Chain termination anionic polymerization

Like other chain-addition processes, anionic chain polymerization is complicated by competing chain transfer and termination steps. However, there are conditions where termina-... 

SBR is prepared by anionic polymerization initiated by lithium alkyls in cycloaliphatic media as a solvent. The main feature of this class of anionic chain polymerization is that it is a living polymerization, i.e., the polymeric chain ends are able to survive even when monomer is completely depleted and to reinitiate the polymerization reaction when monomer is newly added. Due to the absence of termination reactions, pol5mier active chain ends do not inherently terminate, continuously growing up to tiie complete depletion of monomers this in turn means that the average pol5uner molecular weight can be predicted from the amount of starting material and tiie quantity of the initiator. 

In ionic polymerizations termination by combination does not occur, since all of the polymer ions have the same charge. In addition, there are solvents such as dioxane and tetrahydrofuran in which chain transfer reactions are unimportant for anionic polymers. Therefore it is possible for these reactions to continue without transfer or termination until all monomer has reacted. Evidence for this comes from the fact that the polymerization can be reactivated if a second batch of monomer is added after the initial reaction has gone to completion. In this case the molecular weight of the polymer increases, since no new growth centers are initiated. Because of this absence of termination, such polymers are called living polymers. 

In anionic polymerization, as in carbonium ion polymerization, termination does not involve bimolecular reaction between two growing chains. Neither can recombination of ions lead to termination, since a carbon-metal bond is highly polar, in the case of alkali metals frequently completely ionized, and in every case very reactive. The termination step leading to the formation of a terminal C=C double bond is not too probable. This reaction involves the formation of a metal hydride, and this does not contribute greatly to the driving force. Consequently, such a termination is observed at higher temperatures only and it is probably more common in coordination polymerization where the metals involved are less electropositive. 

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

The addition of the anion takes place at the unsubstituted carbon atom, which, in this case, carries a partial positive charge. Since the growing chain end is a genuine anion, chain termination can occur by addition of a reactive cation. As in cationic polymerization, combination of two growing ends is not possible. Chain transfer with electrophiles can also occur. 

On the basis of the nature of the initiation step, polymerization reactions of unsaturated hydrocarbons can be classified as cationic, anionic, and free-radical polymerization. Ziegler-Natta or coordination polymerization, though, which may be considered as an anionic polymerization, usually is treated separately. The further steps of the polymerization process (propagation, chain transfer, termination) similarly are characteristic of each type of polymerization. Since most unsaturated hydrocarbons capable of polymerization are of the structure of CH2=CHR, vinyl polymerization as a general term is often used. 

In the 1960s, anionic polymerized solutron SBR (SSBR) began to challenge emulsion SBR in the automotive tire market. Organolithium compounds allow control of the butadiene microstructure, not possible with ESBR. Because this type of chain polymerization takes place without a termination step, an easy synthesis of block polymers is available, whereby glassy (polystyrene) and rubbery (polybutadicnc) segments can be combined in the same molecule. These thermoplastic elastomers (TPE) have found use ill nontire applications. 

C olvents have different effects on polymerization processes. In radical polymerizations, their viscosity influences the diffusion-controlled bimolecular reactions of two radicals, such as the recombination of the initiator radicals (efficiency) or the deactivation of the radical chain ends (termination reaction). These phenomena are treated in the first section. In anionic polymerization processes, the different polarities of the solvents cause a more or less strong solvation of the counter ion. Depending on this effect, the carbanion exists in three different forms with very different propagation constants. These effects are treated in the second section. The final section shows that the kinetics of the... 

Lastly, another type of reaction which also belongs to the general type al is the synthesis of polymers with azo groups along their main chain by terminating anionic polymerizations with AIBN 68>69> ... 

Figure 5.14. Reactions involved in anionic addition polymerization. Shown are (a) generation of a carbanion from a Lewis basic initiator, (b) propagation of the polymer chain through the combination of the carbanionic polymer chain and additional monomers, and (c) termination of the polymer growth through the addition of a Lewis base. UnUke the other addition polymerization schemes, termination does not occur in situ, but must be initiated deUberately.

Termination. As in anionic polymerization, termination by coupling or disproportionation cannot occur, leaving chain transfers as the most likely mechanisms. 

Termination reactions cannot be eliminated in radical polymerizations because termination reactions involve the same active radical species as propagation therefore, eliminating the species that participates in termination would also result in no polymerization. Termination between active propagating species in cationic or anionic processes does not occur to the same extent because of electrostatic repulsions. Equation (1) represents the rate of polymerization, Rp, which is first order with respect to the concentration of monomer, M, and radicals, P, while Eq. (2) defines the rate of termination, Rt, which is second order with respect to the concentration of radicals. To grow polymer chains with a degree of polymerization of 1000, the rate of propagation must be at least 1000 times faster than the rate of termination (which under steady state condition is equal to the rate of initiation). This requires a very low concentration of radicals to minimize the influence of termination. However, termination eventually prevails and all the polymer chains produced in a conventional free radical process will be dead chains. Therefore they cannot be used in further reactions unless they contain some functional unit from the initiator or a chain transfer agent. 

Anionically Initiated Polymerization. The disadvantages of radical polymerization of cyanoprene result from the operating conditions (temperatures) too many side reactions, chain-terminating reactions, and consecutive reactions occur. Because of this and the dimerization tendency of cyanoprene, catalysts had to be found that could fulfill two contradictory requirements. They should be so reactive that it would be possible to work at temperatures that exclude dimerizations as com-... 

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