene at 0°C. by varying the total concentration of the two monomers while keeping their mole ratio constant. As pointed out above, theory predicts a dependence on absolute monomer concentration when depropagation occurs. Table I summarizes some of Ivin and Spensley s data. 

Studies by many authors, e. q. on copolymerizations of styrene with a-methylstyrene (characterized by low Tc, 334 K), appear to agree with the ideas of Lowry. Some author claim, however, that even copolymerization of this pair can be described by the simple copolymerization equation [221], Johnston and Rudin are of the opionion that the depropagation reaction is not as important in this case because only short sequences of a-methylstyrene are produced. The formation of short blocks is accompanied by relatively high polymerization enthalpy. They are thermodynamically more stable than the homopolymer and have a higher Tc. Only at considerably higher copolymerization temperatures (with the pair styrene—a-methylstyrene > 420 K) does the depropagation effect become important. 

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