Hydrocarbons living polymerization

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. 

Alkyllithium initiators are primarily used as initiators for polymerizations of styrenes and dienes. They effect quantitative living polymerization of styrenes and dienes in hydrocarbon solution. In general, these alkyllithium initiators are too reactive for alkyl methacrylates and vinylpyridines. n-Butyllithium is used commercially to initiate anionic homopolymerization and copolymerization of butadiene, isoprene, and styrene with linear and branched structures. Because of its high degree of association (hexameric), n-butyllithium-initiated polymerizations are often effected at elevated temperatures (>50 °C) and in the presence of small amounts of Lewis base to increase the rate of initiation relative to propagation and thus obtain polymers with narrower molecular weight distributions [55, 57]. 

In contrast to the polymerization of D4, the anionic polymerization of hexamethyl cyclotrisiloxane (D3) with lithium as counterion is a living polymerization which produces polydimethylsiloxanes with well-defined structures. Useful initiators include lithium silanolates or the product from the reaction of 3 mol of butyllithium with D3 in a hydrocarbon solvent as shown in Scheme 7.19. It is noteworthy that no polymerization occurs in the absence of a Lewis base promoter such as THF, glymes, DMSO, or HMPA. 

The effect of varying the nature of the spacer from hydrocarbon to siloxane is starting to be investigated using living polymerizations [28]. However, these polymers have not been characterized yet. 

Despite a number of investigations, the efficient synthesis of co-epoxide-functionalized polymers in hydrocarbon solution with lithium as counterion at room temperature had not been reported. Takenaka et developed an efficient synthesis of epoxide-functionalized polystyrenes and poly-isoprenes by terminating the corresponding living polymeric anions with (2-bromoethyl)oxirane at -78 °C in THE as shown in eqn [27]. Termination of either PSLi or PlLi by inverse addition to 2-3 equivalents of (2-bro-moethyl)oxirane reportedly produced the corresponding epoxy-functionalized polymers in quantitative yields as determined by NMR and TLC-FID chromatography. However, the requirements of -78 °C and THF as solvent limit the usefulness of this procedure. 

Anionic Polymerization of Hydrocarbon Monomers - Living Polymerization... 

Pure living polymerization of hydrocarbon monomers usually occurs at low temperatures, where side reactions are not important. At higher temperatures, however, two side reactions do become important, elimination of LiH and transfer to compounds with an acidic hydrogen [5, 6, 31, 86, 87]. 

Turning our attention first to the polymerization of styrene by sodium naphthalene, we showed, in collaboration with the group at the National Bureau of Standards, the type of narrow distribution of molecular weights which could be obtained by this method. We then showed that such living" polymerizations were not unique with sodium naphthalene, which is restricted to ether solvents, but can be generalized to other homogeneous anionic systems, such as the polymerization of styrene, butadiene, isoprene, etc, by organo-lithi im, both in ether and hydrocarbon media. 

Formation of block polymers is not limited to hydrocarbon monomers only. For example, living polystyrene initiates polymerization of methyl methacrylate and a block polymer of polystyrene and of polymethyl methacrylate results.34 However, methyl methacrylate represents a class of monomers which may be named a suicide monomer. Its polymerization can be initiated by carbanions or by an electron transfer process, the propagation reaction is rapid but eventually termination takes place. Presumably, the reactive carbanion interacts with the methyl group of the ester according to the following reaction... 

Polymer Synthesis and Characterization. This topic has been extensively discussed in preceeding papers.(2,23,24) However, we will briefly outline the preparative route. The block copolymers were synthesized via the sequential addition method. "Living" anionic polymerization of butadiene, followed by isoprene and more butadiene, was conducted using sec-butyl lithium as the initiator in hydrocarbon solvents under high vacuum. Under these conditions, the mode of addition of butadiene is predominantly 1,4, with between 5-8 mole percent of 1,2 structure.(18) Exhaustive hydrogenation of polymers were carried out in the presence of p-toluenesulfonylhydrazide (19,25) in refluxing xylene. The relative block composition of the polymers were determined via NMR. 

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