Organolithium-initiated polymerization kinetics

It would be expected that the kinetics of organolithium-initiated polymerization in hydrocarbon solvents would be simplified because of the expected correspondence between the initiator concentration and the concentration of propagating anionic species, resulting from the lack of termination and chain transfer reactions. However, in spite of intensive study, there is... 

The dependence of the propagation rate on the concentration of growing chains is illustrated in Figures 6 and 7, and is listed in Table II. The first-order rate constant from Table II are plotted as a function of the initiator concentration. Although the kinetics of organolithium polymerization in nonpolar solvents have been subjected for intensive studies, the results were still somewhat controversial. In view of the strong experimental evidence for association between the organolithium species, the kinetic order ascribed to this phenomenon was postulated (30,31) as shown in Equations (5) and (6). 

In the alkyllithium initiated polymerizations of vinyl monomers, Lewis bases such as ethers and amines alter the kinetics, stereochemistry, and monomer reactivity ratios for copolymerization. In general, the magnitude of these effects has been directly or indirectly attributed to the extent or nature of the interaction of the Lewis base with the organolithium initiator or with the organolithium chain end of the growing polymer. Unfortunately, all of these observed effects are kinetic in nature, and therefore the observed effects of solvent represent a composite effect on the transition-state versus the ground state as shown below in Eq. (6), where 5 represents the differential... 

Nonpolar Media. Because organolithium initiators are soluble in hydrocarbons, the kinetics of these polymerizations have also been studied in these nonsolvating media. A large number of such studies have been carried out (3, 41) mainly on styrene and the dienes. Again the propagation rate is first order with respect to monomer, in accordance with Reaction 13. However, the rate dependence on growing chain concentration has been found to show marked variation from one system to another with the orders varying from one half to much lower values (3, 41). These systems pose... 

Many interesting and important synthetic applications of 1,1-diphenylethylene and its derivatives in polymer chemistry are based on the addition reactions of polymeric organolithium compounds with 1,1-diphenylethylenes. Therefore, it is important to understand the scope and limitations of this chemistry. In contrast to the factors discussed with respect to the ability of 1,1-dipheny-lalkylcarbanions to initiate polymerization of styrenes and dienes, the additions of poly(styryl)lithium and poly(dienyl)lithium to 1,1-diphenylethylene should be very favorable reactions since it can be estimated that the corresponding 1,1-diphenylalkyllithium is approximately 64.5kJ/mol more stable than allylic and benzylic carbanions as discussed in Sect. 2.2 (see Table 2). Furthermore, the exothermicity of this addition reaction is also enhanced by the conversion of a tt-bond to a more stable a-bond [51]. However, the rate of an addition reaction cannot be deduced from thermodynamic (equilibrium) data an accessible kinetic pathway must also exist [3]. In the following sections, the importance of these kinetic considerations will be apparent. 

Fractional kinetic orders of homogenous reactions in solution may point to association of a particular reagent. The kinetics of the initiation step of styrene polymerization in the presence of n-BuLi (equation 33) is in accordance with the assumption that this organolithium compound in a nonbonding solvent forms aggregates of six molecules on the average" . 

Magnin and coworkers 173) also studied the kinetics of the polymerization of ethylene in hexane using n-BuLi/TMEDA as initiator and obtained results very different from those of Hay et al. 17l,172). The reaction was found to be first order in ethylene and to exhibit a square root dependence upon whichever of the substances TMEDA or RLi was present in the smaller quantity. With a constant concentration of organolithium, the rate increased on increasing the concentration of TMEDA until a limiting value was reached when [TMEDA] = [RLi]. These observations were rationalized by the scheme ... 

Initiation The mechanism of initiation of anionic polymerization of vinyl monomers with alkyllithium compounds and other organometallic compounds is complicated by association and cross-association phenomena in hydrocarbon solvents and by the presence of a variety of ionic species in polar media [3, 4, 45, 48, 55, 56]. The kinetics of initiation is complicated by competing propagation and the occurrence of cross-association of the alkyllithium initiator with the propagating organolithium [55]. Thus, only the initial rates provide reliable kinetic data. 

Zhong, X. F. Francois, B. Kinetics of 1,3-cyclohexadiene polymerization initiated by organolithium compounds in anon-polar medium, 1. Pure propagation step. Mafcrc>mc>/. Chem. 1990,797,2735-2741. 

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