Chain carbanionic

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

One of the principal features of living anionic procedures is that the carbanion chain end is very stable in the absence of terminating species. This permits the... 

The alkyllithium-initiated, anionic polymerization of vinyl and diene monomers can often be performed without the incursion of spontaneous termination or chain transfer reactions (1). The non-terminating nature of these reactions has provided methods for the synthesis of polymers with predictable molecular weights and narrow molecular weight distributions (2). In addition, these polymerizations generate polymer chains with stable, carbanionic chain ends which, in principle, can be converted into a diverse array of functional end groups using the rich and varied chemistry of organolithium compounds (3). 

Under suitable conditions, anionic polymerization is faster than free-radical polymerization and so can be conducted at lower temperatures. The main reasons are fast initiation by an ionic reaction and absence of an effective termination mechanism. However, the sensitivity to impurities is much greater and choice and control of reaction conditions are more delicate. Water, oxygen, carbon dioxide, and other substances able to react with carbanion chain carriers must be strictly excluded. 

One interesting aspect of this PS-MA synthesis is the possibility of modifying the amphipolarity of these macromonomers in the end capping reaction of the polystyryl anion with ethylene oxide (EO), as shown schematically in Scheme 16. Contrary to the literature report [181] that only one EO unit is added to the carbanion chain end in benzene solution under the reaction conditions used (40 °C), we have found that a higher conversion of... 

In general, these anions are associated with a counterion, typically an alkali metal cation. The exact nature of the anion can be quite varied depending on the structure of the anion, counterion, solvent, and temperature [3-5]. The range of possible propagating species in anionic polymerization is depicted in terms of a Winstein spectrum of structures as shown in Equation 7.2 for a carbanionic chain end (R ) [3, 6]. In addition to the aggregated (associated) (I) and unaggregated (unassociated) (2) species, it is necessary to consider the intervention of free ions (5), contact... 

The rate constant for spontaneous decomposition was reported to be 40 x i0 s at 65 °C in cyclohexane [101, 103]. The rate of decomposition of PSLi in cyclohexane at 150 °C is 0.205 min corresponding to a 3.5-min half-life [104]. In the presence of 2 equivalents of n,sec-dibutylmagnesium at 100 °C, the rate of decomposition of PSLi is 1.9 X 10 min while it is 6.4 x 10 " in the absence of additive, corresponding to half-lives of 102 and 3 h, respectively [105]. Similar decomposition reactions have been observed for poly(styryl)sodium [102]. The thermal stability of poly(a-methylstyryl)lithium is much lower than that of poly(styryl)lithium. The observed half-lives for spontaneous termination are 5 h and a few minutes at 25 and 60 °C, respectively [106]. The relative thermal stability of styryl carbanionic chain ends follows the order K Na > Li for the alkali metal counterions. 

Chain Transfer Reactions Chain transfer reactions to polymeric organoalkali compounds can occur from solvents, monomers, and additives that have p/f values lower than or similar to those of the conjugate acid of the carbanionic chain end [3]. Relatively few monomers that undergo anionic polymerization exhibit chain transfer to monomer. Chain transfer has been well documented for the anionic polymerization of 1,3-cyclohexadiene. The chain transfer constant was calculated to be 2.9 x 10 at... 

Polar Vinyl Monomers The anionic polymerization of polar vinyl monomers is often complicated by side reactions of the monomer with both anionic initiators and growing carbanionic chain ends, as well as chain termination and chain transfer reactions. However, synthesis of polymers with well-defined structures can be effected under carefully controlled conditions. The anionic polymerizations of alkyl methacrylates and 2-vinylpyridine exhibit the characteristics of living polymerizations under carefully controlled reaction conditions and low polymerization temperatures to minimize or eliminate chain termination and transfer reactions [118, 119]. Proper choice of initiator for anionic polymerization of polar vinyl monomers is of critical importance to obtain polymers with predictable, well-defined structures. As an example of an initiator that is too reactive, the reaction of methyl methacrylate (MMA)... 

The kinetics of copolymerization provides a partial explanation for the copolymerization behavior of styrenes with dienes. One useful aspect of living anionic copolymerizations is that stable carbanionic chain ends can be generated and the rates of their crossover reactions with other monomers measured independently of the copolymerization reaction. Two of the four rate constants involved in copolymerization correspond at least superficially to the two homopolymerization reactions of butadiene and styrene, for example, and k, respectively. The other... 

One of the unique and important synthetic applications of living polymerizations is the synthesis of block copolymers by sequential monomer addition [225-228, 192]. The ability to prepare block copolymers is a direct consequence of the stability of the carbanionic chain ends on the laboratory time scale when all of the monomer has been consumed. Since a living polymerization and the ability to prepare well-defined block copolymers require the absence (or reduction to a negligible level) of chain termination and chain transfer reactions, monomer purity, and the absence of side reactions with the monomer are necessary... 

This marked sensitivity of the stereochemistry of anionic polymerization to the nature of the counterion and solvent can be traced to the structure of the propagating chain end. The latter involves a carbon-metal bond which can have variable characteristics, ranging all the way from highly associated species with covalent character to a variety of ionic species (Hsieh and CJuirk, 1996). The presence of a more electropositive metal and/or a cation-solvating solvents, such as ethers, can effect a variety of changes in the nature of the carbanionic chain end (a) the degree of association of the chain ends can decrease or be eliminated (b) the interaction of the cation with the anion can be decreased... 

Proton NMR spectra show that solvation shifts the structures of the carbanionic chain ends from... 

One of the most useful and Important characteristics of anionic polymerization is the generation of polymer chains with stable carbanionic chain... 

Since anionic polymerization provides for the initiation of all chains to occur at the same time, there can be no new chains started during the course of polymerization, as would be the case in free-radical polymerization. This means that the precipitated system is set and that chain growth must occur by Incorporation of monomer at the particle. If polymerization is to occur at any reasonable rate, then the active living" ends of the chains must be accessible to the monomer. The data in Table 5 demonstrates that the chain ends are readily accessible to the monomer. In this experiment, trimethylchlorosilane was used to terminate the reaction. The amount of silicon measured was compared to that expected if all the carbanion chain ends were converted to the silane derivative. Based upon the close agreement between the measured and theoretical silicon values, it appears that the carbanions in the particle are readily accessible up to 85% conversion. 

The active carbanion chain ends are contained in the polymer particle and are very accessible to the monomer which remains soluble in the pol niierization medium. 

---