Chain microstructure of polydienes

TABLE 2.14 Microstructure of Polydienes Prepared by Anionic Polymerizatioi  

This marked sensitivity of the stereochemistry of anionic polymerization to the nature of the counterion and solvent can be traced to the structure of the propagating chain end. The latter involves a carbon-metal bond which can have variable characteristics, ranging all the way from highly associated species with covalent character to a variety of ionic species (Hsieh and CJuirk, 1996). The presence of a more electropositive metal and/or a cation-solvating solvents, such as ethers, can effect a variety of changes in the nature of the carbanionic chain end (a) the degree of association of the chain ends can decrease or be eliminated (b) the interaction of the cation with the anion can be decreased 

The possibilities inherent in the anionic copolymerization of butadiene and styrene by means of organolithium initiators, as might have been expected, have led to many new developments. The first of these would naturally be the synthesis of a butadiene-styrene copolymer to match (or improve upon) emulsion-prepared SBR, in view of the superior molecular weight control possible in anionic polymerization. The copolymerization behavior of butadiene (or isoprene) and styrene is shown in Table 2.15 (Ohlinger and Bandermann, 1980 Morton and Huang, 1979 Ells, 1963 Hill et al., 1983 Spirin et al., 1962). As indicated earlier, unlike the free radical type of polymerization, these anionic systems show a marked sensitivity of the reactivity ratios to solvent type (a similar effect is noted for different alkali metal counterions). Thus, in nonpolar solvents, butadiene (or isoprene) is preferentially polymerized initially, to the virtual exclusion of the styrene, while the reverse is true in polar solvents. This has been ascribed (Morton, 1983) to the profound effect of solvation on the structure of the carbon-lithium bond, which becomes much more ionic in such media, as discussed previously. The resulting polymer formed by copolymerization in hydrocarbon media is described as a tapered block copolymer it consists of a block of polybutadiene with little incorporated styrene comonomer followed by a segment with both butadiene and styrene and then a block of polystyrene. The structure is schematically represented below  

The data in Table 2.15 illustrate the problems encountered in such copolymerizations, since the use of polar solvents to assure a random styrene-diene 

TABLE 2.15 Monomer Reactivity Ratios for Organolithium Copolymerization of Styrene and Dienes 

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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 crucial point of the procedure is the control of the stoichiometry of the reaction between the living A chains and the DPE derivative, otherwise a mixture of stars is produced. A major problem is the fact that the rate constants for the reaction of the first and second polymeric chain with the DPE derivative are different. This results in bimodal distributions because of the formation of both the monoanion and dianion. In order to overcome this problem polar compounds have to be added, but it is well known that they affect dramatically the microstructure of the polydienes that are formed in the last step. However the addition of lithium sec-butoxide to the living coupled DPE derivative, prior to the addition of the diene monomer, was found to produce monomodal well defined stars with high 1,4 content. Finally another weak point of the method is that, as in the case of the DVB route, the B arms cannot be isolated from the reaction mixture and characterized separately. It is therefore difficult to obtain unambiguous information about the formation of the desired products. 

In conclusion, FMC has developed a viable, commercial synthesis of a family of omega-(/-butyldimethylsilyloxy)-l-alkyllithiums that are valuable anionic initiators. A variety of chain lengths are available between the protected hydroxyl ftmction and the carbon-lithium bond. These hydrocarbon soluble initiators afford very high 1,4-microstructure in the polymerization of polydienes, such as... 

A comprehensive hypothesis has been proposed to explain the effects of the concentrations of active chain ends and monomer on polydiene microstiucture [163], Based on studies with model compounds and the known dependence of polydiene microstructure on diene monomer... 

The anionic polymerization of dienes is also a subject of long-term, continuous interest. Using a variety of initiators, Lewis base additives, and solvent systems, a wide range of polydiene microstructures can be prepared. Several reports have appeared regarding the relationships between polydiene microstructure, monomer concentration and chain-end concentra-tion. In general, the highest cis-1,4-microstructures for either polybutadiene or polyisoprene can be obtained at high ratios of [monomer] to... 

The peculiar features of polydienes are due not only to the presence of unsaturated double bonds in the polymer chain, but also to their particular microstructural characteristics (chemo-, regio-, and stereoselectivity). Owing to the complexity of polydiene structures, before going into detail concerning the different stereoregular polymers that can be obtained from a given monomer, it is... 

It follows from the existence of conformational scissors (Fig. 4) that in the polymerization of symmetric or quasisymmetric dienes in hydrocarbon media on an active center with a slightly polar carbon-metal bond the primary acts of monomer attachment lead to the cis-conformation of the end unit. This conclusion is in good agreement with the modern concepts of the formation mechanism of the cis-structure of anionic polydienes [70]. According to these concepts this is followed by either the attachment of the next monomer molecule or by the cis-trans isomerization of the end unit. The microstructure is fixed at the moment of the attachment of a new monomer unit to the active center, the configuration (cis- or trans-) of the end unit being retained in the polymer chain ... 

Difunctional Initiators. Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, are efficient difunctional initiators (see scheme under Radical Anions). However, the need to use polar solvents for their formation limits their utility for diene polymerization since the unique ability of lithium to provide high 1,4-polydiene microstructure is lost in polar media. The methodology for preparation of hydrocarbon-soluble, dilithium initiators is generally based on the reaction of an aromatic divinyl precursor with 2 moles of butyllithium. Unfortunately, because of the tendency of organolithium chain ends in hydrocarbon solution to associate and form electron-deficient dimeric, tetrameric, or hexameric aggregates, most attempts to prepare dilithium initiators in hydrocarbon media have generally resulted in the formation of insoluble, three-dimensionally associated species (40). 

Results for the structurally more complex, mixed microstructure polydienes, which have substantial in-chain and side-chain unsaturation, are more difficult to interpret. Clearly, mixed microstructure polybutadiene (PBD-57) exhibits a value (6.0) intermediate to that found for high 1,4 (C = 5.1) and high 1,2 (C = 7.0) materials. Conversely, a mixed microstructure polyisoprene (PI-51) of similar microstructure to PBD-57 yields a C o of 5.3, a result virtually identical to that found for the high 1,4 material. Both mixed microstructure polymyrcene (PMYRC-64) and poly(2,3-dimethyl butadiene) (PDMB-55) yield C values (7.6 and 8.4, respectively) that are considerably larger than those for their high 1,4 counterparts. Interpretation of these results at the present level of theoretical development would be speculative. 

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