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 ... 

II. The polymerization takes place without an initiator, or the initiator is a true catalyst. The monomer is first converted into an active species M [e.g., diradical in xylylene polymerization, equation (16-28), or zwit-terion, etc.], which is in equilibrium with the inactive form ... 

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. 

It is also possible to iaterfere with the polymerization by attaching at the alpha positions either too many groups, or groups which are too bulky. Four chlorine atoms (12) or four methyl groups (13) seem to be sufficient to hinder the production of polymer. These crowded -xylylene monomers can be polymerized, but not through a VDP process. 

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. 

In estimating the enthalpy of polymerization, the physical state of both starting monomer and polymer must be specified. Changes in state are accompanied by ethalpy changes. Therefore, they also affect the level of the polymerization enthalpy. The AfT forN ylylene previously mentioned is apphcable to the monomer as an ideal gas. To make comparisons with other polymerization processes, most of which start with condensed monomer, a heat of vaporization for N ylylene is needed. It is assumed herein that it is the same as that for N ylene, 42.4 kJ /mol (10.1 kcal/mol). Thus the AfT of the hquid monomer -xylylene is 192.3 kJ/mol (46.0 kcal /mol). 

Polymerization by Gycloaddition. Bisimides and oligoimides capped with reactive unsaturations such as maleimide, acetylene, and xylylene groups, can be chain-extended by a cycloaddition reaction with proper bisdienes. 

A xylylene-fc/.v-phosphonium salt 11 gave films of PPV 1 upon clectropolymer-ization. The absorption and emission spectra of the resultant material were blue-shifted with respect to PPV produced by other routes, suggesting that the electro-polymerized material has a shorter effective conjugation length, possibly because of incomplete elimination of phosphonium groups [22]. 

Poly(para-phenylenevinylene)s (PPVs) represent one of the most intensively investigated classes of rr-conjugated materials. Many synthetic procedures to generate unsubstituted and substituted PPVs have been developed. They include 1,6-polymerizations of 1,4-xylylene intermediates as well as several polycondensation methods. Parallel to the polymer syntheses, several series of PPV oligomers (OPVs) have been synthesized and characterized. Such model oligomers of different molecular size allow for a study of the dependence of electronic and optical properties on the length of the conjugated Ti-system. 

However, not all types of para-xylylene analogues are able to polymerize under the Wessling conditions to give polyelectrolyte precursors, as shown by failure to polymerize the anthracene homologue, 65 [771. 

The generation of PPV and corresponding derivatives via the dihalide approach is possible not only in solution reaction, but also - via the gas phase -in a so-called chemical vapor deposition (CVD) process. In this process, the vapor of a dichlorinated para-xylene (a,a or a,a) is pyrolyzed at moderately low pressures (0,1-0,2 torr) to form a chlorinated para-xylylene intermediate, which then condenses and polymerizes on a suitable, cooled substrate. The coating of the chlorinated precursor polymer can be heated to eliminate HCl, to form PPV 60 (or a PPV derivative) [88]... 

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]. 

The compounds in this section—fulvenes, isotoluenes, xylylenes—are characterized by trivial names. Our various reference citations document their nontrivial chemistry isomerization, polymerization and oxidation befall the unwary experimentalist who would study them. 

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... 

Using a fluorinated benzocyclobutene-based monomer (Figure 18.4) should provide at least one advantage over the already promising properties of fluorinated poly(j9-xylylene). All the desirable properties such as low dielectric constant and low affinity for water should remain but the thermal stability should be enhanced because of the cross-linking that would accompany the generation of these films. The synthesis and polymerization paths for poly(octafluorobisbenzocyclobutene) are depicted in Scheme 5. 

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>. 

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