Coordination polymerization conjugated dienes

Studied in coordination polymerization. Cyclic dienes have also been polymerized by coordination catalysts however, the activity is lower compared to acyclic conjugated dienes, and the versatility of their isomeric structures is limited. 

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... 

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 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. 

Polymerization of isobutylene, in contrast, is the most characteristic example of all acid-catalyzed hydrocarbon polymerizations. Despite its hindered double bond, isobutylene is extremely reactive under any acidic conditions, which makes it an ideal monomer for cationic polymerization. While other alkenes usually can polymerize by several different propagation mechanisms (cationic, anionic, free radical, coordination), polyisobutylene can be prepared only via cationic polymerization. Acid-catalyzed polymerization of isobutylene is, therefore, the most thoroughly studied case. Other suitable monomers undergoing cationic polymerization are substituted styrene derivatives and conjugated dienes. Superacid-catalyzed alkane selfcondensation (see Section 5.1.2) and polymerization of strained cycloalkanes are also possible.118... 

Conjugated dienes (1,3-butadiene, isoprene) have suitable nucleophilicity to undergo cationic polymerization. There is, however, not much practical interest in these processes since the polymers formed are inferior to those produced by other (free-radical, coordination) polymerizations. A significant characteristic of these polymers is the considerably less than theoretical unsaturation due to cyclization processes.132 A fully cyclized product of isoprene has been synthesized163 by constant potential electrolysis in CH2C12. 

The mechanism of coordination polymerization of 1,3-butadiene and, in general, that of conjugated dienes follows the same pathway discussed for alkene polymerization that is, monomer insertion into the transition metal-carbon bond of the growing polymer chain occurs. One important difference, however, was recognized very early.47,378,379 In the polymerization of dienes the growing chain end is tt-allyl complexed to the transition metal ... 

Attempts have been made to copolymerize conjugated dienes with olefins but there are no data on polymerization rates or reactivity ratios. They are ill suited for copolymerization in that polymerization rates are markedly reduced by the presence of the conjugated diene and the copolymers are heterogeneous in composition and may be crosslinked. The reasons for this behaviour have not been established but a possible explanation is that the conjugated diene coordinates preferentially with the catalyst and so excludes the olefin, but has a slow insertion rate compared with the more reactive olefin. 

The structures of three of the compounds have been determined they show essentially octahedral coordination of the lanthanide with bond angles around 90°. Average lanthanum-carbon distances are 2.563(18) A (Ho) 2.57(2) A (Er), and 2.53(2)A (Lu). More complicated lanthanide methyl species have been synthesized by another route, involving reaction of main-group methyls, Lewis acids, with lanthanide alkoxides " and amides, " a process of the type implicated in the lanthanide-catalyzed polymerization of conjugated dienes... 

This chapter describes the coordination polymerization of acyclic and cyclic vinylic monomers, conjugated dienes, and polar vinylic monomers with the most important catalytic systems known in this area. A chronological classitication for the development of the main coordination catalyst types is outlined, as well as polymerization kinetics and mechanisms and applications of polymers obtained through different metallic complexes. 

The initiations of the polymerizations of the conjugated dienes in inert hydrocarbons are also believed to be preceded by coordination of the organometallic compounds with the 7r-electron clouds of the monomers ... 

Many different polymers of conjugated dienes are prepared conunercially by a variety of processes, depending upon the need. They are formed by free-radical, ionic, and coordinated anionic polymerizations. In addition, various molecular weight homopolymers and copolymers, ranging from a few thousand for liquid polymers to high molecular weight ones for synthetic rubbers, are on the market. 

The lanthanide coordination catalysts are known to be highly stereospecific for producing high-cis polybutadiene and high-cis polyisoprene as well as high-cis copoljnnerization of the two monomers. In addition, the polymerizations of other conjugated dienes such as trans-piperlene, 2,4-hexadiene,... 

Transition Metal-Catalyzed Coordination Polymerization of Cyclic Conjugated Dienes 479... 

TRANSITION METAL-CATALYZED COORDINATION POLYMERIZATION OF CYCLIC CONJUGATED DIENES... 

A wide variety of transition metal catalysts, such as Ti-, Mo-, W-, Ni-, and Pd-based complexes, have been investigated since the late 1950s for cyclic conjugated diene polymerization. Because this chapter focuses on the coordination polymerization of CHD and its derivatives, cationic polymerization initiated by transition metal compounds will be excluded in this section. 

NMR. These polymers should have >95% 1,4-linked structures because there are only three types of carbon atoms in their NMR spectra." The stereochemistry of polymerization is believed to be iyn-coordinative addition to CHD, resulting in the formation of a cis structure. The iyn-addition mechanism was originally proposed by Porri and Aglietto for linear conjugated diene polymerization (see Chapter 17). ... 

One of the key technologies needed to make cyclic conjugated diene polymers useful is an expansion of monomer availability. Presently, neither 1,3-cycloheptadiene nor 1,3-cyclooctadiene has been coordinatively polymerized, even with highly active cationic Ni complexes. The polymerization of functionalized CHDs is, so far, limited to ANiTFA. In order to provide processability and functionality to cyclic conjugated diene polymers, these problems must be overcome. The progress of transition metal-catalyzed polymerization may make this possible in the near future. 

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