Alkyllithium initiated butadiene-styrene random

EVALUATION OF ALKYLLITHIUM INITIATED BUTADIENE-STYRENE RANDOM COPOLYMER (SOLPREN 1204) IN COMPOUNDED STOCK... 

The copolymerization with alkyllithium to produce uniformly random copolymers is more complex for the solution process than for emulsion because of the tendency for the styrene to form blocks. Because of the extremely high rate of reaction of the styryl-lithium anion with butadiene, the polymerization very heavily favors the incorporation of butadiene units as long as reasonable concentrations of butadiene are present. This observation initially was somewhat confusing because the homopolymerization rate of styrene is seven times that for butadiene. However, the cross-propagation rate is orders of magnitude faster than either, and it therefore dominates the system. For a 30 mole percent styrene charge the initial polymer will be almost pure butadiene until most of the butadiene is polymerized. Typically two-thirds of the styrene charged will be found as a block of polystyrene at the tail end of the polymer chain ... 

Langer (13) has also disclosed the use of alkyllithium and dialkyl-magnesium tertiary diamine complexes as catalysts for copolymerization of ethylene and other monomers such as butadiene, styrene, and acrylonitrile to form block polymers. Examples are given in which polybuta-dienyllithium initiates a polyethylene block, as well as vice-versa. Random copolymers of these two were also prepared, and other investigators have used not only tertiary diamines but hexamethylphosphoramide (14) and tetramethylurea (15) as nitrogenous base cocatalysts in such polymerizations. Antkowiak and co-workers (11) showed the similarity of action of diglyme and TMEDA in copolymerizations of styrene and... 

Commercial random SBR polymers (solution SBR) prepared by alkyllithium-initiated polymerization typically have 32% cis-, A-, 41% trans-, A-, and 27% vinyl-microstructure compared to 8% cw-1,4-, 74% trans-, A-, and 18% vinyl-microstructure for emulsion SBR with the same comonomer composition [3, 221]. Solution SBRs typically have branched architectures to eliminate cold flow [17, 49]. Compared to emulsion SBR, solution random SBRs require less accelerator and give higher compounded Mooney, lower heat buildup, increased resilience, and better retread abrasion index [3]. Terpolymers of styrene, isoprene, and butadiene (SIBR) have been prepared using a chain of single-stirred reactors whereby the steady-state concentration of each monomer and Lewis base modifier at any degree of conversion could be controlled along the reactor chain [3, 222-224]. 

In general, random SBR with a low amount of block styrene and low amounts of 1,2-butadiene enchainment (<20%) can be prepared in the presence of small amounts of added potassium or sodium metal alkoxides. " For example, at 50 °C in the presence of as little as 0.067 equivalents of potassium t-butoxide in cyclohexane, the amormt of bound styrene was relatively independent of conversion, in contrast to the heterogeneity observed in the absence of randomizer, that is, tapered block copolymer formation. " The polybutadiene microstmcrnre obtained tmder these conditions corresponds to about 15% 1,2-microstmctrue. Using 0.2 equivalents of hydrocarbon-soluble sodium 2,3-dimethyl-2-pentoxide, the monomer reactivity ratios for alkyllithium-initiated SBR were fotmd to be of re = 1.1 and rs = 0.1. The resulting copolymer had only 5% block styrene and 18% 1,2-vinyl microstmctrue. It was found that there is a very narrow compositional window ([RONa]/[RLi]) at... 

An increasing amount of styrene-butadiene rubber is being manufactured by solution processes using alkyllithium catalysts. Production techniques resemble those used for the polymerization of isoprene (section 20.3.3) and butadiene (section 20.4.3). There is a tendency for alkyllithium initiation to lead to block copolymers since the butadiene in the mixture polymerizes first to the virtual exclusion of the styrene. In order to obtain random copolymers it is necessary to add the butadiene incrementally so that the molar ratio of... 

It was also discovered at Phillips. that the four rate constants discussed above can be altered by the addition of small amounts of an ether or a tertiary amine resulting in reduction or elimination of the block formation. Figures 13 and 14 illustrate the effect of diethyl ether on the rate of copolymerization and on the incorporation of styrene in the copolymer. Indeed, random copolymers of butadiene and styrene or isoprene and styrene can be prepared by using alkyllithium as initiator in the presence of small amounts of an ether or a tertiary amine. 

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