Organolithium initiated

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). 

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

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. 

Commercial polymers from organolithium initiator systems cover a wide range, and Phillips commitment in this field on a world-wide basis is illustrated in Table 1. 

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 second approach to the synthesis of tele-chelic polymers involves the use of functionally substituted anionic initiators. An example of a functional organolithium initiator is p-lithiophenoxide (40). Unfortunately, p-lithiophenoxide shows low solubility even in polar solvents like THF. It is difficult to control polymer molecular weights with such a heterogeneous initiator (41). Trepka (42) has improved the solubility of D-lithiophenoxide by adding alkyl groups to the ring, nevertheless, only moderate yields of the alkylated initiator have been reported. Hirshfield... 

Anionic Copolymerization of Butadiene and Isoprene with Organolithium Initiators in Hexane... 

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... 

The first successful use of lithium metal for the preparation of a of-l,4-polyisoprene was announced in 1955 (50) however, lithium metal catalysis was quickly phased out in favor of hydrocarbon soluble organ olithium compounds. These initiators provide a homogeneous system with predictable results. Organolithium initiators are used commercially in the production of ai- 1,4-polyisoprene, isoprene block polymers, and several other polymers. 

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... 

Table 17. Microstructure of Polydienes Obtained using Organolithium Initiators...

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. 

When butadiene and styrene are mixed in the presence of an organolithium initiator, the resulting copolymerization process and product will be governed by the reaction conditions. The rate of copolymerization, the relative composition of the copolymer, and the distribution of monomer units (i.e., block, random, etc.) will be determined by such factors as solvent, temperature, and monomer feed ratio. 

Foss and co-workers (88) reported ABA copolymers obtained from a new dilithium reagent this organolithium initiator was formed by the addition of sec-butyllithium to m-diisopropenylbenzene in the presence of a small proportion of triethylamine, followed by reaction with isoprene to improve the hydrocarbon solubility. Unfortunately, the starting ma-... 

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. 

When external forces act on HIPS, the rubber particles have a stress-relaxing action. To do this job, they must be bound sufficiently to the matrix and have a certain elasticity. This means that the rubber used must be capable of grafting and crosslinking. Both properties can be controlled within certain limits via the microstructure. These requirements mean that nowadays use is predominantly made of medium-m and sometimes also high-cw rubbers, which can be produced using organolithium initiators or coordinate Ziegler catalysts. 

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. 

It is interesting to note that a review of the literature shows no reference to the successful synthesis of polybutadiene, by organolithium initiators, with a cis-1,4 content as high as... 

Functional groups can be attached to the living polymer chains by choosing suitable termination reagents, resulting in linear or star-shaped molecules. With bi- or multifunctional organolithium initiators , a greater variety of polymer products can be synthesized,... 

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

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

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