Organolithium polymerization

Waack and Doran [26] reported on the relative reactivities of 13 structurally different organolithium compounds in polymerization with styrene in tetrahydro-furan at 20°C. The reactivities were determined by the molecular weights of the formed polystyrene. The molecular weights are inversely related to the activity of the respective organolithium polymerization initiators. Reactivities decreased in the order alkyl > benzyl > allyl > phenyl > vinyl > triphenylmethyl as shown in Table 3.1. 

In view of the evidence described above, and elaborated below, there is no reason to-day to relate the reaction order in the organolithium polymerization of the dienes to the state of association, as exemplified by the scheme described by Equations... 

In the case of the much-studied organolithium polymerization of butadiene and isoprene, the effect of ethers and other polar solvents is to change the chain structure from one containing... 

B. Structure Studies of the Propagating Chain End in Organolithium Polymerization of Dienes. The remarkable effects of solvents on the chain mierostructure of the lithium-polymerized conjugated dienes made it of great interest to study the structure of the propagating chain ends in these systems. 

So far the discussion was focused on copolymers derived from a mixture of styrene and a diene. In view of the "living" nature of organolithium polymerization, it is also possible to synthesize block polymers in which the sequence and length of the blocks are controlled by incremental (or sequential) addition of monomersr This general method of preparing block polymers is readily adaptable to commercial production, and, indeed, a number of block copolymers are manufactured this way. Those that have received the most attention in recent years are the diene-styrene two-phase... 

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). 

The living character of organolithium polymerizations makes such processes ideally suited for the preparation of pure as well as tapered-block copolymers. Diene-olefin pure-block copolymers have become important commodities because of their unique structure-property relationships. When such copolymers have an ABA or (AB) X [A = polyolefin, e.g., polystyrene or poly(a-methylstyrene) B = polydiene, e.g., polybutadiene or polyisoprene and X = coupling-agent residue] arrangement of the blocks, the copolymers have found use as thermoplastic elastomers (i.e., elastomers that can be processed as thermoplastics). 

There is no inherent termination step in organolithium polymerizations of hydrocarbon monomers, and this method of initiation yields living polymers. Living polymerizations are defined as those in which there is no inherent termination reaction (as described in Section 6.3.3 for free-radical polymerizalions) and in which the macrospecies continue to grow as long as monomer is supplied. 

Factors Affecting the Isomeric Chain Unit Structure in Organolithium Polymerization of Butadiene and Isoprene... 

It appears from this work that the chain unit structure in the organolithium polymerization of butadiene and isoprene is sensitive to certain reaction parameters even in non-polar media. This is especially true for the effect of initiator concentration and the amount and type of hydrocarbon solvent present. These effects apparently influence mainly the cis/trans ratio of 1,4 units in the chain, being largely ineffective in changing the side-vinyl content. 

In contrast, in anionic systems in which the solvent may not actually interrupt the propagation process, it may play an active role in controlling both the rate and mode of the chain growth step. This control is perhaps most dramatically illustrated in the case of the organolithium polymerizations in connection with two specific aspects chain microstructure of polydienes and copolymerization of dienes and styrene. 

The dramatic effect of even traces of ethers on the microstructure of polybutadiene and polyisoprene in organolithium polymerizations in hydrocarbon media was demonstrated very effectively by Tobolsky et al. (16. 17) They showed that highly solvating ethers, such as H -furan, when present in approximately... 

Morton M, Wu M. Organolithium polymerization of 8-caprolactone. In McGrath JE, editor. Ring-opening Polymerization Kinetics, mechanisms, and Synthesis. ACS Symposium Series. Volume 286. Washington (DC) American Chemical Society 1985. p 175-182. 

Morton M, Rupert JR. Factors affecting the isomeric chain unit structure in organolithium polymerization of butadiene and isoprene. In Bailey FE Jr editor. Initiation of Polymerization. ACS Symposium Series. Volume 212. Washington (DC) American Chemical Society 1983. p 283. 

The kinetics of the propagation reaction in organolithium polymerization of styrenes and dienes in nonpolar solvents (i.e., hydrocarbons) have also been subjected to intensive study. For styrene polymerizations, a kinetic order dependence on chain end concentration is observed (Eq. (2.75)). Since it has been... 

The control of chain structure and molecular weight afforded by the organolithium polymerization of dienes, has, of course, been of great technological interest [161,162,209]. Such product developments have been mainly in the form of (1) polybutadiene elastomers of various chain structures [162, 198,209] and functional end groups [210], (2) liquid polybutadienes [211], (3) butadiene-styrene copolymers (solution SBR) [69, 161, 162, 209], and (4) styrene-diene triblock copolymers (thermoplastic elastomers) [212]. 

As discussed above, the amount of 1,2-addition in polybutadiene rubbers (BRs) can be varied from 8 up to 100% depending on the type of modifier and conditions used in organolithium polymerizations. This process versatility provides an important way to control 7 of BR and SBR rubbers. Figure 3 shows the relationship between increasing vinyl... 

Retrospective View of Vinyl-BR Properties In the 1950s, the Phillips Petroleum Company and the Firestone Tire and Rubber Company started commercial production of polybutadienes by organolithium polymerization for use in tyres. These solution BRs, having low vinyl contents (8-10%), were used in blends with emulsion SBR in tyre treads for balancing traction and wear performance properties. In the early 1970s when styrene monomer was in short supply, developments from Phillips Petroleum Company and EniChem (formerly the International Synthetic Rubber Company) showed that vinyl-BRs with 50-55% vinyl content behaved like emulsion polymerized SBR in tyre tread formulations and exhibited very similar tread wear and wet skid resistance. Tread compounds containing 45%-vinyl polybutadiene showed lower heat build-up and better blow-out resistance than E-SBR and blends of E-SBR with cw-BR. EniChem introduced trial quantities of a medium-vinyl butadiene rubber (MVBR) under the name Intolene 50 in 1973. 

Morton, M, Fetters, L. J. Homogeneous anionic polymerization. V. Association phenomena in organolithium polymerization. J. Polymer Sci. A 2,3311 (196 ... 

Studies of the Nature of the Propagating Chain End in Organolithium Polymerization of Dienes... 

Solvation of the organolithium polymeric species, with possibly some disruption of the polymers to less-associated species, leads to adducts which may function as a source of carbanions in this sense the adducts behave as bases in furnishing a nucleophilic moiety. It is important to realize, however, that such basic behavior can occur only in the presence of a basic solvent, or it requires a strong Lewis add. Examples of the latter situation are found in the reaction of alkyllithium compounds vrith other organometals, as described in the following section. 

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