Organolithium-initiated polymerization

More recently, a number of different copolymer structures have been prepared from butadiene and styrene, using modified organolithiums as polymerization initiators ( 4). Organolithium initiated polymerizations have gained prominence because stereo-control is combined with excellent polymerization rates, and the absence of a chain termination reaction facilitates control of molecular weights and molecular weight distributions ( 5). 

Evidence for the absence of termination or transfer reactions in the organolithium-initiated polymerization of styrene and iso-prene is shown in Table I for representative examples of these polymers. It can be seen that these polymers exhibit the expected low kfo/Mn values, except in the case of the isoprene polymerized in the H4furan, where a slow side reaction seems to occur between the solvent, on the one hand, and both the initiator (Ifo vs Mg) and the growing chains (1% vs Mn)>on the other hand. 

The phenomenal growth in commercial production of polymers by anionic polymerization can be attributed to the unprecedented control the process provides over the polymer properties. This control is most extensive in organolithium initiated polymerizations and includes polymer composition, microstructure, molecular weight, molecular weight distribution, choice of functional end groups and even monomer sequence distribution in copolymers. Furthermore, a judicious choice of process conditions affords termination and transfer free polymerization which leads to very efficient methods of block polymer synthesis. 

Block polymers were prepared by organolithium-initiated polymerization in cyclohexane solution by using the sequential monomer addition technique (3). Polymers were both of the linear-SBS and radial -branched (SB) type. Blends were prepared in cyclohexane solution, either before or after coupling the initially linear SBLi precursor. Coupling agents investigated were ethyl acetate (for linear coupling), epoxi-aized soybean oil (ESO), and SiCh. 

The important influence of temperature on the microstructure of organolithium-initiated polymerizations of butadiene, both with and with-... 

General Functionalization Reactions General functionalization reactions are reactions of organolithium compounds that proceed efficiently to introduce a variety of different functional groups [3]. The most useful reactions can be utilized at elevated temperatures in hydrocarbon solution so that the unique characteristics of organolithium-initiated polymerizations can be preserved. 

Kim J, Choi GJ, Lee SB. Organolithium-initiated polymerization of carbazole derivative retaining blue light photoluminescence. Abstracts of Papers, 228th ACS National Meeting, Philadelphia, PA, United States, August 22-26,2004 PMSE-275. 

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 characteristics of the organolithium-initiated polymerization of the dienes was clearly established by Morton and co-workers. It was demonstrated that the polymerizations are of the living type, which take place without a chain termination reaction in the absence of impurities.The implications of this behaviour in preparing polymers of controlled structure are well recognized today. 

The most common Lewis bases used in organolithium-initiated polymerizations are aprotic amines and ethers. More strongly complexing agents include hexamethylphosphoramide, polyglycol dimethyl ethers, cyclic ethers (crown ethers), and dipiperidinoethane (DIPIP)... 

Alkyl derivatives of the alkaline-earth metals have also been used to initiate anionic polymerization. Organomagnesium compounds are considerably less active than organolithiums, as a result of the much less polarized metal-carbon bond. They can only initiate polymerization of monomers more reactive than styrene and 1,3-dienes, such as 2- and 4-vinylpyridines, and acrylic and methacrylic esters. Organostrontium and organobarium compounds, possessing more polar metal-carbon bonds, are able to polymerize styrene and 1,3-dienes as well as the more reactive monomers. 

The anionic polymerization of styrene using the organolithium initiators can be described as a termination-free polymerization, as shown in Eqs. (1)... 

Lewis bases effect dramatic changes in microstructure, initiation rates, propagation rates, and monomer reactivity ratios for alkyllithium—initiated polymerizations of vinyl monomers (1-6). Some insight into the molecular basis for these observations has been provided by a variety of NMR, colligative property, and light-scattering measurements of simple and polymeric organolithium compounds in hydrocarbon and basic solvents... 

Anionic polymerization has also been used to make telechelic polymers (Greek telos, end, and chele, claw), i.e., polymers with reactive terminal groups.We coined the term, telechelic in 1957 and it has been accepted ever since in technical as well as patent literature. Liquid carboxy- and hydroxy telechelic polybutadienes initiated with difunctional organolithium initiators are commercially produced since 1962. Some of the physical properties,production details and uses as in solid rockets... 

The commerical polybutadiene (a highly 1,4 polymer with about equal amounts of cis and trans content) produced by anionic polymerization of 1,3-butadiene (lithium or organolithium initiation in a hydrocarbon solvent) offers some advantages compared to those manufactured by other polymerization methods (e.g., it is free from metal impurities). In addition, molecular weight distributions and microstructure can easily be modifed by applying appropriate experimental conditions. In contrast with polyisoprene, where high cis content is necessary for suitable mechanical properties, these nonstereoselective but dominantly 1,4-polybutadienes are suitable for practical applications.184,482... 

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... 

The lithium and alkyllithium initiation of diene polymerization has, from the earliest times, remained in the shadow of other, apparently more important, initiator systems. However, it has now become clear that the alkyllithium catalyst is the most efficient, initiator system at present available for diene polymerization. That organolithium initiators are not used much more widely is due largely to economic considerations,... 

This review is limited to the polymerization of hydrocaibon dienes and olefins by means of organolithium initiators. It is not intended to include activated olefins or dienes that can be polymerized by bases of far lower reactivity or that do not involve direct caibon-lithium bonding. 

An example of a functional organolithium initiator is p-lithiophenoxide. It acts both as a carrier of hydroxyl functionality and as an initiator for polymerization (122). Unfortunately, p-lithiophenoxide has a low solubility, even in such polar solvents as THF, and up to 30% of the initiator is initially inactive toward polymerization. It is difficult to control polymer molecular weights when such initiators are used (123). Trepka (124) improved the solubility of p-lilhiophenoxide by the introduction of alkyl... 

The existence of such associated organolithium compounds has been estabhshed in various cases (19, 20, 24), In addition to isotactic polystyrene, a considerable amoimt of atactic material is always present it is formed by starting the polymerization on the nonassociated part of the organolithium compounds which probably promote a nonstereospecific anionic polymerization. The stereoregulation of the polymerization of styrene by heterogeneous alkali metal aUcyl initiators is limited by the forces on the surface of the catalyst while the dissolved organolithium initiators in their associated form cause the stereospecific polymerization. 

The overall polymerization reaction in a homogeneous medium involving, for example, an organolithium initiator, may be written as follows ... 

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... 

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