Copolymerization of Styrenes and Dienes

Pellecchia et al. copolymerized isoprene and styrene [35] and examined the copolymerization rate. They found a value for the product of the reactivity ratios of r r2 = 2.3. The difference in the catalytic activity of styrene and isoprene may be due to the difference in coordination strength. 

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GopolymeriZation Initiators. The copolymerization of styrene and dienes in hydrocarbon solution with alkyUithium initiators produces a tapered block copolymer stmcture because of the large differences in monomer reactivity ratios for styrene (r < 0.1) and dienes (r > 10) (1,33,34). In order to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such as tetrahydrofuran or an alkaU metal alkoxide (MtOR, where Mt = Na, K, Rb, or Cs). In contrast to Lewis bases which promote formation of undesirable vinyl microstmcture in diene polymerizations (57), the addition of small amounts of an alkaU metal alkoxide such as potassium amyloxide ([ROK]/[Li] = 0.08) is sufficient to promote random copolymerization of styrene and diene without producing significant increases in the amount of vinyl microstmcture (58,59). 

While the majority of SBC products possess discrete styrene and diene blocks, some discussion of the copolymerization of styrene and diene monomers is warranted. While the rate of homopolymerization of styrene in hydrocarbon solvents is known to be substantially faster that of butadiene, when a mixture of butadiene and styrene is polymerized the butadiene is consumed first [21]. Once the cross-propagation rates were determined (k and in Figure 21.1) the cause of this counterintuitive result became apparent [22]. The rate of addition of butadiene to a growing polystyryllithium chain (ksd) was found to be fairly fast, faster in fact than the rate of addition of another styrene monomer. On the other hand, the rate of addition of styrene to a growing polybutadienyllithium chain (k s) was found to be rather slow, comparable to the rate of butadiene homopolymerization. Thus, until the concentration of butadiene becomes low, whenever a chain adds styrene it is converted back to a butadienyllithium chain before it can add more styrene. Similar results were found for the copolymerization of styrene and isoprene. Monomer reactivity ratios have been measured under a variety of conditions [23]. Values for rs are typically <0.2, while values for dienes (rd) typically range from 7 to 15. Since... 

Copolymerizations analogous to free-radical reactions occur between mixtures of monomers which have more or less the same e values [Table 9-1). The copolymerizations of styrene and dienes have been particularly studied in this connection. The simple copolymer equation (Eq. 7-13) applies to most of these systems, but the reactivity ratios will vary with the choice of solvent and positive counterion because these factors influence the nature of the propagating ion pair. 

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

AlkyUithium compounds are primarily used as initiators for polymerizations of styrenes and dienes (52). These initiators are too reactive for alkyl methacrylates and vinylpyridines. / -ButyUithium [109-72-8] is used commercially to initiate anionic homopolymerization and copolymerization of butadiene, isoprene, and styrene with linear and branched stmctures. Because of the high degree of association (hexameric), -butyIUthium-initiated polymerizations are often effected at elevated temperatures (>50° C) to increase the rate of initiation relative to propagation and thus to obtain polymers with narrower molecular weight distributions (53). Hydrocarbon solutions of this initiator are quite stable at room temperature for extended periods of time the rate of decomposition per month is 0.06% at 20°C (39). 

The copolymerization of styrene and the dienes in hydrocarbons was first investigated by Korotkov 43) who reported an unexpected phenomenon. The polymeriza-... 

Copolymerization of styrene and conjugated dienes is another attractive subject which provides the most commonly used styrene-butadiene rubbers (SBRs). Boisson reported that by using neodymium amide Nd N(SrMe3)2 3 and TIBA and DEAC, SBRs with 10-15 mol% of styrene were produced [189], although drops in both activity and molecular weight were observed as compared with those of... 

Triblock polymers are generally made in sequential addition processes with the aid of sodium naphthalene or alkyl lithium. They can also be produced in a "coupling" process wherein a dlblock is prepared first and is coupled with the aid of phosgene or alkyl dlhalides to form the ABA triblock. Tapered blocks can be made by starting with a polystyrene block and then copolymerizing a mixture of styrene and diene. 

The alkyllithium-initiated copolymerizations of styrene with dienes, especially isoprene and butadiene, have been... 

Tapered Block Copolymers. The alkyllithium-initiated copolymerizations of styrene with dienes, especially isoprene and butadiene, have been extensively investigated and illustrate the important aspects of anionic copolymerization. As shown in Table 15, monomer reactivity ratios for dienes copolymerizing with styrene in hydrocarbon solution range from approximately 8 to 17, while the corresponding monomer reactivity ratios for styrene vary from 0.04 to 0.25. Thus, butadiene and isoprene are preferentially incorporated into the copolymer initially. This type of copolymer composition is described as either a tapered block copolymer or a graded block copolymer. The monomer sequence distribution can be described by the structures below ... 

Although 1,1-diphenylethylene does not hompolymerize, it will copolymerize with styrene and diene monomers. The anionic copolymerization behavior of 1,1-diphenylethylene with these monomers can be described in terms of the Mayo-Lewis copolymerization equation (Eq.22) [123, 124] ... 

Studies of the copolymerizations of 1,1-diphenylethylene and dienes showed rather different behavior compared with the copolymerizations of styrene and 1,1-diphenylethylene [125, 133-136]. The monomer reactivity ratios for copolymerizations of dienes with DPE are shown in Table 7. When butadiene was copolymerized with 1,1-diphenylethylene in benzene at 40 °C with -butyl-lithium as initiator, the monomer reactivity ratio for butadiene, ri, was 54 this means that the addition of butadiene to the butadienyl anion is 54 times faster than addition of 1,1-diphenylethylene to the butadienyl anion [133]. This unreactivity of poly(butadienyl)lithium towards addition to DPE was also observed in studies of end-capping of poly(butadienyl)lithium with DPE in hydrocarbon solution (see Sect.3.3) [109, 111]. Because of this unfavorable monomer reactivity ratio, few DPE units would be incorporated into the co-... 

The anionic copolymerization of styrene and l-(4-dimethylaminophenyl)-1-phenylethylene in benzene has been investigated [188]. As discussed previously in Sect. 5, Yuki and coworkers [125, 126, 129, 133-136] have developed the formalism for analyzing the kinetics of copolymerization of 1,1-diphenylethylene (M2) with styrene and diene monomers (Mi). It was assumed that the 1,1-diphenylethylene derivative, M2, does not add to itself due to steric effects, i.e., k22=0, as discussed previously in Sect 5. Thus, the monomer reactivity ratio for M2 is zero, i.e., r2- 22l ii- - It was also assumed that the styrene monomer is completely consumed at the end of the polymerization... 

In polar media, the preference for diene incorporation is reduced as shown by the monomer reactivity ratios in Table 13. In THF, the order of monomer reactivity ratios is reversed compared to hydrocarbon media. The monomer reactivity ratios for styrene are much larger than the monomer reactivity ratios for dienes. Thus, although it is apparent that polar solvents such as THF can alter the copolymerization behavior of styrenes and dienes, they have the disadvantage of concurrently increasing the amount of vinyl microstmcture for polybutadiene, an undesirable feature. 

Sodium is a catalyst for many polymerizations the two most familiar are the polymerization of 1,2-butadiene (the Buna process) and the copolymerization of styrene—butadiene mixtures (the modified GRS process). The alfin catalysts, made from sodium, give extremely rapid or unusual polymerizations of some dienes and of styrene (qv) (133—137) (see Butadiene Elastomers, synthetic Styrene plastics). 

Lithium diethylamide has been shown to be an effective initiator for the homopolymerization of dienes and styrene llr2). It is also known that such a polymerization process is markedly affected by the presence of polar compounds, such as ethers and amines (2,3). However, there has been no report of the use of a lithium amide containing a built-in polar modifier as a diene polymerization initiator. This paper describes the preparation and use of such an initiator, lithium morpholinide. A comparison between polymerization with this initiator and lithium diethyl amide, with and without polar modifiers, is included. Furthermore, we have examined the effects of lithium-nitrogen initiators on the copolymerization of butadiene and styrene. 

The reversal of reactivity of styrene and the dienes in copolymerizations has been explained on a kinetic basis 253 263-268), i.e., that the rate constants for the four possible reactions decrease in the sequence ... 

As in the case of olefin or diene homopolymerization by RLi, copolymerization is particularly sensitive to solvent effects. Initial-charge (all monomers added together) copolymerization of butadiene and styrene tends to result in a tapered block copolymer (a block of butadiene with increasing levels of styrene, followed by a block of styrene) in hydrocarbon solvents and a random copolymer (a uniform distribution of butadiene and styrene) in polar media. 

In addition to the polymerization of dienes the versatility of NdP-based catalysts is exceptional regarding the number of different non-diene monomers which can be polymerized with these catalysts. Acetylene is polymerized by the binary catalyst system NdP/AlEt3 [253,254]. Lactides are polymerized by the ternary system NdP/AlEt3/H20 [255,256]. NdP/TIBA systems are applied in the copolymerization of carbon dioxide and epichlorhy-drine [257] as well as for the block copolymerization of IP and epichloro-hydrin [258]. The ternary catalyst system NdP/MgBu2/TMEDA allows for the homopolymerization of polar monomers such as acrylonitrile [259] and methylmethacrylate [260]. The quaternary system NdP/MgBu2/AlEt3/HMPTA is used for the polymerization of styrene [261]. 

From an industrial point of view the other aspects dealt with in this section are less exciting. Therefore, the homopolymerization of substituted dienes other than IP, the copolymerization of BD and styrene and the copolymerization of BD with ethylene and higher 1-alkenes are only briefly summarized. The Nd-catalyzed homo- and copolymerization of monomers with polar entities are not dealt with in this review. 

Binary neodymium alk(aryl)oxide/dialkylmagnesium diene polymerization catalysts were reported by J.-F. Carpentier and coworkers [181,192], The homopolymerization of butadiene, and copolymerization with styrene and... 

Copolymerization of styrene with diolefins provides further support that monomer coordinates with the cationic site prior to addition. Korotkov (218) showed that in homopolymerizations styrene is more reactive than butadiene, but in copolymerization the butadiene reacted first at its homopolymerization rate and when it was exhausted the styrene reacted at its homopolymerization rate. This interesting result has been duplicated by Kuntz (245) and analogous results have been obtained by Spirin and coworkers (237) for the styrene-isoprene system. Presumably, the diene complexes more strongly than styrene with the lithium and excludes styrene from the site. That the complex occurs at a cationic site, rather than at the anion or the metal-carbon bond, is indicated by the fact that dienes form more stable complexes than styrene with Lewis acids (246). It should be emphasized that selective monomer coordination is not the only factor influencing reactivities in copolymerizations. Of greatest importance are the relative reactivities of the different polymer anions. The more resonance-stabilized anion is more readily formed and is less reactive for polymerizing the co-monomer. 

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