Cyclopentene/cyclooctene, ROMP

After Kaminsky, Brintzinger, and Ewen discovered homogeneous metallocene/ methylaluminoxane (MAO) catalysts for stereospecific a-olefin polymerizatiOTi (for reviews on olefin polymerization, see [13-21]), the first report [22, 23] rai addition cycloolefin polymerization without ROMP appeared. This stimulated a great interest in these polymers and in catalysts for cycloolefin polymerization (Fig. 1). Cycloolefins such as cyclopentene, cyclooctene, and norbomene can be polymerized via addition (Fig. 2). Polycycloolefins by metallocenes are difficult to process due to their high melting points and their low solubility in common organic solvents. However, metallocenes allow the synthesis of cyclic olefin copolymers (COC), especially of cyclopentene and norbomene with ethene or propene, which represent a new class of thermoplastic amorphous materials (Scheme 1) [24, 25]. 

It has also been exploited in the synthesis of phenylenevinylene polymers and copolymers. (Scheme 12.2). In general, these studies focused on the preparation of precursors that could be converted to insoluble phenylenevinylene homopolymer 8 (Thom-Csmyi et a/., 1993 Thorn-Csanyi and Pleug, 1993 Thorn-Csanyi et al., 1994), for example ROMP of (Friend et al, 1999) paracyclophane-1,9-diene 9 gives PPV as an insoluble, yellow fluorescent powder. Soluble phenylenevinylene copolymers have been prepared by ROMP of (Friend et al, 1999) paracyclophane-1,9-diene with cyclopentene, cyclooctene and cyclocta-1,5-diene comonomers (Thorn-Csanyi etal, 1993 Thorn-Csanyi and Pleug, 1993 Thorn-Csanyi et al, 1994). Unfortunately, the incorporation of more than 5% of phenylenevinylene units gives insoluble polymers, limiting the potential application of these materials. 

One of the first applications envisioned of ROMP was the preparation of polyenes with rubber-like properties by ROMP of cyclopentene. Although this polymer did not have the optimum product profile, the polymer obtained by ROMP of cyclooctene can indeed be used in blends with other rubbers, conferring on the final product improved mechanical properties and simplifying its processing. This polymer, called Vestenamer 8012, is being produced by Chemische Werke Hiils, Marl, Germany, and the annual capacity in 1990 reached 12000t. 

At one time it seemed likely that the high-tra 5 polymer of cyclopentene formed in reaction (2) would find use as an all-purpose elastomer. Raw materials were cheap and the product had properties akin to those of natural rubber. However, its early promise has not been fulfilled. Other high-/ran5 polymers, such as those formed from norbomene, eqn. (11), and cyclooctene, eqn. (12), have found their way on to the market as components of elastomeric products, and the ROMP of enc o-dicyclopentadiene, eqn. (13), is being used to produce large objects by reaction injection moulding (RIM) see Ch. 17. 

The ROMP of [2.2]paracyclophane-1,9-diene (73) yields poly(p-phenylene-vinylene) (74) as an insoluble yellow fluorescent powder. Soluble copolymers can be made with an excess of cyclopentene (Thom-Csanyi 1992b), cycloocta-1,5-diene (Thom-Csanyi 1993a), or cyclooctene (Thom-Csanyi 1994c). The UV-visible absorption spectra of the copolymers with cyclooctene show separate peaks for sequences of one, two, and three /7-phenylene-vinylene units at 290, 345, and about 390 nm, respectively, with a Bemoullian distribution. The formation of the odd members of this series evidently involves dissection of the two halves of the original monomer units by secondary metathesis reactions. 

It was first observed in 1969 that the ROMP of cycloocta-1,5-diene and cyclooctene produced not only linear polymer but also a series of cyclic oligomers, detectable by GC [67]. These are the products of back-biting metathesis reactions in competition with the propagation reaction. Eqn. (10) is an example of the formation of cyclic tetramer during the ROMP of cyclopentene, [68]. The back-biting reaction is thus the reverse of the propagation step for the cyclic oligomer itself. 

Monocyclic alkenes, like cyclopentene or cyclooctene give ROMP reactions with the catalysts A and B but not with C. The ROMP activities from A and B depend on the solvents and the molar ratios of monomer and catalyst [5], Table 1. 

ROMP is a powerful technique for the preparation of polymers with high molecular weights and low dispersity [38]. A number of computational studies on ROMP with various olefin substrates have been reported. For example, a direct comparison of the computed energies of the intermediates and transition states in the ROMP of norbornene, cyclopentene, cycloheptene, and Z-cyclooctene was reported by Cramer and Hillmyer [39] in 2012 (Table 7.1). In these reactions, a fused bicyclic metallacylobutane intermediate is formed, and the ring strain... 

Table 7.1 Gas-phase, Gibbs free energies at 298 K of the intermediates and transition states in the ring-opening metathesis polymerization (ROMP) of norbornene, cyclopentene, cycloheptene, and Z-cyclooctene. Adapted from Ref [39] . ...

These complexes showed higher thermal stabihty in toluene at 80 °C than the Hoveyda first-generation catalyst, with half-lives ranging from 3 to 6 h, depending on the nature of the Schiff base-derived hgand. They also showed latent catalyst behavior, as only moderate-to-low olefin metathesis activity was observed at room temperature in CM and RCM [44]. On the other hand, these complexes were active in the ROMP of cyclooctene and cyclopentene. The NHC-containing catalyst was found to be especially efficient, leading to a TOP of 667 min at room temperature [43]. 

As mentioned above, Calderon recognized in 1972 that metathesis polymerization and metathesis of acyclic olefins are two aspects of the same reaction [10]. As early as 1968 he had identified the double bonds as the reactive centers in the metathesis of acyclic olefins. Apart from the educts the metathesis reaction of dg-2-butene with 2-butene yielded only d4-2-butene, so he could exclude the cleavage of any single bond [39,40]. Dali Asta and Motroni drew an analogous conclusion for ROMP by copolymerization of 1- C-cyclopentene and cyclooctene (3). After ozonolytic degradation of the polymers the complete radioactivity was found in the C5-fraction, showing the exclusive cleavage of the double bonds (pathway (3b)) [41,42]. 

---