P-xylylene monomers

Successjul p-Xylylene VDP Monomers, within the limits mentioned above, it is frequently possible, and often desirable, to modify the p-xylylene monomer by attaching to it certain substituents. Limitations on such modifications lie in the three areas reactivity, performance in the coaler (deposition equipment) and cost. 

Scheme 1. Preparation of p-xylylene monomers and their polymerization after deposition together with metal vapors...

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 ... 

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. 

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. 

It is also possible to interfere with the polymerization by attaching at the alpha positions either too many groups, or groups which are too bulky. Four chlorine atoms (13) or four methyl groups (14) seem to be sufficient to hinder the production of polymer. These crowded p-xylylene monomers can be polymerized, but not through a VDP process. 

Table 3. Threshold Condensation Temperatures Ttc for Substituted p-Xylylene Monomers...

Parylene is a completely different class of coating. The p-xylylene monomer is stable as a gas at low pressure but polymerizes spontaneously on any surface on which it condenses. The polymer formed has excellent moisture, chemical, mechanical, and electrical properties, but expensive application equipment is required. 

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. 

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)... 

A particularly useful property of the PX monomer is its enthalpy of formation. Using a scmicmpirical molecular orbital technique, the heat of formation of p-xylylene has been computed to be 234.8 kJ/mol (56.1 keal/mol). 

Only one exception to the clean production of two monomer molecules from the pyrolysis of dimer has been noted. When a-hydroxydi-p-xylylene (9) is subjected to the Gorham process, no polymer is formed, and the 16-carbon aldehyde (10) is the principal product in its stead, isolated in greater than 90% yield. This transformation indicates that, at least in this case, the cleavage of dimer proceeds in stepwise fashion rather than by a concerted process in which both methylene—methylene bonds are broken at the same time. This is consistent with the predictions of Woodward and Hoffmann from orbital symmetry considerations for such [6 + 6] cycloreversion reactions in the ground state (5). 

The thermodynamic ceiling temperature (26) T for a polymerization is computed by dividing the Afi°polym by the standard entropy of polymerization, A+°polym. The T is the temperature at which monomer and polymer are in equilibrium in their standard states at 25°C (298.15 K) and 101.3 kPa (1 atm). (In the case of p-xylylene, such a state is, of course, purely hypothetical.) The T quantifies the binding forces between monomer units in a polymer and measures the tendency of the polymer to revert back to monomer. In other systems, the T indicates a temperature above which the polymer is unstable with respect to its monomer, but in the case of parylene it serves rather as a means of comparing the relative stability of the polymer with... 

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 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. 

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 earliest work on the copolymerization of p-xylylene type monomers was reported in the brilliant pioneering studies of M. Szwarc (17). He and Roper (5) studied the pyrolysis of mixtures of p-xylylene and pseudocumene and concluded that mixtures of p-xylylene and 2-methyl-p-xylylene were formed which copolymerized on condensation. Apparently no extensive work on copolymerization or the nature of the products was conducted. 

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... 

In considering the copolymerization of substituted p-xylylenes, such as X and XI, an important question is whether the growing polymer chain XII (which has just added a unit of monomer X) shows any preference for reacting with X or XI. The chain-propagating step involves addition of a radical to a highly reactive monomer to form a covalent bond and a... 

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. 

Copolymerization of Chloro- and Butyl- -xylylene. Mixtures of dichloro-di-p-xylylene (XIII) and butyl-di-p-xylylene (XIV) were pyro-lyzed to form chloro-p-xylylene (XV), butyl-p-xylylene (XVI), and p-xylylene (VII). In the initial polymerization zone (ca. 90°C.) chloro-p-xylylene (XV) and butyl-p-xylylene (XVI) condensed and polymerized. Since this temperature is above the Tc of p-xylylene at 0.3 mm., p-xylylene molecules passed through and condensed and polymerized in the final, air-cooled zone. In this way it was possible to study the copolymerization of chloro- and butyl-p-xylylenes starting from the mixtures of the three monomers XV, XVI, and VII. 

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... 

Unique Capabilities. A number of unique features and capabilities of the p-xylylene polymerization process have been uncovered and elucidated. The reactive monomers will condense and polymerize on any solid surface placed in the condensation (deposition) zone. The chemical nature of the surface is unimportant, and many materials, including strong acids, bases, the alkali metals, metal hydrides, and chemically reactive compounds such as resorcinol, have been coated when placed in a deposition chamber. 

The polymerization of p-xylylenes on condensation is extremely rapid and appears to proceed from gaseous monomer to solid polymer without passing through a viscous stage. The monomer behaves as a reactive plasma which surrounds solid objects placed in the deposition chamber. 

In a further elaboration of this feature the ability of the monomer to penetrate between closely positioned glass slides and into deep crevices was investigated. In the initial experiment, several pairs of 3 inch X 3 inch glass slides were placed in a polymerization zone, the distance between the parallel surfaces of the plates varied from 0.017 to 0.125 inch, and sufficient chloro-p-xylylene was introduced to deposit 1.0 mil poly(chloro-p-xylylene) on the walls of the chamber and exposed surfaces. The plates were placed perpendicular to the flow of the gaseous monomer. At the end of the experiment the samples were removed, and the thickness of the coating at the center point on the inside surface of the plates was examined with the following results ... 

In a second series of experiments, 3 inch X 3 inch glass slides were joined at one end, the side openings were closed with tape, and the angle of the open end (0) varied from 90° to 1°. This experiment was designed to investigate the degree of penetration of the monomer-polymer into a crevice. The polymerization was conducted to deposit 1.0 mil polv-(chloro-p-xylylene) on the walls of the chamber. The samples were removed at the end of the run, and the thickness of the film at the bottom point of the 3-inch crevice was measured. 

Particle Encapsulation. A unique capability of the p-xylylene vapor deposition process has been uncovered in the area of encapsulation (12) of particulate solids. The particles or granules to be encapsulated are placed in a container which in turn is placed in the deposition chamber, and the nozzle from the pyrolysis tube is inserted in the mouth of the bottle. During the run the monomers pass from the pyrolysis zone through a nozzle into the bottle and polymerize on the surface of the tumbling particles or granules. Polymer is also formed on the inner surface of the bottle which is rotated at 50-150 r.p.m. Relatively simple equipment (Figure 4) has been used to study this phenomenon. 

Another striking example concerns the polymer poly(p-xylylene) (PPX). It is obtained by condensation at room temperature of the gaseous monomer. A clear foil may be obtained in that way. The melting point of PPX is 427 °C and it crystallises immediately after polymerisation, so that entanglement cannot be formed. Van der Werff and Pennings (1988, 1991) have shown that hot drawing of such a material at 420 °C yields a material with a tensile strength of 3 GPa and a tensile modulus of 100 GPa. 

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... 

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-... 

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