Di-p-xylylene

Paralene [para-xylene] Also called Gorham and also spelled parylene. A process for coating articles with poly-p-xylene. The vapor of di-p-xylylene is pyrolyzed at 550°C, yielding p-xylyl free radicals, -CHj-CgH CH, which deposit and polymerize on cooled surfaces. Developed by W. F. Gorham at Union Carbide Corporation. 

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... 

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... 

In the commercial Gorham process. PX is generated by the thermal cleavage of its stable dimer, cyc/o-di-p-xylylene (DPX). a [2.2]paracyclo-phane (3). In many instances, substituents attached to the paracyclophane framework are carried through the process unchanged, ultimately becoming substituents of the polymer in the coating. 

Purification. Unsubsliluled di-p-xylylene (DPXN) is readily purified by recrystallization front xylene. It is a colorless, highly crystalline solid. The principal impurity is polymer, which is insoluble in the recrystallization solvent and easily removed by hot filtration. In purifying DPXC and DPXD, care is taken not to disturb the homologue composition, so that product uniformity is maintained. 

Manufacture. For the commercial production of DPXN (di-p-xylylene) (3), two principal synthetic routes have been used the direct pyrolysis of -xylene (4, X = Y = H) and the 1,6-Hofmann elimination of ammonium (HNR3) from a quaternary ammonium hydroxide (4, X = H, Y = NR ). Most of the routes to DPX share a common strategy PX is generated at a controlled rate in a dilute medium, so that its conversion to dimer is favored over the conversion to polymer. The polymer by-product is of no value because it can neither be recycled nor processed into a commercially useful form. Its formation is minimized by careful attention to process engineering. The chemistry of the direct pyrolysis route is shown in equation 1 ... 

The structure of di-p-xylylene was first shown to be (59) by Brown and Farthing s (1949) X-ray analysis. The compound crystallizes in the tetragonal system, space group P42/mwm, with two C16H16 molecules in the unit cell. A consequence of this is that mmm molecular symmetry is a crystallographic requirement and the three carbon atoms in the asymmetric unit are completely specified by seven coordinates. The conformation of the molecule is shown in Fig. 6. If the benzene rings... 

The process for preparing linear poly-p-xylylenes by pyrolytic polymerization of di-p-xylylenes has been extended to include the formation of p-xylylene copolymers. Pyrolysis of mono-substituted di-p-xylylenes or of mixtures of substituted di-p-xylylenes results in formation of two or more p-xylylene species. Copolymerization is effected by deposition polymerization on surfaces at a temperature below the threshold condensation temperature of at least two of the reactive intermediates. Random copolymers are produced. Molecular weight of polymers produced by this process can be controlled by deposition temperature and by addition of mercaptans. Unique capabilities of vapor deposition polymerization include the encapsulation of particulate materials, the ability to replicate very fine structural details, and the ability of the monomers to penetrate crevices and deposit polymer in otherwise difficultly accessible structural configurations. 

A new general synthetic method for preparing linear poly-p-xylylenes - was reported recently (6, 7, 8, 9, 10, 11, 13). This new method involves the vacuum pyrolysis of di-p-xylylene or substituted di-p-xylyl-enes at temperatures of 600°-700°C. to form p-xylylenes and the subsequent condensation and spontaneous polymerization of these reactive species to form a family of linear polymers. The over-all reaction scheme is illustrated at the top of p. 644. 

While all substituted di-p-xylylenes are pyrolyzed under substantially identical conditions, the temperatures of condensation and polymerization varied substantially, depending on the p-xylylene derivative under study. It was established that there is a threshold condensation temperature, Tc, above which the rate of condensation-polymerization was very slow under the system conditions (50-100 fi) normally used. The Tcs for several p-xylylene monomers were established as follows ... 

Growth is terminated by coupling of radicals from two growing polymer molecules or by the reactive sites becoming buried in the polymer matrix. Polymers formed by the di-p-xylylene process have been shown (6, 7,8, 9,10, II, 13) to be living polymers and exhibit radical concentrations of 5-10 X 10 4 mole of free electrons per mole of p-xylylene. 

Pyrolysis of monosubstituted di-p-xylylenes, such as acetyl-di-p-xylylene (V) results in formation of two reactive p-xylylenes with different Tcs. The two species were separated as their polymers by using the principle of threshold condensation temperature. The pyrolysis vapors containing the two monomers VI and VII were passed initially through a zone maintained at a temperature low enough to permit rapid condensation and polymerization of acetyl-p-xylylene, but substantially above the Tc of p-xylylene which passed through the first zone and polymerized in a final zone maintained at ambient temperature. In a sense, the monomers were fractionated on the basis of volatility, and the monomers were isolated in the form of their polymers. These transformations are illustrated at the top of p. 646. 

The formation of p-xylylene monomers by pyrolysis of substituted di-p-xylylenes is a quantitative, clean process and provides an excellent starting point for generating mixtures of monomers and for studying their copolymerization. The formation of linear poly-p-xylylenes by the di-p-xylylene route is also a major advantage in characterizing the products formed. 

Considerable flexibility is available in the di-p-xylylene process with regard to preparation of intermediates for generating monomers. Introduction of a substituent on one ring of di-p-xylylene (as, for example, acetyl-di-p-xylylene) provides a starting material which on pyrolysis yields two distinct monomers. Alternatively, disubstituted products, for... 

Assuming the above statements are correct, any two p-xylylene species should be capable of copolymerization in any desired ratio. This is somewhat of a simplification since it has been observed that for each substituted p-xylylene there is a definite ceiling condensation temperature above which it will not condense and polymerize at any appreciable rate. Thus, if the monomer does not condense, it is not available for copolymerization. This was demonstrated in the studies described earlier of the pyrolysis of acetyl-di-p-xylylene and separation of the monomers VI and VII on the basis of widely differing Tc s. 

Preparation of Mixtures of -Xylylenes and Separation into Polymer on the Basis of Tc. The distillation and pyrolysis steps were conducted as described above for di-p-xylylene and substituted di-p-xylylenes. The pyrolysis gases were led immediately into a deposition zone which consisted of a 24-inch section of 1-inch i.d. glass tubing. The initial 15-inch section of this tubing (Zone A) was heated to 90°—100°C. The final 9-inch section of the tubing (Zone B) was maintained at room temperature. The end of the deposition zone was connected via rubber tubing through the dry ice trap to the pump. 

Preparation of Copolymer of Chloro- and Butyl- -xylylene. Mixtures of dichloro-di-p-xylylene and butyl-di-p-xylylene were prepared and melted to ensure homogeneity. The mixture was placed in the distillation zone, and the reaction was carried out in the usual fashion. The copolymers formed in the initial polymerization zone which was maintained at 90°-100°C. Poly-p-xylylene formed in the final, air-cooled zone. At... 

Dichloro-di-p-xylylene (gram) Butyl-di-p-xylylene (gram)... 

Copolymerization of Chloro- and Dichloro-/>-xylylene. Trichloro-di-p-xylylene (XVIII) was obtained by chlorination of di-p-xylylene with three molar equivalents of chlorine. Pyrolysis yielded monomers XV and XIX, which were condensed and polymerized on a 90 °C. surface. A quantitative yield of product was obtained. The product was transparent, tough, self-extinguishing, had a softening point above 280°C., and exhibited the correct elemental analysis for copolymer XX. Owing to the low solubility of the chlorinated poly-p-xylylenes, no attempts... 

Copolymers of ethyl- and chloro-p-xylylene were also prepared by pyrolytic polymerization of mixtures of dichloro-di-p-xylylene and diethyl-di-p-xylylene at 50°C. This is an example of preparation of a copolymer by pyrolysis of a mixture of two disubstituted di-p-xylylenes. 

Control of Molecular Weight. Studies have been conducted on techniques for controlling the molecular weight of poly-p-xylylenes produced from di-p-xylylenes by the vacuum pyrolysis route. Earlier work by Szwarc (17), Errede (3), and Auspos (I) indicated that very reactive chain transfer agents were required to achieve a significant effect in the polymerization of p-xylylene derived from p-xylene. This general picture was confirmed in the present study. 

Example 1 Encapsulation of Lithium Aluminum Hydride with Poly(chloro-p-xylylene). In the distillation zone were placed 5.0 grams of dichloro-di-p-xylylene. In a 4-oz. polyethylene bottle were placed 10.0 grams (400 pellets) of lithium aluminum hydride (LAH). LAH was obtained from Metal Hydrides, Inc., as 1/8-inch diameter pellets. The bottle was positioned in the coating chamber, the system was... 

Example 2 Encapsulation of 3/16-inch Sodium Hydroxide Pellets with Poly( chloro-p-xylylene ). Fifty grams of sodium hydroxide pellets were encapsulated with polymerizing chloro-p-xylylene generated by pyrolysis of 5.0 grams of dichloro-di-p-xylylene over a 15-minute period. The bottle was rotated at 60 r.p.m. during the run. A pyrolysis temperature of 660°C. and system pressure of 50 //, were employed. A total of 51.97 grams of encapsulated pellets was recovered at the end of the run. 

Polymer films can also be deposited on solid particles by vapor phase reaction or from a melt. The best example of vapor phase reaction is the deposition of Union Carbide s Parylene , a derivative of p-xylylene. In this process, di-p-xylylene, or more commonly a halogenated derivative of it, is vaporized in a vac and therm-mally dissociated into the very reactive monomer, a diradical. The monomer is allowed to condense on the surface of the particles to be coated, where it instantaneously polymerizes to form a high molecular weight, polymeric film (Ref 10). Less reactive, vaporizable or meltable polymers can be applied by hot spraying onto agitated particulates or by deposition in a fluidized bed or in liq suspension (Ref 2). Wax is a common example of wall material applied in all three ways... 

The quinonoid form of p-xylylene reacts as a diradical would. It can be produced in quantitative yield by vacuum vapor phase pyrolysis of di-p-xylylene at 550 to 600° C (30). Condensation of the monomer to crystalline polymer does not occur in the gas phase under the vacuum con-... 

The most common conformal coatings are derived from polyurethanes, acrylics, and epoxies the more special formulations for high-temperature performance are based on silicones, diallyl-phthalate esters, and polyimides. An example of a vapor deposited conformal coating is Parylene. It is obtained by vapor deposition of p-xylylene, which is formed as a transient by dehydrogenation of p-xylene at high temperature, and polymerization on the surface of the object to be coated. Because p-xylylene monomer is not stable, it is advantageous to work with the cyclic dimer, di-p-xylylene (paracyclophane), which, upon heating under reduced pressure, will produce the transient monomer which converts to the polymer at low temperatures. 

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