Butadiene reactivity ratios

Distribution of the monomer units in the polymer is dictated by the reactivity ratios of the two monomers. In emulsion polymerization, which is the only commercially significant process, reactivity ratios have been reported (4). IfMj = butadiene andM2 = acrylonitrile, then = 0.28, and r2 =0.02 at 5°C. At 50°C, Tj = 0.42 and = 0.04. As would be expected for a combination where = near zero, this monomer pair has a strong tendency toward alternation. The degree of alternation of the two monomers increases as the composition of the polymer approaches the 50/50 molar ratio that alternation dictates (5,6). Another complicating factor in defining chemical stmcture is the fact that butadiene can enter the polymer chains in the cis (1), trans (2), or vinyl(l,2) (3) configuration ... 

The composition curve In the co-graft polymerization of CTFE and butadiene is shown in Figure 7. From the result, the monomer reactivity ratios were obtained as rQ, j. =0.10 0.06 and rButadiene=16 3 from the Finemann-Ross plot. Since the product of CTFE rButadiene is nearly equal to one, a random co-grafting takes place in the reaction system. The high reactivity of butadiene in the co-grafting results in the remarkable acceleration of the gel formation by the addition of a small amount of butadiene to CTFE (Figure 6). 

It is evident from Table 11 that the rate of addition of the triplet bis(methoxy-carbonyl) carbene is somewhat slower than that of the singlet. Another important general rule may also be deduced from Table 11 the triplet carbene adds to dienes about 3—4 times faster than to olefins the reactivity ratio of 2,3-dimethyl-butadiene-1,3/pentene-l is 9.6 for triplet and 2.8 for the singlet. This ratio may be compared with that for diphenylcarbene (1,3-butadiene/hexene-l), which is > 100. 

Compositionally uniform copolymers of tributyltin methacrylate (TBTM) and methyl methacrylate (MMA) are produced in a free running batch process by virtue of the monomer reactivity ratios for this combination of monomers (r (TBTM) = 0.96, r (MMA) = 1.0 at 80°C). Compositional ly homogeneous terpolymers were synthesised by keeping constant the instantaneous ratio of the three monomers in the reactor through the addition of the more reactive monomer (or monomers) at an appropriate rate. This procedure has been used by Guyot et al 6 in the preparation of butadiene-acrylonitrile emulsion copolymers and by Johnson et al (7) in the solution copolymerisation of styrene with methyl acrylate. 

There are few studies of the effect of temperature on monomer reactivity ratios [Morton, 1983]. For styrene-1,3-butadiene copolymerization by r-butyllithium in rc-hexane, there is negligible change in r values with temperature with r — 0.03, r2 = 13.3 at 0°C and n = 0.04, r% = 11.8 at 50°C. There is, however, a signihcant effect of temperature for copolymerization in tetrahydrofuran with r — 11.0, r2 = 0.04 at —78°C and r — 4.00, r2 = 0.30 at 25° C. The difference between copolymerization in polar and nonpolar solvents is attributed to preferential complexing of propagating centers and counterion by 1,3-butadiene as described previously. The change in r values in polar solvent is attributed to the same phenomenon. The extent of solvation decreases with increasing temperature, and this results in... 

Using the Q and e values in Table 6-7, calculate the monomer reactivity ratios for the comonomer pairs styrene-1,3-butadiene and styrene-methyl methacrylate. Compare the results with the r and rx values in Table 6.2. 

Reactivity ratios for all the combinations of butadiene, styrene, Tetralin, and cumene give consistent sets of reactivities for these hydrocarbons in the approximate ratios 30 14 5.5 1 at 50°C. These ratios are nearly independent of the alkyl-peroxy radical involved. Co-oxidations of Tetralin-Decalin mixtures show that steric effects can affect relative reactivities of hydrocarbons by a factor up to 2. Polar effects of similar magnitude may arise when hydrocarbons are cooxidized with other organic compounds. Many of the previously published reactivity ratios appear to be subject to considerable experimental errors. Large abnormalities in oxidation rates of hydrocarbon mixtures are expected with only a few hydrocarbons in which reaction is confined to tertiary carbon-hydrogen bonds. Several measures of relative reactivities of hydrocarbons in oxidations are compared. 

Reactivity Ratios and Relative Rates of Oxidation. Table VI summarizes reactivity ratios for all combinations of butadiene, styrene, Tetralin, and cumene reported here and by Hendry (12) for butadiene. The underscored r values in the table are calculated from the base point in the same column (1.00) and the reciprocals (because of the definition of r in Equation 2) of the relative reactivities of the hydrocarbons toward an average R02 radical, B S T C = 1.0 2.2 4.5 5 30 (all underscored values in the butadiene column). The calculated and experimental values always agree within 50%, and in seven out of 12 systems, within 15%. Thus, among these four hydrocarbons a change of R— in RC>2 affects relative reactivities by less (often much less) than 50%. 

Most of the largest differences between observed and calculated r values in Table VI occur for cumene combinations. Such discrepancies are inevitable if cumene tends to alternate with butadiene and Tetralin but not with styrene. (This distinction could be partly experimental error, which tends to be greatest in combinations of the least reactive hydrocarbon—cumene—with the other hydrocarbons.) These alternation tendencies are measured by the products of the reactivity ratios, the last number in each group of three in Table VI, 1 corresponding to no effect and 0 to inability of one or both peroxy radicals to react with the hydrocarbon from which it is derived. If we take cumene to be 1/40 as reactive as butadiene (instead of 1/30, as in the table), agreement is better for the two styrene-cumene reactivity ratios and poorer for the other cumene ratios. If instead we take cumene to be 1/20 as reactive as butadiene, agreement is much better for the butadiene-cumene and Tetralin-cumene... 

Table VI. Reactivity Ratios in Co-Oxidations of Butadiene, Styrene, Tetralin, and Cumene at 60°C. ...

In the copolymerization of butadiene or isoprene and styrene, the reactivity ratios are influenced by the type of solvent usedJLi Typical conversion curves of a 75/25 butadiene/... 

Relatively little information is available for the copolymerization of butadiene with isoprene. In an early paper by Rakova and Korotkov (8), it was concluded that in n-hexane with n-butyllithium as the initiator, the reactivity ratios for butadiene and isoprene were rg = 3.38 and rj = 0.47, respectively. 

This observation is corroborated with what has been found in Figures 8-10. There is more of an inversion phenomenon occurance at 20°C. However, the difference between 30°C and 40°C is small and apparently similar, within experimental error. Nevertheless, the new established reactivity ratios of butadiene and isoprene at all three temperatures differ by a smaller factor than what were reported by the work of Korotkov (8) (e.g. rj - 3.38 and 2 = 0.47). Moreover, butadiene is more reactive and initial copolymer contains a larger proportion of butadiene randomly placed along with some incorporation of isoprene units. The randomness of the copolymer via direct copolymerization has been confirmed by the comparison with pure diblock copolymer produced by sequential monomer addition. Both copolymers have similar chemical composition (50/50) and molecular weight. Their... 

In copolymerization butadiene reacts preferentially, however, significant concentration of isoprene units are also incorporated in a random manner during the early stage of reaction. Butadiene has higher reactivity ratio than isoprene. 

In hydrocarbon solvents it is known that most of the growing chains are associated and it is necessary to enquire what effect this has on the copolymerization mechanism. The reactivity ratios measured from copolymer composition are unaffected because they refer to a common ion-pair. The equilibrium constants for association cancel and the reactivity ratios measured give a true measure of the relative propagation constants of the two monomers. No assessment can be made of the real reactivity of two types of active chain with the same monomer, however. In this case the observed rates are a function of the relative reactivities of the free ion-pairs and also of the relative extents of association. For example in hydrocarbon solvents polystyryllithium reacts with butadiene much more rapidly than does polybutadienyllithium. Until we know the two equilibrium constants for self-association we cannot find out if the increased rate is due to greater intrinsic reactivity or to a higher concentration of free polystyryllithium. In polar solvents or in hydrocarbon solvents in the presence of small amounts of ethers, these difficulties do not arise as self-association is no longer important. 

A parallel situation is encountered for the copolymerization of 1,3-butadiene with isoprene. McGrath et al. 251) have shown that in homopolymerizations, under equivalent conditions, isoprene exhibits a rate constant which is more than five times larger than that observed for butadiene. However, butadiene is favored in the copolymeriza-tion. The available reactivity ratios for various diene and styrenyl monomer pairs in hydrocarbon solvents are listed in Table 24. 

An alternative rationale for the unusual RLi (hydrocarbon) copolymerization of butadiene and styrene has been presented by O Driscoll and Kuntz (71). Rather than invoking selective solvation, these workers stated that classical copolymerization kinetics is sufficient to explain this copolymerization. They adapted the copolymer-composition equation, originally derived from steady-state assumptions for free-radical copolymerizations, to the anionic copolymerization of butadiene and styrene. Equation (20) describes the relationship between the instantaneous copolymer composition c/[M,]/rf[M2] with the concentrations of the two monomers in the feed, M, and M2, and the reactivity ratios, rt, r2, of the monomers. The rx and r2 values are measures of the preference of the growing chain ends for like or unlike monomers. 

As previously noticed, butyl rubber (HR), poly(methylpropene-co-2-methyl-1,3-butadiene), is a random copolymer of isobutene and 0.7-2.2 mol% of isoprene. The industrial slurry process used all over the world consists in a low-temperature copolymerization initiated by A1C13 in meth-ylchloride. In contrast to 1,3-butadiene, isoprene copolymerizes readily with the more reactive isobutene. Reactivity ratios of the pair isobutene-isoprene, ri = 2.5 0.5 and r2 = 0.4 0.1, measured at the conditions of industrial process [10], show that the copolymerization behaves ideally (ri-r2 = 1), and, at the used low concentration of isoprene, isolated units of this latter comonomer are randomly distributed along the chain with 90% M-p-aiw-enchainment [52,53] ... 

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... 

Rank the following monomers in order of their increased tendency to alternate in copolymerization with butadiene and explain your reasoning vinyl acetate, styrene, acrylonitrile, and methyl methacrylate, Hint Use Q-e values if reactivity ratios are not readily available.)... 

An examination of reported reactivity ratios (Table 6) shows that the behaviour rj > 1, r2 1 or vice versa is a common feature of anionic copolymerization. Only in copolymerizations involving the monomers 1,1-diphenylethylene and stilbene, which cannot homopolymerize, do we find <1, r2 <1 [212—215], and hence the alternating tendency so characteristic of many free radical initiated copolymerizations. Normally one monomer is much more reactive to either type of active centre in the order acrylonitrile > methylmethacrylate > styrene > butadiene > isoprene. This is the order of electron affinities of the monomers as measured polarographically in polar solvents [216, 217]. In other words, the reactivity correlates well with the overall thermodynamic stability of the product. Variations of reactivity ratio occur with different solvents and counter-ions but the gross order is predictable. 

Successive addition of 1,3-butadiene and isocyanide to the solution of the Ni catalyst forms a product with flexible polybutadiene blocks and rigid polyisocyanide blocks. Although the reaction of butadiene and isoprene in the presence of CoCl2/MAO causes homopolymerization of butadiene, the reaction catalyzed by CoCl2/MAO/PPh3 affords a copolymer with 1,2-butadiene and 3,4-isoprene units [94]. The monomer reactivity ratios indicate higher reactivity of butadiene than isoprene. 

A value of unity (or nearly unity) for the monomer reactivity ratio signifies that the rate of reaction of the growing chain radicals towards each of the monomers is the same, i.e. kn ki2 and 22 — A 2i and the copolymerization is entirely random. In other words, both propagating species and M2 have little or no preference for adding either monomer. The copolymer composition is the same as the comonomer feed with a completely random placement of the two monomers along the copolymer chain. Such behavior is referred to as Bemoullian. Free-radical copolymerization of ethylene and vinyl acetate and that of isoprene and butadiene are examples of such a system, but this is not a common case. Random monomer distributions are obtained more generally in a situation where both types of radicals have exactly the same preference for the same type of monomer as represented by the relationship... 

Some commercially important examples of random free-radical copolymerizations include styrene (ri = 0.8)-butadiene (r2 = 1.4) for which rir2 = 1.1 and vinyl chloride (ri = 1.4)-vinyl acetate (r2 = 0.65) for which rir2 = 0.9. In these products the proportion of a given monomer in the copolymer depends on the feed concentrations and reactivity ratios [Eq. (7.11)]. 

The copolymers of o- and p-chlorostyrene with butadiene can be prepared by anionic initiators (21). Since the reactivity ratios of 1,3-butadiene or o- and p-chlorostyrene are close to unity, the resulting copolymers have a constant composition. 

The simplest diblock polymer, styrene/butadiene block polymer, is formed when the two monomers are charged into a batch reaction along with the catalysts. The reactivity ratios are such that the butadiene polymerizes first and with almost total exclusion of any styrene present. Only after all of the butadiene monomer has been consumed does the bulk of the styrene enter the polymer chain. 

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