Diene cyclopolymerization

Cyclopolymerization of Nonconjugated Dienes. Cyclopolymerization is an addition polymerization that leads to introduction of cyclic structures into the main chain of the polymer. Nonconjugated dienes are the most deeply studied monomers for cyclopolymerization and for cyclocopolymerizations with alkene monomers 66 In general, (substituted and unsubstituted) dienes with double bonds that are linked by less than two or more than four atoms cannot undergo efficient cyclization and result in crosslinked materials.12 In fact, efficient cyclopolymerization processes have been described, for instance, for a,oo-dienes like 1,5-hexadiene, 2-methyl-l,5-hexadiene, 1,6-heptadiene, and 1,7-octadiene,67 73 which lead to formation of homopolymers and copolymers containing methylene-1,3-cycloalkane units. 

Diene monomers with suitably disposed double bonds may undergo intramolecular ring-closure in competition with propagation (Scheme 4.12). The term cyclopolymcrization was coined to cover such systems. Many systems which give cyclopolymerization to the exclusion of normal propagation and crosslinking are now known. The subject is reviewed in a series of works by Butler.98 102... 

Geometric considerations in cyclopolymerization are optimal for 1,6-dienes (see 4.4.1.1). Instances of cyclopolymerization involving formation of larger rings have also been reported (see 4.4.1.4), as have examples where sequential intramolecular additions lead to bicyclic structures within the chain (see 4.4.1.2). Various 1,4- and 1,5-dienes are proposed to undergo cyclopolymerization by a mechanism involving two sequential intramolecular additions (see 4.4.1.3). 

Cyclopolymerizations of other 1,6-dienes afford varying ratios of five- and six-membered ring products depending on the substitution pattern of the starting diene. Substitution of the olefinic methine hydrogen (e.g. 11, R- CH3) causes a shift from five- to six-membered ring formation. More bulky R substituents can prevent efficient cvclization and cross-linked polymers may result. 

A vast range of symmetrical and unsymmetrieal 1,6-diene monomers has now been prepared and polymerized and the generality of the process is well established.98,1 A summary of symmetrical 1,6-dienc structures, known to give cyclopolymerization, is presented in Table 4.4 In many cases, the structure of the repeat units has not been rigorously established. Often the only direct evidence for cyclopolymerization is the solubility of the polymer or the absence of residual unsaturalion. In these cases the proposed repeat unit structures are speculative. 

The observation by Matsumoto et al. (see 4.3.1.4) that significant amounts of head addition occur in polymerization of simple ally] monomers brings into question the origin of the small amounts of six-membered ring products that arc formed in cyclopolymerization of simple diallyl monomers (Scheme 4.14). If the intcrmolecular addition step were to involve head addition, then the intramolecular step should give predominantly a six-membered ring product (14) (by analogy with chemistry seen for 1,7 dienes - see 4.4.1.4). Note that the repeat units 14 and 16, like 12 and 17 are the same however, they are oriented differently in the chain. 

Table 4.4 Ring Sizes Formed in Cyclopolymerization of Symmetrical 1,6-Diene...

Geometric considerations would seem to dictate that 1,4- and 1,5-dicncs should not undergo cyclopolymerization readily. However, in the case of 1,4-dienes, a 5-hexenyl system is formed after one propagation step. Cyclization via 1,5-backbiling generates a second 5-hexenyl system. Homopolymerization of divinyl ether (22) is thought to involve such a bicyclization. The polymer contains a mixture of structures including that formed by the pathway shown in Scheme 4.18. 

It has been suggested that certain 1,5-dienes including o-divinylbenzene (23),156 vinyl acrylate (24, X 11) and vinyl methacrylate (24, X CH )120 may also undergo cyclopolymerization with a monomer addition occurring prior to cyclization and formation of a large ring. However, the structures of these cyclopolymers have not been rigorously established. 

To explain the formation of non-crosslinked polymers from the diallyl quaternary ammonium system, Butler and Angelo proposed a chain growth mechanism which involved a series of intra- and inter-molecular propagation steps (15). This type of polymerization was subsequently shown to occur in a wide variety of symmetrical diene systems which cyclize to form five or six-membered ring structures. This mode of propagation of a non-conjugated diene with subsequent ring formation was later called cyclopolymerization. 

Recently, a metallocene/MAO system has been used for the polymerization of non-conjugated dienes [204, 205]. The cyclopolymerization of 1,5-hexadiene has been catalyzed by Zieger-Natta catalyst systems, but with low activity and incomplete cyclization in the formation 5-membered rings [206]. The cyclopolymerization of 1,5-hexadiene in the presence of ZrMe2Cp2/MAO afforded a polymer (Mw = 2.7 x 107, Mw/Mn = 2.2) whose NMR indicated that almost complete cyclization had taken place. One of the olefin units of 1,5-hexadiene is initially inserted into an M-C bond and then cyclization proceeds by further... 

As for olefins different from propene, molecular modeling studies have also been able to rationalize the dependence on metallocene symmetry of E-Z selectivity for 2-butene copolymerization as well as the stereoselectivity of the cyclization step, which determines the cis or trans configuration of the rings, for cyclopolymerization of nonconjugated 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. 

Cyclopolymerizations yielding more complex ring structures have also been reported [Butler, 19896, 1989]. For example, 1,4-dienes such as divinyl ether yield uncrosslinked products with little or no unsaturation and possessing different bicylic structures. The formation of one of the bicyclic structures is shown in Eq. 6-107 [Tsukino and Kunitake, 1979]. 

Although the polymerization of diene monomers is most familiar for 1,3-dienes, as in the production of rubbers, the polymerization of 1,6-dienes to yield polymers containing six-membered rings ( cyclopolymerization ) has been well established for many years43. Gibson et al.441 have used cyclopolymerization of 1,6-diynes to prepare polymers which are effectively substituted polyacetylenes, the archetype being the polymerization of 1,6-heptadiyne ... 

Cyclopolymerizations of functionalized 1,6 dienes such as 4-trimethyl-silyoxy-1,6-heptadiene are also possible, with B(C6F5)3 as cocatalyst and bis-(pentamethylcyclopentadienyl)zirconocenes as catalysts. Hydrolysis of the product with HC1 gives polymethylene-3-hydroxycyclohexane. With the same catalyst, Kesti et al. (269) succeeded in polymerizing 5-/ /V-diisoprop-lyl-amino-l-pentene and 4-tert-butyldimethylsiloxy-l-pentene. 

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