Temperature effects diene monomers

As was found for the polymerization of styrene, CpTiCT/M AO and similar half-sandwich titanocenes are active catalysts for the polymerization of conjugated 1,3 dienes (Table XX) (275). Butadiene, 1,3-pentadiene, 2-methyl-l,3-pentadiene, and 2,3-dimethylbutadiene yield polymers with different cis-1,4, trans-1,4, and 1,2 structures, depending on the polymerization temperature. A change in the stereospecificity as a function of polymerization temperature was observed by Ricci et al. (276). At 20°C, polypen-tadiene with mainly ds-1,4 structures was obtained, whereas at -20°C a crystalline, 1,2- syndiotactic polymer was produced. This temperature effect is attributed to a change in the mode of coordination of the monomer to the metallocene, which is mainly cis-rf at 20°C and trans-rj2 at -20°C. 

The copolymerization of a,p-unsaturated ketones has been studied extensively in order to improve the poor chemical and thermal stability exhibited by the homopolymers. The vinyl ketones have been copolymerized with most of the common vinyl and diene monomers. The data are given in Ref. [326]. For initiation, the same reagents could be used as for free-radical homopolymerization. Copolymerization was carried out in bulk [371] and in emulsion systems [372]. In copolymerization with methyl methacrylate, vinyl acetate [373], and styrene [371] it was concluded that the relative reactivities of the vinyl ketones increase with the increasing electron-withdrawing nature of the vinyl ketone substituent. Polar and steric effects are not observed. Most of the work has been directed toward the preparation of oil- and solvent-resistant rubbers to replaee styrene-butadiene rubber. Emulsion eopolymerization of butadiene with methyl isopropenyl ketone yielded rubbers with good solvent resistance and low temperature flexibility, but the products tended to harden on storage and were not compatible with natural rubber [374]. The reactive earbonyl function caused sensitivity to alkine reagents. Copolymers of butylacrylate and methyl vinyl ketone, for example, can be erosslinked by treatment with hydrazine [375]. 

Dimerisation is highly exothermic, the rate increasing rapidly with temperature, and may cause rupture of a closed uncooled container. The monomer may largely be prevented from dimerising by storage at — 80°C or below [1]. The polymerisation of the undiluted diene may become explosive within the range 0-40°C and at pressures above 2300 bar. The effect of diluents was also studied [2], A polymerisation was... 

The 3,3 -bis-triarylsilyl BINOL ligands 98 were first introduced by Maruoka, Itoh, Shirasaka, and Yamamoto for the heteroatom Diels-Alder reaction of electron-rich dienes with unactivated aldehydes [76,77], The catalyst was prepared by treating the BINOL 97 with trimethylaluminum in dichloromethane at room temperature for 1.5 h. The catalyst is pink to wine red and was found, by freezing point depression, to be a monomer. A typical procedure for the Diels-Alder reaction is illustrated for the reaction of the trisubstituted diene 371 and cyclohexylcarboxaldehyde. The reaction is complete with 10 mol % catalyst at -20 °C in 2 h in toluene and, after treatment with TFA to effect elimination of the methoxyl group, the product of the reaction is predominately the cii-pyrone 373. The triphenylsilyl substituted catalyst gives the product in 93 % ee (Sch. 47). 

Diels-Alder reactions. Unlike cyclopentadienone, this substance is stable as the monomer, probably because of the bulky substitutents. However, this dienone undergoes Diels-Alder reactions with alkenes, sometimes at room temperature or below. In reactions with 1,3-dienes, it functions as the diene rather than the dienophile, probably because of steric effects. 

Cyclopentadiene is formed in considerable amounts during the refining of petroleum. It exists as its dimer at room temperature but can be dissociated into the monomer on heating—the effect of the increased importance of entropy at higher temperatures (Chapter 12). It can be chlorinated to give hexachlorocyclopentadiene, and the Diels-Alder product of this diene with maleic anhydride is a flame retardant. 

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