1,4-Polybutadiene metathesis

The most recent work in our group on metathesis depolymerization has focused on tlie solvent-free depolymerization of 1,4-polybutadiene.51 In our previous work using catalyst 14, a solvent is required in order to bring the catalyst and polymer into the same phase for depolymerization to take place. However, we have found that catalyst 12 can effectively depolymerize 1,4-polybutadiene with no solvent... 

Among cyclic polyenes, cyclic dienes, trienes and tetraenes have been ring-open polymerised via the metathesis reaction. Representative of the cyclodienes most commonly used for polymerisation are 1,5-cyclooctadiene, norbornadiene (bicyclo[2.2.1]hept-2,5-diene) and dicyclopentadiene as mono-, bi- and tricyclic diolefins respectively. Cycloocta-1,5-diene metathesis polymerisation is another approach to the preparation of 1,4-polybutadiene ... 

Indicate cycloolefin monomers that will be polymerised by metathesis catalysts to polymers with a structure equivalent to polybutadiene, an alternating ethylene/ butadiene copolymer, an alternating butadiene / isoprene copolymer and polyacetylene. 

The ruthenium catalyst RuCl2(= CHPh)(PCy3)2 is able to promote both alkene metathesis polymerization (ROMP) and atom transfer polymerization (ATRP) [80,81]. The bifunctional catalyst A was designed to promote both ROMP of cyclooctadiene (COD) and ATRP of methyl methacrylate (MMA). Thus, catalyst A was employed to perform both polymerizations in one pot leading to diblock polybutadiene/polymethylmethacrylate copolymer (58-82% yield, PDI = 1.5). After polymerization the reaction vessel was exposed to hydrogen (150 psi, 65 °C, 8h), under conditions for Ru(H2)(H)Cl(PCy3)2 to be produced, and the hydrogenation of diblock copolymer could attain 95% [82] (Scheme 36). 

As with all polycondensation reactions, the formation of cychc ohgomers by ADMET is possible and has been demonstrated in a variety of cases [31-33]. This occurs by intramolecular back-biting metathesis of an active metal carbene with an internal olefin of the polymer (Scheme 6.5) to hberate cyclooctadiene, for example, from ADMET polybutadiene, although larger cychcs have also been observed. A related undesired cyclization is the intramolecular cyclization of the monomer by RCM. 

Scheme 6.5 (a) Back-biting metathesis of ADMET polybutadiene to produce cyclooctadiene (b)... 

We can visualise the capability of suitable lanthanide (Ln) compounds (J, 6), e.g. as homogeneous catalysts with respect to olefins, by invoking similar intermediates. Although the series of reportedly catalytically active Ln-complexes spans from the pure trihalide via tris(B-diketonato)complexes to the organo-metallic tris(cyclopentadienyl) and tetra(allyl)complexes (8), respectively, no really optimal combination of ligands on a Ln-element has been found so far. Promising aspects are, however, based on some evidence for "reaction steering" in that either cis- or trans-polybutadienes can be obtained from 1,3-dienes, and either polymers or metathesis products from monoolefins, respectively (Table I). 

Fig. 16.4 Degradation of 1,4-polybutadiene by metathesis with oct-4-ene. Decline of the fraction of unreacted double bonds in the polymer, (1—i), with time. Catalyst WCl6/EtAlCl2 in tetrachloroethene (Stelzer 1977).

Cross-metathesis of dimethyl hex-3-enedioate with polybutadiene containing 20-30% 1,2 units has also been studied, with similar results (Campistron 1986). 

Scheme 16.1 Products of metathesis degradation of polybutadiene containing 1,2 units.

Polybutadiene is readily cross-linked by heating with 1% dicumyl peroxide for 10 min at 145°C. If the product is first extracted with C2CI4 to remove any non-cross-linked material and then placed in eontact with WCl6/EtA.lCl2/EtOH in C2CI4 at 45°C, it rapidly goes into solution (Hummel 1970 Kumar 1981). An added acyclic olefin assists this process but is not essential. It is clear that the network can be readily broken down by the occurrence of either intramolecular or intermolecular metathesis reactions. 

Degradation of Unsaturated Polymers by Metathesis 16.3.5 Polybutadienes modified by partial addition reactions... 

Fig. 16.5 Degradation of partially (25%) cyclopropanated di-1,4-polybutadiene by metathesis with troni-oct-4-ene. GC of a reaction mixture in the early stages of reaction. n, m above the peaks correspond to the number of Mj and M2 units, respectively, in Q(Mi/2) (Mi) (M2)m(M /2)Q (see text). S = internal standard, tetradecane (Hummel 1984).

Cyclooctadienyl substituents at the methylene groups in c/s-1,4-polybutadiene can be introduced by the radical-induced reaction with cycloocta-1,5-diene. Metathesis degradation of the product with tra 5-oct-4-ene shows that the substitution reaction is accompanied by double-bond shift reactions (Hummel 1990a). 

Bielawski CW, Benitez D, Grubbs RH. Synthesis of eyebe polybutadiene via ting-opening metathesis polymerization the importance of removing trace bnear contaminants. J Am Chem Soc. 2003 125 8424-8425. 

The catalytic polymerization of butadiene with a cobalt catalyst was performed, producing particles of 150-200 nm diameter that consisted of crystalline 1,2-polybutadiene [80]. Metathesis polymerization reactions in aqueous miniemulsion were carried out when water-resistant metathesis catalysts became available. 

Another example is ring-opening olefin metathesis polymerization (ROMP), as can be used for synthesis of polybutadiene from 1,5-cyclooctadiene. 

ADMET polymerization and depolymerization methods have been used in the synthesis of telechelic oligomers. The metathesis depolymerization of 1,4-polybutadiene is accomplished in the presence or absence of a monofunctional diene by using either of the catalysts presented in Figure 13. Telechelic oligomers with terminal alkene, ester, and silyl ether and imide functional groups may be... 

Cyclooctene is obtained by partial reduction of 1, 5-cyclooctadiene, which in turn is available from butadiene (p. 369). Co-metathesis between the polyoctene and cis-1,4-polybutadiene gives a rubber which is very stable to heat, oxygen and light, is easily moulded and is readily combined with other rubbers by vulcanization. 

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