Homopolymerization butadiene

Copolymerization. The copolymerization of butadiene-styrene with alkyllithium initiator has drawn considerable attention in the last decade because of the inversion phenomenon (12) and commercial importance (13). It has been known that the rate of styrene homopolymerization with alkyllithium is more rapid than butadiene homopolymerization in hydrocarbon solvent. However, the story is different when a mixture of butadiene and styrene is used. The propagating polymer chains are rich in butadiene until late in reaction when styrene content suddenly increases. This phenomenon is called inversion because of the rate of butadiene polymerization is now faster than the styrene. As a result, a block copolymer is obtained in this system. However, the copolymerization characteristic is changed if a small amount of polar solvent... 

Kuntz (33) reported on the copolymerization of butadiene and styrene in n-heptane at 30° using n-butyl lithium. Although styrene homopolymerized six times faster than butadiene, the copolymerization rate was initially the same as that of butadiene homopolymerization and then increased markedly. It was found that about 80% of the styrene remained when 90% of the butadiene was consumed and that the increase in rate coincided with the almost complete consumption of butadiene. With added tetrahydrofuran, the rate of polymerization was faster and about 30% styrene was found in the initial copolymer. 

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... 

The most important polymerization variables which affect the molecular structure of poly butadienes, prepared by the Ba/Mg/Al catalyst, are the ratio of barium salt to Bu2Mg (Ba /Mg ), the polymerization temperature and catalyst concentration. The effect of these variables on -content and molecular weight is summarized in Table 7. Whilst the trends are shown for butadiene homopolymerization, essentially equivalent responses have been obtained for copolymerizations of styrene and butadiene. 

However, the addition of butadiene into the polymerization systems substantially lowers the catalytic activity, though this catalyst is quite efficient for the butadiene homopolymerization. For the EB and PB copolymerizations, the changes of the catalytic activities against butadiene content both appear as the saddle curves, as shown in Fig.6. The reductions of the catalytic activities in the presence of butadiene are attributable to the stronger action of coordination of butadiene monomer towards Ti active centers and lower chain-growing rate of the addition butadiene. The BPB terpolymerization showed a similar result. 

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). 

Homopolymerization of butadiene can proceed via 1,2- or 1,4-additions. The 1,4-addition produces the geometrically distinguishable trans or cis stmctures with internal double bonds on the polymer chains, 1,2-Addition, on the other hand, yields either atactic, isotactic, or syndiotactic polymer stmctures with pendent vinyl groups (Eig. 2). Commercial production of these polymers started in 1960 in the United States. Eirestone and Goodyear account for more than 60% of the current production capacity (see Elastomers, synthetic-polybutadiene). 

All this evidence suggests that the radical produced from 2-vinylfuran is a rather strongly stabilized entity, compared with those of more common monomers, and is therefore, not very active in homopolymerization. On the other hand, because of its relative stability, it does not add easily to monomers like styrene, vinylidene chloride or butadiene, and thus the copolymerization rates are also low. Aso and Tanaka86) calculated the values of Q and e as 2.0 and 0.0, respectively. 

This paper is concerned with some of our experiments in this field. Our purpose was to obtain polymers with extremely high stereoregularity. In the first part we will report on the homopolymerization of butadiene with f-transition metal catalysts. 

Homopolymerization of Butadiene. It appeared to us that catalysts based on f-transition metals were the ones most likely to enable us to prepare polybutadiene with an extremely high cis content. We began by investigating catalysts based on uranium compounds. Two such systems were known at the beginning of our work. 

The stereochemistry of the 1,3-diene units in copolymerization is expected to be the same as in homopolymerization of 1,3-dienes. This expectation has been verified in styrene-1,3-butadiene copolymerizations at polymerization temperatures of —33 to 100°C [Binder, 1954],... 

In the presence of a dissolved polymer, the radical polymerization of a monomer by thermal decomposition of an initiator results in a mixture of homopolymerization and graft polymerization [Brydon et al., 1973, 1974 Ludwico and Rosen, 1975, 1976 Pham et al., 2000 Russell, 2002], Polymer radicals (XXX), formed by chain transfer between the propagating radical and polymer, initiate graft polymerization of styrene. The product (XXXI) consists of polystyrene grafts on the 1,4-poly-1,3-butadiene backbone. Polymer radicals are also formed... 

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 homopolymerizations were all run in 28 oz. beverage bottles. The bottles were baked for at least 24 hrs. and then capped with crown, three-hole caps and rubber liners. Cooling of the bottles was effected while purging with nitrogen. After cooling, the bottles were charged with the butadiene blend, the heterogeneous initiator-hexane suspension and modifiers. 

In homopolymerization initiated by sec-butyl-lithium in hexane, isoprene is a more active monomer than butadiene (with kj 5.53 x 10-5 sec l vs. ki 0.98 x 10 sec- at 20eC). This is also true for reactions at 30° and 40°C. The apparent activation energy for both monomers has been found to be roughly the same,... 

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... 

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. 

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 ... 

The monomer addition scheme, shown at the top, requires an initiator which is capable of removing a hydrogen atom from the allylic position of the butadiene, resonance stabilization of the radical from AIBN does not permit this initiator to effect this reaction while benzoyl peroxide is capable of reaction to remove a hydrogen atom and initiate the reaction. On the other hand the polymeric radical addition scheme requires that homopolymerization of the monomer be initiated and this macroradical then attack the polymer and lead to the formation of the graft copolymer. Huang and Sundberg explain that the reactivity of the monomer... 

The butadiene-styrene system alone has received the detailed study required to give a clearer picture of the mechanism. The results should, however, be similar for the other systems. The two homopolymerization rates are easily measured. The exchange rate between two active centres can be measured by forming a solution of polybutadienyllithium or of polystyryllithium and allowing it to react with the other monomer. It is convenient to measure the rate spectroscopically from the rate of... 

Similar results are obtained in cyclohexane at 40° (rx = 26 r2 < 0.04). Under these conditions the reaction of polystyryllithium with butadiene occurs virtually instantaneously and with comparable concentrations of both monomers present the homopolymerization rate of styrene can be neglected (46). Hence ... 

An interesting effect of the ionic factors of the polymerization was found by Kuntz (59). He has shown that the homopolymerization of styrene using butyllithium catalysts is six times as rapid as that of butadiene. However, in copolymerization, butadiene polymerized initially at its own rate with relatively small amounts of the styrene being consumed. Only after 90% of the butadiene had been consumed, the styrene began to polymerize at its own rate. THF increased the rate of the polymerization but had little effect on the rate of butadiene to styrene which is polymerized. The butadiene structure is little influenced by copolymerization. The homopolymer contained 44% cis-1.4, 7% 1.2 and 49% trans-1.4 while the butadiene units of the butadiene copolymers contained 40% cis 1.4, 7% 1.2 and 53% trans-1.4 groups. 

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. 

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. 

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