Poly p-xylylene

Pyrolysis of p-xylene yields poly(p-xylylene) [lUPAC poly(l,4-phenyleneethylene)] [Errede and Szwarc, 1958 Humphrey, 1984 Itoh, 2001 Lee, 1977-1978 Lee and 

Wunderlich, 1975 Surendran et al., 1987]. The reaction is carried out by pyrolyzingp-xylene at temperatures of up to 950 C (in tbe absence of oxygen) to yield di-p-xylylene (Lm) (also named [2,2]-paracyclophane), which is isolated as a stable solid. Subsequent polymerization of di-p-xylylene to poly(p-xylylene) involves vaporization of di-p-xylylene by heating to about 160°C at 1 toir (1.3 x 10 Pa) followed by cleavage to p-xylylene diradical (LIV) by heating at 680°C at 0.5 torr. The diradical nature of p-xylylene was shown by ESR and 

Polymerization is initiated by couphng of two p-xylylene radicals to form a diradical (LV), which then grows by the addition of p-xylylene at both of its radical centers  

Whether the formation of poly(p-xylylene) should be included in this chapter is not clear. Decisive data are not available to indicate the classification of this pol)rmerization as a step or chain reaction. The formation of high polymer occurs instantaneously when p-xyly-lene contacts the cool surface, precluding the evaluation of polymer molecular weight versus conversion. Also, the mode of termination for this reaction is unknown. 

One aspect of the polymerization that is well established is the initiation step when di-p-xylylene is pyrolyzed. An alternate initiation mode involving the direct formation of the diradical LV from LIII by cleavage of only one of the two CH2—CH2 bonds is ruled out from experiments with monosubstituted di-p-xylylenes. When acetyl-di-p-xylylene is pyrolyzed and the pyrolysis vapor led through successive condensation surfaces at temperatures of 90 and 25°C, respectively, the result is the formation of two different polymers neither of which is poly(acetyl-di-p-xylylene). Pyrolysis yields acetyl-p-xylylene and p-xylylene 

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PX forms j xylylene when heated above 1200°C. The stmctuie of J-xylylene is represented by a i)-quinoid stmcture or as a i)-ben2enoid brtadical. Condensation yields poly(p-xylylene) (19—22) (see Xylene polymers). 

Moreover, where i r-acetjldi- -xyljlene [10029-00-2]( is pyroly2ed, by adjusting temperatures in the deposition region, it is possible to isolate two different polymeric products, ie, poly(acetyl-/)-xylylene) [67076-72-6] (8) and poly(p-xylylene) (PPX) (2). This of course requires the cleavage of the original dimer into two fragments. 

The linear polymer of PX, poly(p-xylylene) (PPX) (2), is formed as a VDP coating in the parylene process. The energetics of the polymerization set it apart from all other known polymerizations and enable it to proceed as a vapor deposition polymerization. 

Although the first two materials discussed in this chapter, the polyphenylenes and poly-p-xylylenes, have remained in the exotic category, most of the other materials have become important engineering materials. In many cases the basic patents have recently expired, leading to several manufacturers now producing a polymer where a few years ago there was only one supplier. Whilst such competition has led in some cases to overcapacity, it has also led to the introduction of new improved variants and materials more able to compete with older established plastics materials. 

Polybenzyls polyphenethyls Parylenes (poly-p-xylylene) Fusible, soluble, and stable at 400°C (752°F) low molecular weight. Melt above 520°C (968°F) insoluble capable of forming films poor thermal stability in air stable to 400-525° C (752-977°F) in inert atmosphere. 

Zur kathodischen Elektro-polymerisation von l,4-Bis-[difluor-brom-mcthyl]-benzoi unter Debromicrurvg s.Lit.3. Zur Polymerisation von l,4-Bis-[brommethyl]-benzol zu Poly-p-xylylen s.Lit.4. 

The best developed example of a material produced by VDP is poly(p-xylylene) designated as Parylene-N by the Union Carbide Corporation. Poly(/i-xylylene) was discovered by Szwarc12 in 1957 and then commercialized by Gorham at Union Carbide.13,14 (Scheme 1). Gorham has reported that di-p-xylylene is quantitatively cleaved by vacuum vapor-phase pyrolysis at 600°C to form two molecules of the reactive intermediate /i-xylylene, which subsequently polymerizes on the cold substrate. In a system maintained at less than 1 Torr, p-xylylene spontaneously polymerizes on surfaces below 30°C to form... 

The major drawback of poly(p-xylylene) is that it reverts to a monomer when thin films are heated above ca. 400°C and it cracks when the films are annealed at 300-350°C in nitrogen. During module assembly the chip-joining (soldering)... 

The high strain energy of [2.2]paracyclophane (see Section 2.1.) facilitates ring-opening of the molecule via cleavage of the benzyl-benzyl bonds. Pyrolysis at 400 °C affords p,p -dimethylbibenzyl (155) and p,p -di-methylstilbene 109h At 600 °C, p-xylylene (156) is formed it polymerizes spontaneously to the linear poly-p-xylylene (10) on condensation 110>. 

Crystal structure analysis of poly(p-xylylene) (PPX) P-form [2,25]... 

Knapton D, Iyer P, Rowan SJ, Weder C. Synthesis and properties of metaUo-supramolecular poly(p-xylylene)s. Macromolecules 2006 39 4069-4075. 

The photoconductivity of poly-p-xylylenes were investigated under various conditions [218-221]. The fundamental edge absorption of the polymers is at 300 nm. The photocurrents in the visual range of the spectra depended on the electrode nature. So it was interpreted as photoinjection of the charges from electrodes and separation of them at a Schottky type barrier. Suppression of hole injection for the plasma-treated polymer is related to the existence of an oxidized surface layer. 

Poly (p-Xylylene). it was discovered by Szwarc, researched by several companies, and finally commercialized experimentally by Union Carbide. It is prepared by pyrolyzing p-xylene at high temperature and vacuum, then condensing on a cool surface, and is... 

Poly-p-xylylenes were prepared in excellent yield by electrolytic reduction of a,a -dihalo-/>-xylenes at controlled cathode potentials (28). Polymer and halides are formed at the cathode at the anode the halide is oxidized to halogen. It has been known that some of the a-a -dihalo- -xylene type of compounds have been polymerized by a variety of reducing agents, such as zinc, copper, phenyllithium, sodium and iron ... 

L. Alexandrova, R. Vera-Graziano, Poly-p-Xylylenes, in J. C. Salamone (Ed.), Polymeric Materials Encyclopedia, CRC Press, Boca Raton, FL, 1996, p. 7180. 

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