Double networking peroxide radicals

Vulcanization. Generally this is carried out by the action of peroxides, which can cross-link the chains by abstracting hydrogen atoms from the methyl groups and allowing the resulting free radicals to couple into a cross-link. Some varieties of polysdoxanes contain some vinylmethyl siloxane units, which permit sulfur vulcanization at the double bonds. Some Hquid (short-chain) siHcones can form networks at room temperature by interaction between thek active end groups. 

Initiators are used to initiate the curing reaction at elevated temperatures. Cross-linking or polymerization occurs by a free radical mechanism in which the double bond of the polyester chain reacts with the vinyl monomer that is usually styrene, and this reaction provides a three-dimensional network that converts the viscous resin to a hard thermoset solid. The initiators added decompose at elevated temperatures thus providing free radicals to initiate the cross-linking. Peroxyesters and peroxyketals are the most common classes of peroxides used as initiators. 

A model network for dlcumyl peroxide cross-linked natural rubber is proposed in Figure 22. The T represents trans double bonds formed by the rearrangement of the allylic free radicals. 

An alternative method to the step polymerization of a mixture of phenols with methanal to form a copolymerized network is the free radical polymerization of substituted aromatics containing polymerizable, double bonds to produce products with improved properties. In this way, a highly crosslinked network is obtained by copolymerization of allyl Xylok with 4,4 -bismaleimidodiphenyl methane in presence of diisopropylbenzyl peroxide and imidazole (Scheme 25). 

The chains must be crosslinked to form a network (cf. Fig 7.16). In most elastomers containing double bonds, covalent bonds are introduced between chains. This can be done either with sulfur or polysulfide bonds (the well known sulfur vulcanisation of natural rubber is an example), or else by direct reactions between double bonds, initiated via decomposition of a peroxide additive into radicals. Double bonds already exist in the chemical structure of polyisoprene, polybutadiene and its copolymers. When this is not the case, as for silicones, ethylene-propylene copolymers and polyisobutylene, units are introduced by copolymerisation which have the property of conserving a double bond after incorporation into the chain. These double bonds can then be used for crosslinking. This is how Butyl rubber is made from polyisobutylene, by adding 2% isoprene. Butyl is a rubber with the remarkable property of being impermeable to air. It is used to line the interior of tyres with no inner tube. 

UP prepolymers possess a molar mass (A/ ) ranging between 1800 and 2500 g moP and about 5 to 8 double bonds per chain. In order to form a network, they are dissolved in neat styrene whose radical polymerization is most often initiated by a peroxide. Then there is a copolymerization between the bivalent styrene (w = 2) and the plurivalent unsaturated polyester acting as a comonomer. The average valence of the latter is twice the average number of unsaturations per chain. The reaction mechanism of the process is described below ... 

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