Enchainment cyclopentene

The ring-opened polymer of norbomadiene consists of 3,5-disubstituted cyclopentene units (94). When the concentration of these units is kept below 0.2 M the polymer remains soluble, but above this concentration, in the presence of WClg/Me4Sn (1/2), it gels. This is caused by cross-linking, brought about by the ROMP of the enchained cyclopentene... 

C-NMR spectroscopy showed that cyclopentene is incorporated in the copolymer chain through a 1,2-insertion, without ring-opening metathesis. This is in contrast to the homopolymerization of cyclopentene where 1,3-insertion was observed. The 1,2-enchainment by only double bond opening is a result of an easy coordination to the zirconium center when the last insertion was an ethene unit. The p-hydride elimination of a cyclopentene unit at the end of the growing chain, which is needed to form a 1,3-enchained cyclopentene unit, is relatively slow compared to a next ethene insertion. Therefore, it is... 

Although the competing /i-hydrogcn abstraction is suppressed by the relatively fast insertion of carbon monoxide into the Pd C bond, two different modes of cyclopentene enchainment, namely by 1,2- and 1,3-insertion, have been recognised in the copolymers formed [29] ... 

A plausible polymerisation mechanism that accounts for the 1,3-enchain-ment of cyclopentene, leading to both cis- and tram-poly (l,3-cyclopentylene)s, is outlined below [15,19,20] ... 

The microstructures of these polymers have been investigated by using oligomers as models. Norbornene was shown to polymerize via cis exoinsertion (232,233), whereas cyclopentene was shown to give quite unusual cis and trans-1,3 enchainment of the monomer units (234-236). 

Cyclopentene polymerization has been catalyzed by several metallocenes (Table XVII). In each case, the homopolymers were found to contain no 1,2 enchainments. Whereas a mechanism similar to that proposed for the formation of 1,3 enchainments in polypropylene is reasonable, there is no plausible explanation for the formation of trans structures. 

Although homopolymerization of cyclopentene results in 1,3 enchainment of the monomer units in copolymerization, blocks of cyclic monomer units are rarely observed as a consequence of the unfavorable copolymerization parameters. The isolated cyclopentene units maybe incorporated in a cis-1,2 or cis-1,3 fashion, with their ratio dependent on the catalyst used (238-240). Thus, ethylene compensates for the steric hindrance at the a carbon atom of the growing chain after insertion of the cyclopentene. 

In the case of poly(cyclopentene), the configurational base units are cis-and 1,3-enchained (27) while poly(norbomene) shows cis-exo insertion [61, 62]. 

Cyclopentene can be polymerized using a variety of metallocene catalysts to afford poly(cyclopentene)s with 1,3-enchainment of the cyclopentane units [17]. These polymers are invariably of low molecular weight (Mn <2000) and often highly isotactic, and are generally not melt-processable [18]. 

The insolubility of these polymers makes their structural assignments difficult. Kaminsky proposed a 1,2-enchainment of cyclopentene to yield an isotactic microstructure with either cis or trans rings on the basis of IR, X-ray, and C NMR chemical shifts of soluble oligomers produced at higher temperatures. A similar microstructure was proposed for polynorbomene with a cis-exo-, 2 configuration assigned on the basis of comparison with model compounds. ... 

Cycloolefins. The ability of metallocenes to controllably insert previously recalcitrant monomers is further evidenced by the homo- and copolymerization of cycloolefins such as cyclopentene. Initially, enchainment of cyclopentene was assumed (209) to proceed in normal 1,2-fashion, as shown in Figure 14a, and indeed this appears to hold true in copolymerizations with ethylene. However, later work (210) on homopoljrmers showed that isomerization following 1,2-incorporation is obligatory, presumably due to steric considerations, srielding 1,3-inserted units (Fig. 14b)... 

Cyclopentene (CPE) is obtained from one of the C5 fractions of cyclopentadiene, which is a subproduct of cracking naphtha. CPE is prepared by partial hydrogenation of cyclopentadiene or reduction of cyclopentadiene by sodium in liquid nitrogen. CPE is a unique olefin that, owing to its diverse insertion modes, can be polymerized with various transition metal catalysts. Figure 15.1 shows the four types of CPE polymerization insertion and enchainment modes 1,2-cis-enchainment, 1,2-trans-enchainment, 1,3-cis-enchainment, and 1,3-trans-enchainment. 

Metathesis polymers obtained from unsubstituted monocyclic olefins, such as cyclopentene or cyclooctene, can contain two types of repeating units. These are units in which the carbon-carbon double bond has the cis or trans configuration.Sequences of cis- and trans-enchained poly(l-octenylene) are shown in Figure 20.1. 

The first metallocene-based COC material was synthesized from ethene and cyclo-pentene [218]. While homopolymerization of cyclopentene results in 1,3-enchainment of the monomer units [219], isolated cyclopentene units are incorporated into the ethene-cyclopentene copolymer chain by 1,2-insertion. Ethylene is able to compensate the steric hindrance at the ot-carbon of the growing chain after and before the insertion of cyclopentene [220]. 

McLain et polymerized cyclopentene by late transition metal catalysts using MAO and borate-activated nickel and palladium diimine complexes. The nickel diimine complexes produce crystalline materials showing a ds-1,3 enchainment with a melting point of 240-330 °C. The hydroligomers were mainly atactic. Palladium catalysts gave pure atactic polymers. It is also possible to polymerize substituted cyclopentenes such as 3-methyl- or 3-ethyl-cyclopentene. 

Cyclopentene can be copolymerized with ethene or propene by heterogeneous and homogeneous Ziegler-Natta catalysts. Crystalline or elastomeric copolymers are obtained depending on the cyclopentene content and the part of ring-opening or vinyl-type polymerization mechanism. Metallocene/MAO catalysts are very active in the copolymerization of cyclopentene with ethene. In contrast to the homopolymerization of cyclopentene, the cyclic olefin is incorporated into the copolymer chain by 1,2-enchainment. 

The copolymers show small cyclopentene blocks with 1,3-enchained units and isolated 1,2-enchained units. The cisj trans ratio was quantified to be 10% trans of the 1,3-units and 2.9% trans of the 1,2-units, the main part was cis units. 

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