Poly(p-xylylene) s

The original preparation of p-xylylene was carried out by vacuum pyrolysis of p-xylene at 900–950C. The intermediate, p-xylylene polymerizes spontaneously upon condensation on cooler surfaces [106]:

The process was improved by using di-p-xylylene as an intermediate [106]. This dimer converts to a polymer under milder conditions quantitatively. Both methylene bridges cleave to form p-xylylene, which is a reactive intermediate:

The molecular weights of these polymers were estimated to be as high as 500,000. The total process is sometimes called transport polymerization. Poly(p-xylylene) films are produced commer cially. Tm of this polymer, which is crystalline, is 400C and it carries the trade name of Perylene. Films of poly(p-xylylene) have only fair thermal stability and are brittle, but exhibit good chemical resistance and are very good electrical insulators. Pyrolysis of xylene in steam at 950C yields the dimer intermediate. The yield is reported to be 15% [88]. It is possible to form substituted poly(p-xylylene)s by starting with substituted structures. Among the compounds that were reported were chlorinated and brominated compounds as well as some containing alkyl, cyano, acetyl, and carboxymethyl derivatives. When the di-p-xylylenes are unsym metrically substituted, two homogeneous polymers form during pyrolysis because the two condense with spontaneous polymerization at two different temperatures [88].

Poly(p-xylylene) s can also be prepared by other reactions. Among them is the condensation of trimethyl(p-methylbenzyl) ammonium halide in the presence of a base [47]:

A route to a halogen-substituted polymer is through a reaction of di halo-p-xylylenes with caustic. It results in 1,6 elimination of HCl and formation of a chlorine-substituted poly(p-xylylene) [47]:

With excess caustic, however, all chlorine is removed and presumably an all-conjugated poly (phenylene vinylene) polymer forms [47]:

p-Bis(trichloromethyl) benzene can be pyrolyzed over copper gauze at 300–600C temperatures to form a,a,a0,a0-tetrabromo-p-xylylene. This monomer polymerizes at temperatures just below 140C [107]:

p-Xylylene polymers also form electrolytically [88] in dioxane-water solution with mercury or lead cathodes and carbon rod anodes. During the reaction, a quinone dimethide intermediate probably forms [88]. Many substituted polymers also form by this reaction [88]. Following is an example of one of them:

Another interesting synthesis is through an equilibration reaction of suitably substituted styrenes to p-xylylenes that spontaneously polymerize to linear polymers [108]. A suitable substituent can be a p-cyanophenyl methyl group. Such compounds undergo complete ionization and rearrange in the presence of a strong base:

The p-xylylene intermediate then polymerizes:

There are, however, competing side reactions, such as dimerization [108]:

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مواضيع ذات صلة

Polyacetals and Polyketals

Phenoxy Polymers

Polyimides

Aromatic Polyamide-Imides and Aromatic Polyester-Imides

Aromatic Polyamides

Special Reactions for Formation of Polyamides

Fatty Polyamides

Nylons

Polyesters from Lactones

Polycarbonates

Network Polyesters for Surface Coatings

Linear Unsaturated Polyesters