Polybutadiene, 1,5-cyclooctadiene

Cyclic dienes and polyenes, monocyclic as well as bicyclic, can be metathesized in the same way as cyclic monoenes. As expected, cyclobutene 27), 1,5-cyclooctadiene, and 1,5,9-cyclododecatriene 28) yield the same polyalkenamer, in this case polybutenamer (1,4-polybutadiene), since these reactants consist of the same base units, i.e.—(CH2)2CH==CH— ... 

Similarly Stable and Posnjak (87) proposed cyclic formulas of four, five, or possibly more structural units for polystyrene. Lebedev (88) first assigned the cyclooctadiene structure to polybutadiene shown below before expanding the concept of the ring to include several isoprene units. 

The ROMP method was extended to synthesize cyclic polybutadiene using 1,5-cyclooctadiene (COD) or 1,5,9-frans-czs-frans-cyclododecatriene (CDT)... 

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 ... 

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)... 

In the ROMP of cyclooctadiene (Mi) with 4,7-dihydro-2-phenyl-l,3-dioxepin (M2) catalyzed by Ru(=CHR)(Cl)2(PCy3)2, where R = CH=CPh2 or Ph, M2 is about half as reactive as Mi. By using a small proportion of M2, one can produce a polymer of Mi containing occasional M2 units, which may be broken by hydrolysis to yield 1,4-hydroxytelechelic polybutadiene M /M 1.2) (Fraser 1995b). 

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

The synthesis of triblock copolymers requires a telechelic polymer with initiating groups at both ends. Grubbs recently described the synthesis and characterisation of two new telechelic polybutadienes prepared by ROMP of 1,5-cyclooctadiene (Scheme 7) [22]. The... 

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. 

Nickel compounds can also be employed as catalysts [161-170]. A three-component system consisting of nickel naphthenate, triethyl-aluminum, and boron trifluoride diethyletherate is used technically. The activities are similar to those of cobalt systems. The molar Al/B ratio is on the order of 0.7 to 1.4. Polymerization temperatures range from -5 to 40 °C. On a laboratory scale the synthesis of 1,4-polybutadiene with allylchloronickel giving 89% cis, 7.7% trans, and 3.4% 1,2-structures is particularly simple [8]. In nickel compounds with Lewis acids as cocatalysts, complexes with 2,6,10-dodecatriene ligands are more active than those with 1,5-cyclooctadiene (Table 4) [171]. 

As models for polybutadiene, cyclohexene and, more recently, cis,cis-l,5-cyclooctadiene (COD) have been employed (Gregg and Katrenick, 1970). 

The products have superior mechanical properties compared with the random copolymers or blends of homopolymers of the same overall composition. The literature reports block copolymers of polybutadiene with cyclopentene [69a], cyclooctadiene [69b], cyclodo-decene [69c] and substituted norbornenes [69d], of polyisoprene, polychloroprene, polypentenamer, and butyl rubber with norbornene derivatives [69c] and styrene-butadiene copolymers with cyclopentene [69a] and norbornene derivatives [69c]. Graft copolymers of type (103) will arise when unsaturation occurs in branched arms of the polymer to be grafted (e.g., 1,2-polybutadiene with cycloolefins) ... 

The "equibinary" l,4-cis/l,4-trans polybutadiene prepared using a n-allyl nickel trifluoroacetate catalyst [70,71,96,97] and the polybutadiene obtained by polymerization of cyclooctadiene using an olefin metathesis catalyst system where shown by nmr to have random distributions of cis- and trans-ianits, although there is some indication that "equibinary" copolymers with non-Bernoullian structures are obtained in some cases [96]. Polybutadienes prepared using alkyl lithium initiators in hydrocarbon solvents have also been shown to have random distributions of 1,4-cis and 1,4-trans units [20,23,71,90]. 

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