Homopolymerization of Isoprene

In order to gain independence from NR imports the former Soviet Union produced synthetic IR in a large scale whereas in the western world the production of synthetic IR always stayed below these volumes. Two reasons account for this Firstly, in a free market IR faces strong competition with NR and secondly the monomer IP has never been readily available at a price which has allowed competitive IR production. 

The preparation of IR by the use of Nd catalysts was already mentioned in the early patents on Nd-catalyzed diene polymerization [ 154,155,325,326, 472-477]. Because of the limited availability of IP monomer in the western world and due to non-competitive prices of IP the option of manufacturing IR by the application of Nd-catalysis has not been the focus of the rubber producing industry during the past decades. But as a consequence of short NR supplies and strong increases of NR prices the application of Nd catalysis to the polymerization of IP has attracted new interest. In this context the 

Beside this work of Michelin, the application of Nd-catalyzed gas-phase polymerization to IR by Zhang et al. confirm the features elaborated for the gas-phase polymerization of BD (Sect. 3.2) [479,480]. 

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Cationic Polymerization. A small amount of isoprene is cationicaUy copolymerized with isobutjdene in the commercial process for making butyl mbber, wherein the isoprene provides the unsaturation required for sulfur vulcanization. Homopolymerization of isoprene by cationic catalysts can lead to cyclized products and loss of unsaturation (70,71), as for example, during polymerizations initiated by boron trifluoride (72). Cationic polymerization of isoprene with BF, SnCl or AlCl catalysts in pentane, chloroform, or ethjibenzene from —78 to 30°C at around 50% conversion gives about 90% /n7 -l,4-polyisoprene stmcture the balance is 1,2 and 3,4 microstmcture (no microstmcture) (73). An insoluble powder was formed by... 

Polymers account for about 3—4% of the total butylene consumption and about 30% of nonfuels use. Homopolymerization of butylene isomers is relatively unimportant commercially. Only stereoregular poly(l-butene) [9003-29-6] and a small volume of polyisobutylene [25038-49-7] are produced in this manner. High molecular weight polyisobutylenes have found limited use because they cannot be vulcanized. To overcome this deficiency a butyl mbber copolymer of isobutylene with isoprene has been developed. Low molecular weight viscous Hquid polymers of isobutylene are not manufactured because of the high price of purified isobutylene. Copolymerization from relatively inexpensive refinery butane—butylene fractions containing all the butylene isomers yields a range of viscous polymers that satisfy most commercial needs (see Olefin polymers Elastomers, synthetic-butylrubber). 

First-Order Propagation Rate Constants (ki) of the Homopolymerizations of Butadiene and Isoprene... 

It has been emphasized in the copolymerization of styrene with butadiene or isoprene in hydrocarbon media, that the diene is preferentially incorporated. (7,9,10) The rate of copolymerization is initially slow, being comparable to the homopolymerization of the diene. After the diene is consumed, the rate increases to that of the homopolymerization of styrene. Analogously our current investigation of the copolymerization of butadiene with isoprene shows similar behavior. However, the... 

By measuring the kinetic rate of second stage reaction after inflection, one can observe that rate is very analogous to the homopolymerization rate of isoprene. The data are listed in Table III, and can also be detected by the straight portion of Curves 2 and 3 after inflection. The "inversion" phenomenon can be easily explained by the fact that, although the isoprene is... 

Lithium and alkyllithiums in aliphatic hydrocarbon solvents are also used to initiate anionic polymerization of 1,3-butadiene and isoprene.120,183-187 As 1,3-butadiene has conjugated double bonds, homopolymerization of this compound can lead to several polymer structures. 1,4 Addition can produce cis-1,4- or tram-1,4-polybutadiene (19, 20). 1,2 Addition results in a polymer backbone with vinyl groups attached to chiral carbon atoms (21). All three spatial arrangements (isotactic, syndiotactic, atactic) discussed for polypropylene (see Section 13.2.4) are possible when polymerization to 1,2-polybutadiene takes place. Besides producing these structures, isoprene can react via 3,4 addition (22) to yield polymers with the three possible tacticites ... 

Similarly, whereas the Diels-Alder reaction is accelerated at elevated temperatures, under polymerization conditions, the reaction of isoprene and maleic anhydride is extremely exothermic, and the relative amounts of adduct and copolymer are temperature dependent. It has been reported (81) that the rate of copolymerization is very fast compared with the rate of homopolymerization of the diene or the dienophile, and the energy of activation is approximately 5 kcal./mole. Although the rate of copolymerization increases at elevated temperatures, the simultaneous adduct formation which also occurs more readily at elevated temperatures limits the maximum rate to lower temperatures. 

Homopolymerization of complexed acrylonitrile initiated by a one-electron transfer from isoprene monomer ... 

Spontaneous homopolymerization of the (isoprene-complexed acrylonitrile) complex ... 

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... 

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 further possibility consists of subjecting the catalyst to incipient polymerization with small quantities of styrene, isoprene, or butadiene (organic alkali compounds also can be used for this purpose), then adding dropwise a mixture of cyanoprene and selected monomers at low temperatures. If no incipient polymerization takes place, metallo-organic catalysts will only give a homopolymerization of the cyanoprene. 

Strong evidence in support of the carbocationic mechanism in homopolymerizations of isobutene by A, however, is gained by analysis of the product formed on copolymerization of isobutene and isoprene. This commercially important copolymeH is manufactured via a carbocationic process Initiated by AICI3, and contains isoprene ( 1%) incorporated only in a... 

For example, methyl methacrylate block copolymers are much less studied than those of styrene. Anion chain transfer occurs at the pendent ester group, drastically reducing the yield of block copolymers. Poly(methyl methacrylate-b-isoprene) has been prepared, however, by using an ingenious chain cap of l,l -diphenylethyl-ene(27,28). i l diphenylethylene will not anionically homopolymerize, therefore it adds only one mer to the macroanion. This anion is more stable in the presence of methyl methacrylate, but will initiate further polymerization. Other workers have reported the preparation of isoprene-methyl methacrylate block copolymers by sequential addition to "living" polyisoprene anions(29,30),... 

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). 

Monomer reactivity ratios and copolymer compositions in many anionic copolymerizations are altered by changes in the solvent or counterion. Table 6-12 shows data for styrene-isoprene copolymerization at 25°C by n-butyl lithium [Kelley and Tobolsky, 1959]. As in the case of cationic copolymerization, the effects of solvent and counterion cannot be considered independently of each other. For the tightly bound lithium counterion, there are large effects due to the solvent. In poor solvents the copolymer is rich in the less reactive (based on relative rates of homopolymerization) isoprene because isoprene is preferentially complexed by lithium ion. (The complexing of 1,3-dienes with lithium ion is discussed further in Sec. 8-6b). In good solvents preferential solvation by monomer is much less important and the inherent greater reactivity of styrene exerts itself. The quantitative effect of solvent on copolymer composition is less for the more loosely bound sodium counterion. 

In the current study of the homopolymerization and copolymerization of butadiene and isoprene by secondary-butyllithium in hexane the following conclusions can be made. 

Isoprene is a more active monomer than butadiene in homopolymerization, but the apparent activation energy of the propagation reaction is 19.2 kcal/mole for both monomers. 

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. 

Homopolymerization and Copolymerization of Substituted Butadienes (other than Isoprene)... 

Korotkov and Rakova (53) found that butadiene was more active in copolymerization with isoprene with lithium catalyst, although in homopolymerization isoprene is three times faster. Korotkov and Chesnokova (33) studied the copolymerization of butadiene and styrene with n-butyl lithium in benzene. Butadiene polymerized before much of the styrene was consumed. They claimed the formation of block polymers consisting initially of polybutadiene and the polystyrene chain attached. 

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