Copolymerization, anionic ratios

Anionic polymerization of pivalolactone with the polystyrene anion produced only homopolymer mixtures, but the polystyrene carboxylate anion was able to give a block copolymer336. The block efficiency depends on catalyst ratio and conversion because the initiation step is slow compared with propagation337. Tough and elastic films were obtained by graft copolymerization or block copolymerization of pivalolactone onto elastomers containing tetrabutylammonium carboxylate groups338,339. ... 

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). 

Annaka and Tanaka recently found the presence of several phases in polymer gels between the fully swollen and shrunken phases [46]. This interesting phenomenon was observed in a polyampholyte gel, consisting of acrylic acid (anionic constituent, AAc) and methacryl-amido-propyl-trimethyl-ammonium-chloride (cationic, M APT AC). The chemical structures of these constituents are shown in Fig. 32. A series of copolymer gels were prepared by radical copolymerization, where the total molar concentration of the constituents was kept constant at 700 mM and the molar ratio of AAc/MAPTAC was varied systematically. 

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. 

The addition of small amounts of a polar solvent can markedly alter the copolymerization behavior of, for example, the diene-styrene pair. The solvation of the active centers manifests itself in two ways the incorporation of styrene is enhanced and the modes of diene addition other than 1,4 are increased 264,273). Even a relatively weak Lewis base such as diphenyl ether will bring about these dual changes in anionic copolymerizations, as the work of Aggarwal and co-workers has shown 260>. Alterations in polyisoprene microstructure and the extent of styrene incorporation were found for ether concentrations as low as 6 vol. % (diphenyl ether has been shown52) to cause partial dissociation of the poly(styryl)lithium dimers. The findings of Aggarwal and co-workers 260) are a clear demonstration that even at relatively low concentrations diphenyl ether does interact with these anionic centers and further serve to invalidate the repetitive claim 78,158-i60,i6i) tjjat diphenyl ether — at an ether/active center ratio of 150 — does not interact with carbon-lithium active centers. 

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. 

Although the criterion for an anionic initiator given above is very useful, it is now known that other complicating factors can arise. O Driscoll and Tobolsky (79,80) have investigated the copolymerization of styrene and methyl methacrylate using lithium in bulk and in various ratios of tetrahydrofuran and heptane. In all of these systems the authors found considerably more than the less than 1% styrene reported for sodium and potassium. In fact as much as 33% styrene is... 

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. 

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. 

As a result of anionic polymerization of co-hydrolysis products at equimolar ratio of diorga-osiloxane and organosilsesquioxane units, 3D-polymers were synthesized. Polymerization of bicyc-lodimethylsiloxanes with various lengths of dimethylsiloxane chain between two cyclotetrasiloxane rings has given spatially cross-linked polymers [10] copolymerization of octamethylcyclotetra-siloxane with polyphenylsilsesquioxane leads to formation of soluble low-molecular polymers [11],... 

Most data were obtained from copolymerization studies. The copolymerization parameter r (see Chap. 5, Sect. 5.2) is the rate constant ratio for the addition of two different monomers to the same active centre. The inverse values of r j determined for the copolymerization of a series of monomers with the monomer M, define the relative reactivities of these monomers with the active centre from the first monomer, M°,. Thus it is possible to order monomers according to their reactivities in radical, anionic, cationic and coordination polymerizations from the tabulated values of copolymerization parameters [101-103]. 

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