Polymerization of dienes

The polymerization of conjugated dienes is of special interest. Two different types of polymerization reactions occur with 1,3-dienes such as 1,3-butadiene, isoprene(2-methyl- 

One of the polymerization routes involves polymerization of one or the other of the double bounds in the usual manner. The other route involves the two double bonds acting in a unique and concerted manner. Thus addition of an initiating radical to a 1,3-diene such as 1,3-butadiene yields an allylic radical with the two equivalent resonance forms LI and LII 

Propagation may then occur by attachment of the successive monomer units either at carbon 2 (propagation via LI) or at carbon 4 (propagation via LII). The two modes of polymerization are referred to as 1,2-polymerization and 1,4-polymerization, respectively. The polymers obtained have the repeating structures 

The polymerization and copolymerization of 1,3-dienes is of commercial importance in the annual production of over 4 billion pounds of elastomers and about 2 billion pounds of plastics in the United States (Secs. 6-8 and 8-10). 

By the end of the 19th century isoprene had been converted to a rubber-like material by Tilden, Bouchardat and others. During the first decade of the 20th century other dienes such as butadiene and dimethyl butadiene had been polymerized. By 1909 F. Hofmann and his associates had filed patents on the heat polymerization of these monomers. 

In 1910 the observation was made both in Germany (by Harries) and in England (by Strange and Matthews) that sodium metal was an effective catalyst for polymerization at lower temperatures. Whilst this discovery was well publicized the disclosure by Labhardt of BASF that alkali metal alkyls were also effective polymerization catalysts went apparently unnoticed. 

As a consequence the bulk of research effort from 1930 to 1950 was concentrated on free-radical polymerization, a relatively unselective technique which results in various isomeric forms being incorporated into a single chain. Whilst the microstructure could be varied somewhat by, for example, changes in polymerization temperature, the free-radical polymers were by no means stereoregular. 

A notable exception to the emphasis on free-radical polymerization studies was provided by Karl Ziegler and his co-workers who extended the study of the alkali metal polymerization of dienes to include metals other than sodium and various metal alkyls. Of particular interest were the results obtained with the simplest Group I alkali metal, lithium. It was found that when lithium metal was used as a polymerization initiator 1,4- structures predominated over 1,2-polymers. It was also found that polymerization in hydrocarbon solvents further favoured production 1,4- structures whilst polymerization in polar liquids such as ethers and amines often favoured the formation of 1,2- units. It was also found that reaction of lithium with monomer led to the production of an organo-lithium compound which made feasible homogeneous polymerization—a discovery which eventually led to commercial exploitation. 

The commercially viable diene polymers may be classed conveniently into two groups (a) those based on lithium catalyst systems and (b) those based on Ziegler-Natta systems. 

The reaction is stereospecific in yielding a polymer with an all-cts configuration. 

Conjugated dienes undergo nucleophilic attack more easily than simple alkenes because they form more stable allyl carbanions, 

Like the allyl cation, the allylic anion is stabilized by charge delocalization through extended -n bonding. 

Conjugated diene polymers are modified and improved by copolymerizing them with other unsaturated compounds, such as acrylonitrile, H2C==CH—C=N. 

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The final type of isomerism we take up in this section involves various possible structures which result from the polymerization of 1,3-dienes. Three important monomers of this type are 1,3-butadiene, 1,3-isoprene, and 1,3-chloroprene, structures [X]-[XII], respectively ... 

The use of alkaU metals for anionic polymerization of diene monomers is primarily of historical interest. A patent disclosure issued in 1911 (16) detailed the use of metallic sodium to polymerize isoprene and other dienes. Independentiy and simultaneously, the use of sodium metal to polymerize butadiene, isoprene, and 2,3-dimethyl-l,3-butadiene was described (17). Interest in alkaU metal-initiated polymerization of 1,3-dienes culminated in the discovery (18) at Firestone Tire and Rubber Co. that polymerization of neat isoprene with lithium dispersion produced high i7j -l,4-polyisoprene, similar in stmcture and properties to Hevea natural mbber (see ELASTOLffiRS,SYNTHETic-POLYisoPRENE Rubber, natural). 

Analogous principles should apply to ionically propagated polymerizations. The terminus of the growing chain, whether cation or anion, can be expected to exhibit preferential addition to one or the other carbon of the vinyl group. Poly isobutylene, normally prepared by cationic polymerization, possesses the head-to-tail structure, as already mentioned. Polystyrenes prepared by cationic or anionic polymerization are not noticeably different from free-radical-poly-merized products of the same molecular weights, which fact indicates a similar chain structure irrespective of the method of synthesis. In the polymerization of 1,3-dienes, however, the structure and arrangement of the units depends markedly on the chain-propagating mechanism (see Sec. 2b). 

In the free radical polymerization of 1,3-dienes, 1,4 addition dominates 1,2 addition. The proportion of 1,2 (and 3,4 )units decreases in passing from butadiene to its methyl and chlorine substitution products isoprene, 2,3-dimethylbutadiene and chloroprene. The trans configuration of the 1,4 unit from butadiene is formed preferentially, the proportion of trans increasing rapidly with lowering of the polymerization temperature. 

The complex [Cp2Zr(OTf)2(thf)] is a catalyst for the Diels—Alder reactions of 105 compared to the corresponding thermal reactions [82,83] (Scheme 8.45). The isomer ratio of the reaction products (endo/exo or regioisomers) is higher in catalyzed than in thermal reactions. However, because the zir-conocenium triflate is also a catalyst for the polymerization of 1,3-dienes, the Diels—Alder reaction is sometimes completely suppressed in the case of less reactive dienophile-diene combinations. 

Most monomers contain only one polymerizable group. There are some monomers with two polymerizable groups per molecule. Polymerization of such monomers can lead to more than one polymer structure. The polymerization of 1,3-dienes, of large industrial importance, is discussed in Sec. 8-6. 1-Substituted-l,2-dienes (allenes) undergo polymerization through both the substituted and unsubstituted double bonds... 

Infrared spectroscopy has been used for quantitatively measuring the amounts of 1,2-, 3,4-, cis-1,4-, and trans-1,4-polymers in the polymerization of 1,3-dienes its use for analysis of isotactic and syndiotactic polymer structures is very limited [Coleman et al., 1978 Tosi and Ciampelli, 1973]. Nuclear magnetic resonance spectroscopy is the most powerful tool for detecting both types of stereoisomerism in polymers. High-resolution proton NMR and especially 13C NMR allow one to obtain considerable detail about the sequence distribution of stereoisomeric units within the polymer chain [Bovey, 1972, 1982 Bovey and Mirau, 1996 Tonelli, 1989 Zambelli and Gatti, 1978],... 

Traditional Ziegler-Natta and metallocene initiators polymerize a variety of monomers, including ethylene and a-olefins such as propene, 1-butene, 4-methyl-1-pentene, vinylcyclo-hexane, and styrene. 1,1-Disubstituted alkenes such as isobutylene are polymerized by some metallocene initiators, but the reaction proceeds by a cationic polymerization [Baird, 2000]. Polymerizations of styrene, 1,2-disubstituted alkenes, and alkynes are discussed in this section polymerization of 1,3-dienes is discussed in Sec. 8-10. The polymerization of polar monomers is discussed in Sec. 8-12. 

The polymerization of 1,3-dienes is more complicated than that of alkenes because of the greater number of possible stereoisomers (Secs. 8-ld and 8-le). Table 8-8 shows the... 

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... 

TABLE 8-9 Effect of Solvent and Counterion on Stereochemistry in Anionic Polymerization of 1,3-Dienes"... 

There is no mechanism that adequately explains all features of the anionic polymerization of 1,3-dienes. NMR data indicate the presence of it- and cr-bonded propagating chains (L and LI) When reaction occurs in polar solvents, the carbanion center is delocalized as both... 

Later, Tieke reported the UV- and y-irradiation polymerization of butadiene derivatives crystallized in perovskite-type layer structures [21,22]. He reported the solid-state polymerization of butadienes containing aminomethyl groups as pendant substituents that form layered perovskite halide salts to yield erythro-diisotactic 1,4-trans polymers. Interestingly, Tieke and his coworker determined the crystal structure of the polymerized compounds of some derivatives by X-ray diffraction [23,24]. From comparative X-ray studies of monomeric and polymeric crystals, a contraction of the lattice constant parallel to the polymer chain direction by approximately 8% is evident. Both the carboxylic acid and aminomethyl substituent groups are in an isotactic arrangement, resulting in diisotactic polymer chains. He also referred to the y-radiation polymerization of molecular crystals of the sorbic acid derivatives with a long alkyl chain as the N-substituent [25]. More recently, Schlitter and Beck reported the solid-state polymerization of lithium sorbate [26]. However, the details of topochemical polymerization of 1,3-diene monomers were not revealed until very recently. 

Scheme 6 Solid-state polymerization of 1,3-diene compounds providing 1,4-trans polymers...

The polymerization proceeds under photo- [49,50],X-ray [51], and y-ray [52] irradiation in the dark in vacuo, in air, or even in water or organic solvent as the dispersant (nonsolvent) for the crystals, similar to the solid-state polymerization of diacetylene compounds [ 12]. The process of topochemical polymerization of 1,3-diene monomers is also independent of the environment surrounding the crystals. Recently, the thermally induced topochemical polymerization of several monomers with a high decomposition and melting point was confirmed [53]. The polymer yield increases as the reaction temperature increases during the thermal polymerization. IR and NMR spectroscopies certified that the polymers obtained from the thermally induced polymerization in the dark have a stereoregular repeating structure identical to those of the photopolymers produced by UV or y-ray irradiation. 

Scheme 8 Nonconjugating and conjugating polymer formation through topochemical polymerization of 1,3-diene and 1,3-diyne compounds...

The topochemical polymerization of 1,3-diene monomers based on polymer crystal engineering can be used not only for tacticity but also for the other chain structures such as molecular weight [ 102], ladder [84] or sheet [ 103] structures, and also polymer layer structures using intercalation reactions [ 104-107]. Some mechanical and structural properties have already been revealed with well-defined and highly or partly crystalline polymers [ 108-111 ]. A totally solvent-free system for the synthesis of layered polymer crystals was also reported [112]. 

Chiral solid matrices are used for asymmetric synthesis polymerization of 1,3-dienes (inclusion polymerization), although the matrix reaction is not exactly a catalytic synthesis [40,41]. (R)-trans-anti-trans-anti-trans-Perhydrotriphenylene (13) [42,43], deoxyapocholic acid (14) [44,45], and apocholic acid (15) [46,47] are known as effective matrices for the... 

Solution grafting has been the predominant approach for the immobilization of rare-earth metal precatalyst components [288]. The identification of the catalytically active surface species, commonly formed upon interaction with organoaluminum compounds, is difficult and assisted by molecular model complexes. Several types of support materials including magnesium chloride [289], silica [290], and organic (co-)polymers [291,292], were examined both in the gas-phase and the slurry polymerization of 1,3-dienes. 

Coordination Polymerization of 1,3-Dienes Single-Site (or Metallocene) Catalysts Living Radical Polymerizations Other Types of Polymerizations, Polymers Ring-Opening Polymerization... 

This anti-cis and syn-trans correlation is now generally accepted as a fundamental aspect of the mechanism, related to the cis-trans selectivity in the complex-catalyzed 1,4-polymerization of 1,3-dienes [1]. 

Non-Cp ligands [119-127] those containing heteroatom compounds have been extensively explored as spectator ligands by virtue of their strong metal-heteroatom bonds and exceptional and tunable steric and electronic features required for compensating coordinative unsaturation of metal centers and for catalytic activity toward polymerization. The elegant non-Cp-ligated cationic yttrium aluminate species applied in the polymerization of 1,3-dienes was reported by Okuda. Both the yttrium aluminates (31a) and their lithium ate complexes (31b) upon activation... 

Thirty years later, we discovered the topochemical polymerization of various 1,3-diene monomers giving a highly stereoregular polymer in the form of polymer crystals. When ethyl (Z,Z)-muconatc was photoirradiated in the crystalline state, a tritactic polymer was produced [18, 19], in contrast to the formation of an atactic polymer by conventional radical polymerization in an isotropic state. Thereafter, comprehensive investigation was carried out, for example, the design of monomers, the crystal structure analysis of monomers and polymers, and polymerization reactivity control, in order to reveal the features of the polymerization of 1,3-diene monomers [20-23], Eventually, it was revealed that the solid-state photoreaction... 

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