Parylene dimers

Parylene Dimer of para-xylene No initiator Gas phase... 

The process of parylene polymerization is presented schematically in Figure 5.2 using parylene N, unsubstituted poly(para-xylylene). Parylene dimer is heated until it sublimes. The dimer vapor passes through a high temperature pyrolysis zone where it cracks and becomes monomer vapor, i.e., monomer is created in vacuum. The monomer polymerizes and deposits in the deposition chamber, which is usually at room temperature. Parylene polymerization completed in a vacuum is a process involving no solvents, no curing, and no liquid phase. Its use essentially eliminates concern about the operator s health and safety, air pollution, and waste disposal. 

FIGURE 2.1.6 Reactions involved in parylene deposition process (top) sublimation of dimers at 100°C, splitting into monomers at 700°C, and polymerization at room temperature. Inexpensive system for parylene deposition (bottom) consists of a two-zone tube furnace and a 20-mm ID quartz tube containing parylene dimer powder and connected to a one-stage mechanical pump through a liquid nitrogen trap. A sample with prefabricated contacts and attached wire leads is placed in the tube at about 30 cm from the furnace. 

Several types of parylene dimer are commercially available (and many more have been synthesized and reported in the scientific and patent literature), the most popular are C, D, and N. Parylene-C is often used as an encapsulant for organic semiconductors, and is also sometimes used as a gate dielectric. 

C. Lee and D. R. Bassett. Process for the preparation of the parylene dimer. US Patent 4 769 505, assigned to Union Carbide Corporation (Danbury, CT), September 6, 1988. 

In contrast to the extreme reactivity of the monomeric PX (1) generated from it, the dimer DPX (3) feedstock for the parylene process is an exceptionally stable compound. Because of their chemical inertness, dimers in general do not exhibit shelf-life limitations. Although a variety of substituted dimers are known in the Hterature, at present only three are commercially available DPXN, DPXC, and DPXD, which give rise to Parylene N, Parylene C, and Parylene D, respectively. 

This polymer first appeared commercially in 1965 (Parylene N Union Carbide). It is prepared by a sequence of reactions initiated by the pyrolysis of p-xylene at 950°C in the presence of steam to give the cyclic dimer. This, when pyrolysed at 550°C, yields monomeric p-xylylene. When the vapour of the monomer condenses on a cool surface it polymerises and the polymer may be stripped off as a free film. This is claimed to have a service life of 10 years at 220°C, and the main interest in it is as a dielectric film. A monochloro-substituted polymer (Parylene C) is also available. With both Parylene materials the polymers have molecular weights of the order of 500 000. 

In spite of its potential commercial utility, PA-F dimer has not been extensively used as a Parylene-F precursor because the only reported preparative methods for PA-F dimer involve pyrolysis of different precursors at very high temperatures 600-950°C and yields are very low. The conventional way of synthesizing PA-F dimer involves a pyrolysis process as shown in Eq. (3). 

The eight-carbon monomer PX is generated in the first stage of the parylene process by heating gaseous dimer as it passes through a high temperature zone. 

Parylene-C, the trade name of the film formed from the Union Carbide Corp. brand of dichloro-p-xylylene, is a vapor deposited film formed by the reaction shown in Figure 1. The solid dimer(I) is vaporized and pyrolyzed at 650° C to 750 C to the reactive olefinic monomer, chloro-p-xylylene(II), which polymerizes on cool surfaces in the low pressure deposition chamber to form the crystalline linear polymer poly(chloro-p-xylylene) (III) (5). 

The parylene coating process is a three-step procedure that includes vaporization, pyrolysis, and polymerization. The parylene C coating process, shown schematically in Fig. 15.8, begins with the vaporization of the precursor dimer (di-para-xylylene), a granular white powder, at 150 °C and a pressure of 1 Torr. This... 

Chain initiation occurs when two monomer radicals are coupled to form a dimer biradical and proceeds further." This is an endothermic reaction requiring a heat of formation of 16 kcal/mol. Because of energetic concerns, chain initiation is unlikely to happen in the gas phase at low pressure. When the monomers are adsorbed onto the surface of the substrate, it is believed that, the high local concentration of monomers promotes the formation of biradicals assisted by van der waals forces. Models developed for vapor deposition polymerization of parylene-N indicate that initiation is a third order reaction with an activation energy of 24.8 kcal/mol. 

Later researchers followed essentially the same process, substituting monomer precursors instead of the dimer, to produce PPV films. " The monomers were evaporated and the vapors carried into the pyrolysis chamber, where they were pyrolyzed at 800°C. The reactant species were then transported to the substrate maintained at 60°C to form the precursor polymer film. This film was then thermally converted to PPV at 150-320°C. - Scafer et al. have further shown that PPV can be produced via the dehydrogenation route introduced by Iwatsuki et al. They showed that soluble a-phenyl substituted poly-p-xylylene can be prepared by CVP of l-a-chlorobenzyl-4-methylbenzene. The parylene was then dehydrogenated by DDQ resulting in PPV. This process could be used to produce both segmented and unsegmented PPVs. The segmentation ratio was controlled by the molar ratio of the parylene to DDQ, in the... 

Parylene Thermal dissociation of cyclic dimer Difunctional free radicals Recombination of free radicals... 

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