Reactivity Ratios, anionic

Chain-Growth Gopolymerization Theory. The theory of chain-growth (eg, radical, anionic, etc) copolymerisation has received more attention than that of step-growth or other copolymerisations. In the case of chain-growth copolymerisation, growing polymer chains must choose between more than one monomer. Such a choice or relative reactivity has been quantitatively treated by the reactivity ratio (6,7) and the Q-e schemes (8). 

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. 

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). 

Monomer reactivity ratios and copolymer compositions in many anionic copolymerizations are altered by changes in the solvent or counterion. Table 6-12 shows data for styrene-isoprene copolymerization at 25°C by n-butyl lithium [Kelley and Tobolsky, 1959]. As in the case of cationic copolymerization, the effects of solvent and counterion cannot be considered independently of each other. For the tightly bound lithium counterion, there are large effects due to the solvent. In poor solvents the copolymer is rich in the less reactive (based on relative rates of homopolymerization) isoprene because isoprene is preferentially complexed by lithium ion. (The complexing of 1,3-dienes with lithium ion is discussed further in Sec. 8-6b). In good solvents preferential solvation by monomer is much less important and the inherent greater reactivity of styrene exerts itself. The quantitative effect of solvent on copolymer composition is less for the more loosely bound sodium counterion. 

For copolymerizations proceeding by the activated monomer mechanism (e.g., cyclic ethers, lactams, /V-carboxy-a-amino acid anhydrides), the actual monomers are the activated monomers. The concentrations of the two activated monomers (e.g., the lactam anions in anionic lactam copolymerization) may be different from the comonomer feed. Calculations of monomer reactivity ratios using the feed composition will then be incorrect. 

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. 

Molecules having a nucleophilic heteroatom add readily to arynes. In fact, some of these exhibit reactivity comparable to anionic nucleophiles, e.g. the reactivity ratio between phenyllithium and triethyl-... 

Contrary to what has been observed for radical systems, lithium based anionic copolymerizations usually exhibit pronounced sensitivity to solvent type. Thus, the polarity and solvating power of the solvent will influence the copolymer reactivity ratios while mixtures of e.g. ethers and hydrocarbons will lead to effects intermediate with regard to what is observed for the pure solvents. 

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]. 

An alternative rationale for the unusual RLi (hydrocarbon) copolymerization of butadiene and styrene has been presented by O Driscoll and Kuntz (71). Rather than invoking selective solvation, these workers stated that classical copolymerization kinetics is sufficient to explain this copolymerization. They adapted the copolymer-composition equation, originally derived from steady-state assumptions for free-radical copolymerizations, to the anionic copolymerization of butadiene and styrene. Equation (20) describes the relationship between the instantaneous copolymer composition c/[M,]/rf[M2] with the concentrations of the two monomers in the feed, M, and M2, and the reactivity ratios, rt, r2, of the monomers. The rx and r2 values are measures of the preference of the growing chain ends for like or unlike monomers. 

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. 

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. 

Using Al(i-C4H9)3/TiCl4 catalyst for copolymerization of styrene with substituted styrenes, the reactivity ratios showed that cationic copolymerization occurred at Al/Ti< 1 and that stereospecific coordinated anionic copolymerization took place at Al/Ti > 2.5. 

The reactivity ratios observed are markedly different in polar and nonpolar solvents. These differences appear to be determined mainly by the nature of the solvation at the active chain end. Most of the change occurs at quite low concentrations of polar solvent in a primarily hydrocarbon medium hence, the bulk dielectric constant of the solution is not an important factor under conditions where most of the reaction is carried by ion pairs. In solvents such as tetrahydrofuran it might be possible to detect changes in reactivity ratios at different concentrations of active polymer chains as the proportion of free anions increases with dilution. No experiments have been reported yet to check this point. 

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. 

The reactivity ratios for pairs of given monomers can be very different for the different types of chain-growth copolymerization free-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 10.8). 

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