Polymerization butadiene and isoprene

The anionic polymerization of 1,3-dienes yields different polymer structures depending on whether the propagating center is free or coordinated to a counterion [Morton, 1983 Quirk, 2002 Senyek, 1987 Tate and Bethea, 1985 Van Beylen et al., 1988 Young et al., 1984] Table 8-9 shows typical data for 1,3-butadiene and isoprene polymerizations. Polymerization of 1,3-butadiene in polar solvents, proceeding via the free anion and/or solvent-separated ion pair, favors 1,2-polymerization over 1,4-polymerization. The anionic center at carbon 2 is not extensively delocalized onto carbon 4 since the double bond is not a strong electron acceptor. The same trend is seen for isoprene, except that 3,4-polymerization occurs instead of 1,2-polymerization. The 3,4-double bond is sterically more accessible and has a lower electron density relative to the 1,2-double bond. Polymerization in nonpolar solvents takes place with an increased tendency toward 1,4-polymerization. The effect is most pronounced with... 

The pre-eminent interest in lanthanides for the polymerization and copolymerization of conjugated diolefins is demonstrated by a large scientific and patent literature. A recent paper by one of us (24) reviewed the main worldwide contributions on butadiene and isoprene polymerization. Very recent contributions in this field come from Hsieh and Yeh (25). They observed that the butadiene polymerization in n-pentane evolves as a slurry process, the cis-polybutadiene being insoluble in the light hydrocarbon. They also measured a lower activation energy for Nd-based than for conventional Ziegler-Natta catalysts foi- both butadiene and isoprene polymerization. 

The kinetic curves for the polymerization of butadiene and isoprene with the titanium catalytic system in the absence of US irradiation (Method 1) are almost coincident (Fig. 4.9). US irradiation (Method 2) brings about an increase in the initial rates of butadiene and isoprene polymerization and accelerates accumulation of the polymer in the system. In this case, the initial rates of polymerization of butadiene and isoprene increase owing to an increase in the rate constant of chain propagation without any marked changes in the concentration of active sites. This result correlates with the estimation of dispersity of the catalytic system, specifically, with the absence of changes in the most probable size of catalyst particles. In the case of polymerization of butadiene via Method 2, the munerical values of rate constants of chain termination increase. 

Commercially, anionic polymerization is limited to three monomers styrene, butadiene, and isoprene [78-79-5], therefore only two useful A—B—A block copolymers, S—B—S and S—I—S, can be produced direcdy. In both cases, the elastomer segments contain double bonds which are reactive and limit the stabhity of the product. To improve stabhity, the polybutadiene mid-segment can be polymerized as a random mixture of two stmctural forms, the 1,4 and 1,2 isomers, by addition of an inert polar material to the polymerization solvent ethers and amines have been suggested for this purpose (46). Upon hydrogenation, these isomers give a copolymer of ethylene and butylene. 

The refined grade s fastest growing use is as a commercial extraction solvent and reaction medium. Other uses are as a solvent for radical-free copolymerization of maleic anhydride and an alkyl vinyl ether, and as a solvent for the polymerization of butadiene and isoprene usiag lithium alkyls as catalyst. Other laboratory appHcations include use as a solvent for Grignard reagents, and also for phase-transfer catalysts. 

Troublesome amounts of C and Q acetylenes are also produced in cracking. In the butadiene and isoprene recovery processes, the acetylenes in the feed are either hydrogenated, polymerized, or extracted and burned. Acetylene hydrogenation catalyst types include palladium on alumina, and some non-noble metals. 

When polymerizing dienes for synthetic rubber production, coordination catalysts are used to direct the reaction to yield predominantly 1,4-addition polymers. Chapter 11 discusses addition polymerization. The following reviews some of the physical and chemical properties of butadiene and isoprene. 

Emulsion polymerization is the most important process for production of elastic polymers based on butadiene. Copolymers of butadiene with styrene and acrylonitrile have attained particular significance. Polymerized 2-chlorobutadiene is known as chloroprene rubber. Emulsion polymerization provides the advantage of running a low viscosity during the entire time of polymerization. Hence the temperature can easily be controlled. The polymerizate is formed as a latex similar to natural rubber latex. In this way the production of mixed lattices is relieved. The temperature of polymerization is usually 50°C. Low-temperature polymerization is carried out by the help of redox systems at a temperature of 5°C. This kind of polymerization leads to a higher amount of desired trans-1,4 structures instead of cis-1,4 structures. Chloroprene rubber from poly-2-chlorbutadiene is equally formed by emulsion polymerization. Chloroprene polymerizes considerably more rapidly than butadiene and isoprene. Especially in low-temperature polymerization emulsifiers must show good solubility and... 

The distinction between the rates of homo- and copolymerization apparently is misapprehended by some workers. For example, a recent review 141) discusses the results of McGrath 142) who reported butadiene to be more reactive in polymerization in hexane than isoprene, whether with respect to lithium polybutadiene or polyisoprene, although the homopropagation of lithium polyisoprene in hexane was found to be faster than of polybutadiene. The miscomprehension led to the erroneous statement14l) McGrath 142) results regarding the rate constants for butadiene and isoprene place in clear perspective the bizarre assertion 140) that butadiene will be twice as reactive as isoprene (in anionic co-polymerization). 

Propagation constants for butadiene and isoprene were determined from rate of polymerization per particle in emulsion polymerization. 

Conjugated dienes are among the most significant building blocks both in laboratories and in the chemical industry [1], Especially, 1,3-butadiene and isoprene are key feedstocks for the manufacture of polymers and fine chemicals. Since the discovery of the Ziegler-Natta catalyst for the polymerizations of ethylene and propylene, the powerful features of transition metal catalysis has been widely recognized, and studies in this field have been pursued very actively [2-7]. 

Some of the important results for butadiene are summarized in Table XV. The most efficient system identified was for cis-polymerization using 1 1 molar ratio of (XXI) with trifluoroacetic acid. An even more remarkable observation, however, was the almost complete suppression of the cis-polymerization in favor of trans-polymerization processes on addition of triphenylphosphite to the mixture of (XXI) and trifluoroacetic acid. More recently (89), Durand and Dawans have synthesized the trifluoroacetates (XXIII) where R = H and C9H15, and these were shown to be catalytically active as well as exhibiting some specificity in polymerization of butadiene and isoprene. 

Ionic Polymerization. Ionic polymerizations, especially cationic polymerizations, are not as well understood as radical polymerizations because of experimental difficulties involved in their study. The nature of the reaction media is not always clear since heterogeneous initiators are often involved. Further, it is much more difficult to obtain reproducible data because ionic polymerizations proceed at very fast rates and are highly sensitive to small concentrations of impurities and adventitious materials. Butyl rubber, a polymer of isobutene and isoprene, is produced commercially by cationic polymerization. Anionic polymerization is used for various polymerizations of 1,3-butadiene and isoprene. 

Anionic polymerization can be initiated by a variety of anionic sources such as metal alkoxides, aryls, and alkyls. Alkyllithium initiators are among the most useful, being employed commercially in the polymerization of 1,3-butadiene and isoprene, due to their solubility in hydrocarbon solvents. Initiation involves addition of alkyl anion to monomer... 

As to the first route, we started in 1969 (1) in investigating unconventional transition metal complexes of the 5 and 4f block elements of periodic table, e.g., actinides and lanthanides as catalysts for the polymerization of dienes (butadiene and isoprene) with an extremely high cis content. Even a small increase of cistacticity in the vicinity of 100% has an important effect on crystallization and consequently on elastomer processability and properties (2). The f-block elements have unique electronic and stereochemical characteristics and give the possibility of a participation of the f-electrons in the metal ligand bond. 

A common feature of catalysts based on 4 and 5f block elements is that of being able to polymerize both butadiene and isoprene to highly cistactic polymers, independently of the ligands involved. Butadiene, in particular, can reach a cistacticity as high as 99% with uranium based catalysts (3) and cistacticity of > 98% with neodymium based catalysts (4). This high tacticity does not change with the ligand nature (Fig. 1) in contrast to conventional catalysts based on 3-d block elements. A second feature of f-block catalysts is that the cis content of polymer is scarcely... 

This gives rise to dual valency state (+3 and +4) (23). As to the activity of lanthanide based catalysts we confirm a singular behavior that has been already reported by Chinese scientists (22) and that is summarized in Fig. 9. The activity of lanthanides in promoting the polymerization of butadiene and isoprene shows a large maximum centered on neodymium, the only exception being represented by samarium and europium that are not active, reasonably because they are reduced to bivalent state by aluminum alkyls, as pointed out by Tse-chuan and associates (22). 

Butadiene and isoprene have two double bonds, and they polymerize to polymers with one double bond per monomeric unit. Hence, these polymers have a high degree of unsaturation. Natural rubber is a linear cis-polyisoprene from 1,4-addition. The corresponding trans structure is that of gutta-percha. Synthetic polybutadienes and polyisoprenes and their copolymers usually contain numerous short-chain side branches, resulting from 1,2-additions during the polymerization. Polymers and copolymers of butadiene and isoprene as well as copolymers of butadiene with styrene (GR-S or Buna-S) and copolymers of butadiene with acrylonitrile (GR-N, Buna-N or Perbunan) have been found to cross-link under irradiation. 

Diene polymers refer to polymers synthesized from monomers that contain two carbon-carbon double bonds (i.e., diene monomers). Butadiene and isoprene are typical diene monomers (see Scheme 19.1). Butadiene monomers can link to each other in three ways to produce ds-1,4-polybutadiene, trans-l,4-polybutadi-ene and 1,2-polybutadiene, while isoprene monomers can link to each other in four ways. These dienes are the fundamental monomers which are used to synthesize most synthetic rubbers. Typical diene polymers include polyisoprene, polybutadiene and polychloroprene. Diene-based polymers usually refer to diene polymers as well as to those copolymers of which at least one monomer is a diene. They include various copolymers of diene monomers with other monomers, such as poly(butadiene-styrene) and nitrile butadiene rubbers. Except for natural polyisoprene, which is derived from the sap of the rubber tree, Hevea brasiliensis, all other diene-based polymers are prepared synthetically by polymerization methods. 

Hence measurements have been made on unsubstituted allyl alkali-metal compounds, and also on neopentylallyl (I, 5,5-dimethylhexen-2-) and neopentylmethallyl (II, 2,5,5-trimethyl-hexen-2-) alkali-metal compounds which are models of the polymerizing chain end in the anionic polymerization of butadiene and isoprene respectively. 

Many substituents stabilize the monomer but have no appreciable effect on polymer stability, since resonance is only possible with the former. The net effect is to decrease the exothermicity of the polymerization. Thus hyperconjugation of alkyl groups with the C=C lowers AH for propylene and 1-butene polymerizations. Conjugation of the C=C with substituents such as the benzene ring (styrene and a-methylstyrene), and alkene double bond (butadiene and isoprene), the carbonyl linkage (acrylic acid, methyl acrylate, methyl methacrylate), and the nitrile group (acrylonitrile) similarly leads to stabilization of the monomer and decreases enthalpies of polymerization. When the substituent is poorly conjugating as in vinyl acetate, the AH is close to the value for ethylene. 

Alkyllithium compounds are probably the most useful of these initiators, employed com-merically in the polymerizations of 1,3-butadiene and isoprene. Initiation proceeds by addition of the metal alkyl to monomer... 

The stability of polystyryl carbanions is greatly decreased in polar solvents such as ethers. In addition to hydride elimination, termination in ether solvents proceeds by nucleophilic displacement at the C—O bond of the ether. The decomposition rate of polystyryllithium in THF at 20°C is a few percent per minute, but stability is significantly enhanced by using temperatures below 0°C [Quirk, 2002], Keep in mind that the stability of polymeric carbanions in the presence of monomers is usually sufficient to synthesize block copolymers because propagation rates are high. The living polymers of 1,3-butadiene and isoprene decay faster than do polystyryl carbanions. 

In the case of the much-studied organolithium polymerization of butadiene and isoprene, the effect of ethers and other polar solvents is to change the chain structure from one containing... 

Figures 1 and 2 show the dependence of polymer microstructure on the molecular weight of the polymer and therefore on the initial initiator concentration. The polymerization temperature also has an effect on the microstructure as can be seen in Figure 3 for polybutadiene. The overall heat activation energy leading to 1,2 addition is greater than that leading to 1,4 addition.2 IZ In summary, the stereochemistry of polymerization of butadiene and isoprene is sensitive to initiator level, polymerization temperature and solvent. The initiator structure (i.e., organic moiety of the initiator), the monomer concentration and conversion have essentially no effect on polymer microstructure.

A thermoplastic elastomer similar to the above structures was made by utilizing conjugated 1,3 diolefins that can be polymerized anionically. The work of Halasa and co-workers(2 ) illustrate the point. These workers polymerized 1,3-butadiene and isoprene to produce a diblock copolymer of poly(butadiene)-poly(isoprene)... 

More recently ERUSSALIMSKY et al (15) investigated the polymerization of 2,3-dimethylbutadiene induced by oligo-2,3 dimethyl-butadienyllithium/TMEDA. Contrary to butadiene and isoprene, catalytic amounts of TMEDA decrease the propagation rate of dime- thylbutadiene. 

The kinetic results obtained by dilatometry for the polymerization of butadiene and isoprene are shown in Figures 3 and 4. 

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