Anionic polymerization reactivity ratios

Another important consequence of the limitations concerning cross-addition is that anionic polymerization is not suited for the synthesis of random copolymers. If a mixture of two anionically polymerizable monomers is reacted with an initiator, the most electrophilic monomer will polymerize while the other is left almost untouched 30). In other words, a general feature of anionic binary copolymerization is that one of the reactivity ratios is extremely high while the other is close to zero. 

As the polymerization occurs, the reactive ketene silyl acetal group is transferred to the head of each new monomer as it is added to the growing chain (Equation 5.44). Similar to anionic polymerization, the molecular weight is controlled by the ratio of the concentration of... 

For any specific type of initiation (i.e., radical, cationic, or anionic) the monomer reactivity ratios and therefore the copolymer composition equation are independent of many reaction parameters. Since termination and initiation rate constants are not involved, the copolymer composition is independent of differences in the rates of initiation and termination or of the absence or presence of inhibitors or chain-transfer agents. Under a wide range of conditions the copolymer composition is independent of the degree of polymerization. The only limitation on this generalization is that the copolymer be a high polymer. Further, the particular initiation system used in a radical copolymerization has no effect on copolymer composition. The same copolymer composition is obtained irrespective of whether initiation occurs by the thermal homolysis of initiators such as AIBN or peroxides, redox, photolysis, or radiolysis. Solvent effects on copolymer composition are found in some radical copolymerizations (Sec. 6-3a). Ionic copolymerizations usually show significant effects of solvent as well as counterion on copolymer composition (Sec. 6-4). 

The radical reaction mechanism was confirmed by polymerizing a mixture of styrene and methyl methacrylate. The ratio of the monomers in the copolymer (1.15) was nearly equal to the value (1.05) calculated from the reactivity ratio for radical copolymerization and differed considerably from the value of 10.5 for the cationic copolymerization and from the value 0.15 for anionic copolymerization (78). 

Besides the field influence on the monomer reactivity ratio mentioned in the previous sections, living anionic systems present strong evidence against the electroinitiated polymerization mechanism. First of all, the experimental fact, that the apparent rate constant of propagation was increased by the presence of an electric field, rules out a possibility that the observed field-accelerating effect resulted only from the initiation reaction enhanced by the field. The finding that the field had no influence on the dependences of the polymerization rate on monomer and initiater concentrations, but did influence the rate constant, implies that the reaction mechanism was unaltered by the application of the field. These results confirm our very low opinion of the electroinitiated polymerization mechanism. 

Reactivity ratios that differ from those of anionic and radical polymerizations... 

Reactivity Ratios that Differ from those of Anionic and Radical Polymerizations... 

Haddleton determined the reactivity ratios for copolymerization of MMA with BMA by classical anionic as 1.04 0.81 by alkyllithium/trialkylalu-minum initiation, 1.10 0.72 by GTP, 1.76 0.67 by ATRP, 0.98 1.26 by catalytic chain transfer, 0.75 0.98 by classical free radical, 0.93 1.22 [39]. The difference in reactivity ratios between GTP and classical anionic polymerization seems to indicate GTP is an associative process. However, Jenkins has also measured reactivity ratios for the same pair by GTP and reports different results rMMA=0.44 and rBMA=0.26 [40]. 

Narrow distribution in the backbone length as well as in the chemical composition or the branch frequency may be expected from a living-type copolymerization between a macromonomer and a comonomer provided the reactivity ratios are close to unity. This appears to have been accomplished to some extent with anionic copolymerizations with MMA of methacrylate-ended PMMA, 29, and poly(dimethylsiloxane) macromonomers, 30, which were prepared by living GTP and anionic polymerization, respectively [50,51]. Recent application [8] of nitroxide (TEMPO)-mediated living free radical process to copolymerizations of styrene with some macromonomers such as PE-acrylate, la, PEO-methacr-ylate, 27b, polylactide-methacrylate, 28, and poly(e-caprolactone)-methacrylate, 31, may be a promising approach to this end. 

Anionic polymerizations, when carried out in aprotic solvents, are characterized by the long lifetime of the carbanionic (or oxanionic) sites l2). When neither spontaneous transfer nor termination reactions are involved, the polymers obtained exhibit sharp molecular weight distributions, and their number average degree of polymerization is determined by the [Monomer]/[Initiator] molar ratio, provided initiation is fast as compared to propagation. However, the major advantage of these methods, as far as synthesis is concerned, is the socalled living character of the polymers 12) After completion of the polymerization the active sites retain their reactivity and can be used for functionalizations at the chain end. 

Inone, Tsuruta and J. Furukawa (29) have investigated the unusual catalyst system prepared from calcium and diethyl zinc. They claimed that a reaction occurred according to the following equation Ca + 2 ZnEtg -> CaZnEt4 + Zn. Such a catalyst system is heterogeneous in benzene or in bulk, and produces a polystyrene containing 13% of a crystallizable fraction. The catalyst also polymerizes methyl methacrylate, and the anionic nature of these processes is indicated by the reactivity ratios for styrene (Mj) and methyl methacrylate (Mg) copolymerization, rx = 0.31, r2 = 17.1. 

Owing to the stability of the DPE radical, S/DPE polymers cannot be prepared using free radical polymerization, but they can be easily produced using the anionic polymerization technique. The copolymers are, however, limited to a maximum DPE content of 50 mol% because two consecutive DPE units are not possible in the polymer chain for steric reasons. This leads to a reactivity ratio rDPEC dd/ ds) = 0. 

Muller and coworkers studied the anionic polymerization of MMA initiated by RLi in the presence of steadily increasing amounts of LiCl . At LiCl/RLi molar ratios lower than 1, the propagation rate increases, which was accounted for by the formation of a 1 1 complex, more reactive than the uncomplexed species. The situation changes drastically at LiCl/RLi molar ratios higher than 2. Indeed, the propagation rate then decreases with increasing content of LiCl. The same trend was observed for the polymerization of tBMA . According to Muller and coworkers, formation of a less active 2 1 complex would be responsible for the detrimental kinetic effect of an excess of LiCl. 

GTP of methacrylates. Tacticity of PMMA synthesized by either anionic polymerization or mucleophile-mediated GTP is very similar The same conclusion holds for the reactivity ratios in random copolymerization of methacrylates and acrylates for... 

Here [Pf ] is the concentration of growing centres ending in monomer x and kx y is the absolute rate coefficient of reaction of P with monomer y. Two difficulties arise in anionic polymerization. In hydrocarbon solvents with lithium and sodium based initiators, [Pf ] is not the total concentration of polymer units ending in unit x but, due to self-association phenomena, only that part in an active form. The reactivity ratios determined are, however, unaffected by the association phenomena. As each ratio refers to a common active centre, the effective concentration of active species is reduced equally to both monomers. In polar solvents such as tetrahydrofuran, this difficulty does not arise, but there will be two types of each reactive centre Pf, one an anion and the other an ion-pair. Application of eqn. (22) will give apparent rate coefficients as discussed in Section 4 if total concentrations of Pf are used. Reactivities can change with concentration if defined on this basis. 

Recent investigations [259] have indicated that the polymerization is not conventional free radical in character but is likely to be coordinated anionic. In support of this view are the reactivity ratio coefficients in copolymerization of vinyl chloride with vinyl acetate and methyl methacrylate, which are different from those found with free radical initiators. 

Measurement of reactivity ratios under normal free-radical and CCT polymerization conditions indicates that CCT is a modified free-radical polymerization as expected.434 The reactivity ratios for MMA and butyl methacrylate were used as a mechanistic probe. Reactivity ratios were 1.04 and 0.81 for classical anionic polymerization, 1.10 and 0.72 for alkyllithium/trialkylaluminum initiated polymerization, 1.76 and 0.67 for group transfer polymerization, 0.98 and 1.26 for atom transfer radical polymerization, 0.75 and 0.98 for CCT, and 0.93 and 1.22 for classical free-radical polymerization. These ratios suggest that ATRP and CCT proceed via radical propagation. 

The anionic polymerization of B-propiolactone in CH2CI2 led to the dependence of the ratio of reactivities of macro ions and macroion pairs on the solvating power of the medium (53). For higher proportions in the mixture of the more powerfully ion solvating component (B-propiolactone) the ratio of kp/ki decreased (at 350C kp/ki= =210 and 150 for the systems with 1bPL o=1 and 3 mol l respecively). Moreover, k in this system practically does not depend on the composition of the mixture and the decrease of the ratio is due to the decrease of kp (33)). We found also that the ratio of kp/k decreases also when the temperature is lowered, e.g. in a system with bPL o= =3 mol l" from 150 at 35°C to 5.6 at -20OC. ... 

The reactivity ratios rj and T2 can be determined from the composition of the copolymer product. However, a serious complication exists because the propagation rate constants, k j, are composite rate constant, being composed of free-ion contributions and ion-pair contributions, and hence the reactivity ratios also will be composite quantities, having contributions from both ion pairs and free ions. Because the relative abundances of free ions and ion pairs are strongly dependent on the reaction conditions, the reactivity ratios will also depend on these conditions and they can be applied only to systems identical to those for which they were determined. Therefore the utility of such ratios is much more limited in anionic than in free-radical polymerization. 

Compilations of reactivity ratios for various pairs of monomers in radical polymerization have been provided by Eastmond [131] and Odian [132], The reactivity ratios for pairs of given monomers can be very different for the different types of chain-growth copolymerization radical, anionic, cationic, and coordination copolymerization. Although the copolymer equation is valid for each of them, the copolymer composition can depend strongly on the mode of initiation (see Figure 11.8). 

Solvent type plays a very important role in the reactivity ratios of anionic copolymerization pairs. Hydrocarbon solvents, such as C4-C10 alkanes and cycloalkanes, are commonly used. n-Hexane and cyclohexane are employed in many commercial processes. Except for some SBRs with very specific microstructures made at very low temperatures (T < —20°C), the so-called cold rubbers, most anionic polymerization processes occur at relatively high temperatures (T > 30-100°C), isothermally or semiadiabatically. Number average molecular weights for the blocks vary widely but may be most commonly maintained between 30,000 and 100,000 Da. Once all monomer has been consumed via propagation reactions, a short stopper reactant, typically alcohol or water, is added to the mix to kill the living character of the anion and... 

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