P-Xylylene polymerization

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 enthalpy of formation of gaseous p-xylylene of 210 20 kJ mol-1 was obtained from the energetics of gas-phase ion-molecule reactions from S. K. Pollack, B. C. Raine and W. J. Hehre, J. Am. Chem Soc., 103, 6308 (1981). Since gaseous p-xylylene polymerizes... 

It is well known that p-xylylenes (or p-quinodimethanes) and their substituted derivatives are, in general, highly reactive. For example, the parent hydrocarbon, p-xylylene, polymerizes with extreme readiness in the condensed state and reacts with oxygen (Montgomery et al., 1986). 

The original polymerization of p-xylylene was carried out by vacuum pyrolysis of p-xylene at 900-950 °C. The intermediate, p-xylylene, polymerizes spontaneously upon condensation on cooler surfaces ... 

Pent-2-yne-4-ene-l-ol, polymers of, 60 Perchlorocoumalin polymers with 4,4 -diphenylmethane-bis(maleimide), 117 with AT,A( -Hexamethylenebis-(maleimide), 116 Perchloro-4,4 -dimethylbiphenyl, polymers of, 26 Perchloro-p-dipropenylbenzene polymerization of, 11 polymers of, 26 Perchloro-p-xylylene polymerization of, 11,12 polymers of, 15,26 Perfluoro-1,3-pentadiene, polymers of, 58 Perfluoro-1,4-pentadiene,... 

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. 

Benzyl AT-ethyldithiocarbamate (44) and p-xylylene bis(N-ethyldithiocarba-mate) (45) were also prepared as mono- and difunctional photoiniferters, respectively [171,172], consisting of a structure similar to 7 and 8. The polymerization of St with 44 under ultrasonic irradiation was also reported [173]. 

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... 

The thermal stability of PNT from different polymerization methods is presented in Table 18.7. ft appears that the colored (dark brown) but transparent PNT -N film synthesized by VDP is the cleanest film among the polynaphthalenes from other polymerization processes that have been reported. These PNT-N films from VDP also have very low dielectric constants in comparison to poly(tetra-fluoro-p-xylylene) films. PNT-N and PNT-F films have higher dissociation temperatures (>570°C) and better thermal stability (>530°C), and no film cracking was observed until PNT-F was annealed at 600°C in nitrogen. Table 18.8 presents a summary of the different properties ofPNT-N and PNT-F prepared by the VDP process. 

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>. 

Polymerization is initiated by coupling 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 polymerization 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... 

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 ... 

Thermodynamic Considerations. On the basis of the value for the enthalpy of formation of p-xylylene, the enthalpy of polymerization, can be estimated. 

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... 

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

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

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). 

This article surveys the polymerization of quinodimethanes including flash pyrolysis of p-xylene, the synthesis of poly-p-xylylene other than by flash pyrolysis, the vapor-coating process by Gorham, and the polymerizations of halo-p-xylylenes and electron-accepting quinodimethanes. 

In 1947 Szwarc prepared a white polymeric material u by rapid flow pyrolysis of p-xylene under reduced pressure. On the basis of p-xylylene diiodide 2) detected in the reaction mixture of the pyrolysis products with iodine gas he proposed a formation 1,3) of p-xylylene(p-quinodimethane) (QM) in this pyrolysis. He claimed the polymeric material to be poly-p-xylylene(poly-QM)and proposed a mechanism 2) for the formation of poly-QM, involving thermal cleavage of carbon-hydrogen bonds of p-xylene to yield p-xylyl radicals which collide with each other to give p-xylene and QM through disproportionation. QM condenses and polymerizes to produce poly-QM. 

Perchloro-p-xylylene prepared by Ballester et al. 62) exhibits no tendency for polymerization and is very stable even at elevated temperature. 

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