Polymerization of conjugation

The discovery by Ziegler that ethylene and propylene can be polymerized with transition-metal salts reduced with trialkyl aluminum gave impetus to investigations of the polymerization of conjugated dienes (7—9). In 1955, synthetic polyisoprene (90—97% tij -l,4) was prepared using two new catalysts. A transition-metal catalyst was developed at B. E. Goodrich (10) and an alkaU metal catalyst was developed at the Ekestone Tke Rubber Co. (11). Both catalysts were used to prepare tij -l,4-polyisoprene on a commercial scale (9—19). 

The polymerization of conjugated dienes with transition metal catalytic systems is an insertion polymerization, as is that of monoalkenes with the same systems. Moreover, it is nearly generally accepted that for diene polymerization the monomer insertion reaction occurs in the same two steps established for olefin polymerization by transition metal catalytic systems (i) coordination of the monomer to the metal and (ii) monomer insertion into a metal-carbon bond. However, polymerization of dienes presents several peculiar aspects mainly related to the nature of the bond between the transition metal of the catalytic system and the growing chain, which is of o type for the monoalkene polymerizations, while it is of the allylic type in the conjugated diene polymerizations.174-183... 

Several experimental facts have been rationalized in terms of different n-allyl insertion mechanisms, depending on the nature of the catalytic systems and the diene monomer, mainly by the extensive work of Porri and co-workers, as reviewed in Refs. 181 and 182, and Taube and co-workers, as reviewed in Ref. 183. A widely accepted scheme for cis-1,4 and 1,2 polymerizations of conjugated dienes is reported in Scheme 1.5. In particular, it has been... 

Scheme 7.5 Monomer coordinated and monomer free intermediates of a widely accepted mechanism for d.v-l, 4 and 1,2 polymerizations of conjugated dienes.

The acid-catalyzed conversions of hydrocarbons have been extensively studied and are widely reported in chemical literature. Many important petroleum and petrochemical processes involve catalysis by acids. In contrast to this, the use of bases to catalyze hydrocarbon conversions has received little attention except for polymerization of conjugated dienes and styrene to high polymers. 

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-... 

The ionic chain polymerization of unsaturated linkages is considered in this chapter, primarily the polymerization of the carbon-carbon double bond by cationic and anionic initiators (Secs. 5-2 and 5-3). The last part of the chapter considers the polymerization of other unsaturated linkages. Polymerizations initiated by coordination and metal oxide initiators are usually also ionic in nature. These are called coordination polymerizations and are considered separately in Chap. 8. Ionic polymerizations of cyclic monomers is discussed in Chap. 7. The polymerization of conjugated dienes is considered in Chap. 8. Cyclopolymerization of nonconjugated dienes is discussed in Chap. 6. 

Cooper, W., Recent Advances in the Polymerization of Conjugated Dienes, Chap. 3 in Developments in Polymerization—1, R. N. Howard, ed., Applied Science, London, 1979. 

Similar approach has also been taken by Ferain and Legras [133,137,138] and De Pra et al. [139] to produce nanostructured materials based on the template of the membrane with etched pores. Polycarbonate film was also of use as the base membrane of the template, and micro- and nanopores were formed by precise control of the etching procedure. Their most resent report showed the successful formation of ultrasmall pores and electrodeposited materials of which sizes were as much as 20 nm [139]. Another attractive point of these studies is the deposited materials in the etched pores. Electrochemical polymerization of conjugated polymer materials was demonstrated in these studies, and the nanowires based on polypyrrole or polyaniline were formed with a fairly cylindrical shape reflecting the side wall structure of the etched pores. Figure 10 indicates the shape of the polypyrrole microwires with their dimension changes by the limitation of the thickness of the template. 

Anionic polymerization of conjugated dienes and olefins retains its lithium on the chain ends as being active moities and capable of propagating additional monomer. This distinguishing feature has an advantage over other methods of polymerization such as radical, cationic and Ziegler polymerization. Many attempts have been made to prepare block copolymers by the above methods, but they were not successful in preparing the clear characterized block copolymer produced by anionic technique. 

At present, it is common knowledge that not only the photoreactivity, but also the stereochemistry, of the photoproduct is predictable from crystallographic information of starting olefin substrates. This ability of olefinic crystals to dimerize has been widely applied to the topochemical photocycloaddition polymerization of conjugated diolefinic compounds, so called "four-center type photopolymerizations" (7,8). All the photopolymerizable diolefin crystals are related to the center of symmetry mode (centrosymmetric -type crystal) and thus give polymers having cyclobutanes with a 1,3-trans configuration in the main chain on irradiation. 

The influence of tertiary bases, such as TMEDA, upon the polymerization of conjugated dienes is at once more complex than that of olefins because of the variation in chain stereochemistry that accompanies the changes in rate. In an effort to simplify the discussion, the question of polymer stereochemistry is deferred to a separate Section. 

From the early 1960s onwards, the use of lanthanide (Ln) based catalysts for the polymerization of conjugated dienes came to be the focus of fundamental studies [31]. The first patent on the use of lanthanides for diene polymerization originates from 1964 and was submitted by Union Carbide Corporation (UCC) [32,33]. In this patent the use of binary lanthanum and cerium catalysts is claimed. Soon after this discovery by UCC, Throckmorton (Goodyear) revealed the superiority of ternary lanthanide catalyst systems over binary catalyst systems. The ternary systems introduced by Throckmorton comprise a lanthanide compound, an aluminum alkyl cocatalyst and a halide donor [34], Out of the whole series of lanthanides Throckmorton... 

A comprehensive review on the whole field of polymerization of conjugated dienes by transition-metal catalysts was compiled by Porri and Giar-russo in 1989 [50]. 

In addition to the polymerization of conjugated dienes Nd-alcoholates are mainly used for the polymerization of cyclic polar monomers like lactones, lactides, e.g. see [247] and carbonates, e.g. see [248]. 

The adoption of reaction models available for the polymerization of conjugated dienes by Ni- and Ti-catalysts to the polymerization of BD by Nd catalysis is justified by the similarities of the respective metal carbon bonds. In each of these mechanistic models the last inserted monomer is bound to the metal in a 3-allyl mode. The existence of Ni- -allyl-moieties was demonstrated by the reaction of the deuterated nickel complex [ rf- C4D6H)NiI]2 with deuter-ated BD (deuterated in the 1- and 4-position). After each monomer insertion a new 3-allyl-bond is formed [629]. As TT-allyl-complexes are known for Ti and Ni this knowledge has been adopted for Nd-based polymerization catalysts [288,289,293,308,309,630-636,638-645]. 

As was found for the polymerization of styrene, CpTiCT/M AO and similar half-sandwich titanocenes are active catalysts for the polymerization of conjugated 1,3 dienes (Table XX) (275). Butadiene, 1,3-pentadiene, 2-methyl-l,3-pentadiene, and 2,3-dimethylbutadiene yield polymers with different cis-1,4, trans-1,4, and 1,2 structures, depending on the polymerization temperature. A change in the stereospecificity as a function of polymerization temperature was observed by Ricci et al. (276). At 20°C, polypen-tadiene with mainly ds-1,4 structures was obtained, whereas at -20°C a crystalline, 1,2- syndiotactic polymer was produced. This temperature effect is attributed to a change in the mode of coordination of the monomer to the metallocene, which is mainly cis-rf at 20°C and trans-rj2 at -20°C. 

As found for the polymerization of styrene, CpTiCl3/MAO and similar half-sandwich titanocenes are active catalysts for the polymerization of conjugated 1,3-dienes (Table 25) [218], Butadiene, 1,3-pentadiene, 2-methyl-l,3-pentadiene and 2,3-dimethylbutadiene yield polymers with different... 

The polymerization of conjugated dienes to products with a controlled structure usually occurs in the presence of alkylaluminium compounds. The choice not only the transition metal but also of its ligands is of importance. Some systems produce a certain kind of stereochemical structure irrespective of external conditions. So, for example, vanadium compounds yield predominantly the trans-1,4 structure whereas cobalt salts yield the c -1,4 structure. Other catalysts are very sensitive, and a small external effect completely changes their stereochemical activity [267b] [e. g. Cr(acetylacetona-te)3-R3Al]. Examples of several catalytic systems are summarized in Table 7. 

Annex 2 Stereospecific Polymerization of Conjugated Diolefins Butadiene and Isoprene... 

The complexes can initiate the polymerization of conjugated monomers by a very rapid electron transfer process ... 

Figure 14.14 Possible polymer structures from the polymerization of conjugated dienes.

Natta, G., L. Porri, A. Carbonaro, and G. Stoppa, Polymerization of conjugated diolefins by homogeneous aluminum alkyl-titanium alkoxide catalyst systems. I. Cis-1,4 isotactic poly-l,3-pentadiene, Makromolekulare Chemie, 77, 114-125, 1964. 

Polymerization of Conjugated Dienes with Heterogeneous Ziegler-Natta Catalysts... 

The structure of the polymer obtained in the polymerization of butadiene and isoprene with heterogeneous Ziegler-Natta catalysts depends on the nature of the monomer, catalyst system, and reaction conditions. Previously reported results are reviewed and a mechanism is proposed for the stereoregulated polymerization of conjugated dienes. The polymerization of cyclopentadiene with LiAlH -TiCl4 or LiAlR4-TiCl4 catalyst system yields a readily oxidized polymer for which a 1,2-structure is proposed. 

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