Reactivity ratios, for anionic copolymerization

Table 8.3 Some Typical Values of Monomer Reactivity Ratios for Anionic Copolymerization...

A further distinguishing feature of ionic copolymerizations is the strong dependence of reactivity ratios upon the nature of the solvent and the counter-ion (and hence the initiator). These effects can be dramatic and can lead to a reversing of the order of relative monomer reactivities (cf. reactivity ratios for anionic copolymerization of styrene with butadiene and isoprene in different solvents). 

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). 

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... 

Tapered Block Copolymers. The alkyllithium-initiated copolymerizations of styrene with dienes, especially isoprene and butadiene, have been extensively investigated and illustrate the important aspects of anionic copolymerization. As shown in Table 15, monomer reactivity ratios for dienes copolymerizing with styrene in hydrocarbon solution range from approximately 8 to 17, while the corresponding monomer reactivity ratios for styrene vary from 0.04 to 0.25. Thus, butadiene and isoprene are preferentially incorporated into the copolymer initially. This type of copolymer composition is described as either a tapered block copolymer or a graded block copolymer. The monomer sequence distribution can be described by the structures below ... 

Methylthiophene/styrene copolymers Methyl methacrylate does not homopolymerize or copolymerize if present in the monomer feed during the oxidation of 3-methylthiophene. This is the reason that its copolymer with 3-MT is prepared indirectly as described above. Its homopolymerization is generally initiated by anions or free radicals. Styrene, however, undergoes a random copolymerization when present during the chemical oxidation of 3-methylthiophene initiated with anhydrous FeCls [73]. Monomer reactivity ratios for the copolymerizations in methylene chloride and nitrobenzene at 5°C are reported, but there is considerable scatter in the Fineman-Ross plots. The proposed structure of the 3-MT/stryrene copolymer is shown in Figure 11.16, where R = H. 

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]. 

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. 

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). 

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... 

For example, monomer reactivity ratios for styime and methyl methacrylate in a free-radical copolymerization are r, = 0.5, rj = 0.44. This represents a statistical copolymerization. Contrast this with the anionic reaction, where r = 0.12 and 2 = 6.4, or the cationic reaction where r = 10.5 and Z2 = 0.1. Obviously, the propagation rates are no longer similar, and this is represented in Figure 5.3, where it can be seen that the anionic technique produces a copolymer rich in methyl methacrylate, whereas the cationic system leads to a copolymer with a high styrene content. 

The sequence distribution of two copolymerizing monomers depends on the catalyst or initiator used, the method of pol5merization, and the concentration and reactivities of the monomers. Reactivity ratios for many monomer pairs have been measured for free-radical, anionic, and coordination polymerization of butadiene (128). 

Because of the complicating effects of counterion and solvent associated with anionic polymerization, relatively few reactivity ratios have been determined for anionic systems. Typical reactivity ratios for the anionic copolymerization of styrene and a few other monomers are shown in Table 8.3. Most of the values were determined from the copolymer composition equation [Eq. (7.11) or (7.18)]. A dramatic effect of solvent is seen with styrene-butadiene copolymerization, where a change from the nonpolar hexane to the highly solvating THF reverses the order of reactivity. Again in the case of hydrocarbon solvent, the reaction temperature shows a minimal in uence on reactivity ratios, while in the case of polar solvents, such as THF, the reactivity ratios vary considerably, which has been rationalized by considering the solvation of carbon-lithium bond. Thus as the temperature is increased (from -78°C to 25°C), the extent of solvation by THF is expected to decrease, resulting in more covalent carbon-lithium bond. 

Haddleton, D. M., et al. (1997). Identifying the nature of the active species in the polymerization of methacrylates inhibition of methyl methacrylate homopolymerizations and reactivity ratios for copolymerization of methyl methacrylate/n-butyl methacrylate in classical anionic, alkyUithium/trialkylaluminum-initiated, group transfer polymerization, atom transfer radical polymerization, catalytic chain transfer, and classical free radical polymerization. Macromolecules, 30(14) 3992-3998. 

Studies of the copolymerizations of 1,1-diphenylethylene and dienes showed rather different behavior compared with the copolymerizations of styrene and 1,1-diphenylethylene [125, 133-136]. The monomer reactivity ratios for copolymerizations of dienes with DPE are shown in Table 7. When butadiene was copolymerized with 1,1-diphenylethylene in benzene at 40 °C with -butyl-lithium as initiator, the monomer reactivity ratio for butadiene, ri, was 54 this means that the addition of butadiene to the butadienyl anion is 54 times faster than addition of 1,1-diphenylethylene to the butadienyl anion [133]. This unreactivity of poly(butadienyl)lithium towards addition to DPE was also observed in studies of end-capping of poly(butadienyl)lithium with DPE in hydrocarbon solution (see Sect.3.3) [109, 111]. Because of this unfavorable monomer reactivity ratio, few DPE units would be incorporated into the co-... 

The anionic copolymerization of styrene and l-(4-dimethylaminophenyl)-1-phenylethylene in benzene has been investigated [188]. As discussed previously in Sect. 5, Yuki and coworkers [125, 126, 129, 133-136] have developed the formalism for analyzing the kinetics of copolymerization of 1,1-diphenylethylene (M2) with styrene and diene monomers (Mi). It was assumed that the 1,1-diphenylethylene derivative, M2, does not add to itself due to steric effects, i.e., k22=0, as discussed previously in Sect 5. Thus, the monomer reactivity ratio for M2 is zero, i.e., r2- 22l ii- - It was also assumed that the styrene monomer is completely consumed at the end of the polymerization... 

It was anticipated that the copolymerization of substituted 1,1-dipheny-lethylenes with dienes such as butadiene and isoprene would be complicated by the very unfavorable monomer reactivity ratio for the addition of poly(-dienyl)lithium compounds to 1,1-diphenylethylene [133, 134]. Yuki and Oka-moto [133, 134] calculated values of ri=54 and ri=29 in hydrocarbon solutions for the copolymerization of 1,1-diphenylethylene (M2) with butadiene (Mi) and isoprene (Mi), respectively. Although the corresponding values in THE are ri(butadiene)=0.13 and ri(isoprene)=0.12, this would not be an acceptable solution since THE is known to form polymers with high 1,2-microstructures [3]. Anionic copolymerizations of butadiene (Mi) with excess l-(4-dimethyla-mino-phenyl)-l-phenylethylene (M2) were conducted in benzene at room temperature for 24-48 h using scc-butyllithium as initiator [189]. Anisole, triethy-lamine and ferf-butyl methyl ether were added in ratios of [B]/[RLi]=60, 20, 30, respectively, to promote copolymerization and minimize 1,2-enchainment in the polybutadiene units. Narrow molecular weight distribution copolymers with Mn=14xl0 to 32x10 (Mw/Mn=1.02-1.03) and 8, 12, and 30 amine... 

TABLE 2.15 Some typical values of reactivity ratios for cationic and anionic copolymerization of styrene (monomer A)... 

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. 

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