Gorham process

In the commercial Gorham process (2), the extremely reactive PX is conveniendy generated by the thermal cleavage of its stable dimer, Vo-di- -xyljIene (DPX), a [2.2]paracyclophane [1633-22-3] (3). In many instances, substituents attached to the paracyclophane framework are carried through the process unchanged, ultimately becorning substituents of the polymer in the coating. 

Only one exception to the clean production of two monomer molecules from the pyrolysis of dimer has been noted. When a-hydroxydi-Zvxyljlene (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). 

VDP processes using means other than the pyrolytic cleavage of DPX (Gorham process) to generate the reactive monomer are also known, although none are practiced commercially at the time of this writing (ca 1997). 

When -xylene is used as the monomer feed in a plasma polymer process, PX may play an important role in the formation of the plasma polymer. The plasma polymer from -xylene closely resembles the Gorham process polymer in the infrared, although its spectmm contains evidence for minor amounts of nonlinear, branched, and cross-linked chains as well. Furthermore, its solubiUty and low softening temperature suggest a material of very low molecular weight (15). 

The PPXs formed as coatings in tile Gorham process are referred to generically as the parylenes. 

Poly-paraxylylenes are deposited by a thermal CVD process popular in the microelectronics industry as the "Gorham process"." The starting material for this deposition process (precursor) is usually a paracyclophane dimer (DPX). The dimer dissociates to form reactive monomer species, which then undergoes nucleation and growth leading to polymer thin film formation. The following reaction ensues ... 

Although Gorham process is the most popular method of parylene polymerization, there are reports of at least two other polymerization schemes for parylenes. Figure 3 shows the different polymerization schemes used for CVP of parylene thin films. However, in this paper, only the Gorham process will be discussed, the details of alternate polymerization schemes can be found elsewhere. ... 

Chemical vapor polymerization of parylene thin films occurs through free radical polymerization. A generalized schematic of the chemical vapor polymerization reactor, employing the Gorham process is shown in Figure 4. The mechanism of polymerization is as follows ... 

The Gorham process uses [2.2]paracyclophanes as a precursor for deposition. The precursor is evaporated at 150°C in vacuo. In the next stage, the vapors are conducted into a pyrolysis chamber at 700°C. Here the monomeric diradical is formed. Then the reactive vapor reaches a deposition chamber at an ambient temperature. The vapor condenses and polymerizes at the cold surfaces. The technique of CVD for polymer has been reviewed by several authors. ... 

Pinholes that are permeable to ions can be detected by cyclovolta-metric measurements. Films deposited on iron surfaces by the Gorham process show significant ion permeability for film thicknesses below 500 nm, but no permeability at all for films equal to or thicker than 700 nm. The tendency of the formation of pinholes is related to the surface roughness." ... 

In the commercial Gorham process (3), the extremely reactive PX is conveniently generated by the thermal cleavage of its stable dimer, cycZo-di-p-xylylene... 

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