Para-xylylene

Several studies have concerned the microstnicture of lamellae in materials such as the block copolymers polystyrene-h/oc/r-poly-l-vinylpyridine [139] and polystyrene-h/oc/r-polybutadiene [140], as well as single crystals of poly-para-xylylene [139], and reveal features (such as intersecting lamellae (figure Bl.19.29)) that had not been previously observed. 

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]... 

Dehydrochlorination of Unsymmetrically Substituted para-Xylylene Dichorides... 

A PPV derivative which is twofold phenylsubstituted at the vinylene unit, poly(l,4-phenylene-l,2-diphenylvinylene DP-PPV), (71b) (see also the discussion of dehydrochlorination of unsymmetrically substituted para-xylylene dichlorides in Section 3.1) was first synthesized by Smets et al., using acid-catalyzed elimination of nitrogen from l,4-bis(diazobenzyl)benzene 83 [106]. The yellow products obtained are fully soluble in common organic solvents (toluene, chloroform, ethylene chloride, DMF, THF). 

Since the [6]radialenes are triple-diene systems, it comes as no surprise that they have been used in multiple Diels-Alder reactions. In fact, after a first 1 1 addition with 150, leading to 161, has taken place, the reaction could proceed in two fashions—a linear course of addition leading to a para-xylylene 162, and an angular route which produces an ortho-xylylene intermediate 163 (equation 19>I<12 103. 

Fortin, J. B. Lu, T. M., Ultraviolet radiation induced degradation of poly para xylylene (parylene) thin films, Thin Solid Films 2001, 397, 223 228... 

Explain why nucleophilic addition to 23 to give 24 by attack at nitrogen is more likely than the corresponding addition to a quinone by attack at oxygen. At what position would you expect nucleophilic addition to occur most readily to para-xylylene Explain. 

Figure 2. Experimental (A isomer) and computed (A isomer) low energy structures of the dicopper(I) complex of the macrocyclic Schiff-base ligand with para-xylylene spacer groups, and computed energy barrier (confirmed by NMR spectroscopy).125126...

Chemical vapor deposition (CVD) of poly(para-xylylene carboxylic acid pentafluorophenolester-co-para-xylylene] (PPX-PPF) on PDMS can prevent it from solvent swelling [262]. 

The parylene coating process is a three-step procedure that includes vaporization, pyrolysis, and polymerization. The parylene C coating process, shown schematically in Fig. 15.8, begins with the vaporization of the precursor dimer (di-para-xylylene), a granular white powder, at 150 °C and a pressure of 1 Torr. This... 

Y. S. Yeh, W. James, H. Yasuda, Polymerization of para-xylylene derivatives, VI. morphology of parylene N an parylene C films investigated by gas transport characteristics, /. Polym. Sci. Part B 1990, 28, 545. 

Both films with organic FETs and photodiodes are transferred to the vacuum chamber without exposure to air after the manufacturing process and uniformly coated with a 2 pm poly(monochloro-para-xylylene) (parylene) passivation layer. Spots of parylene on electrodes are removed by a CO2-laser drilling machine for... 

The process of parylene polymerization is presented schematically in Figure 5.2 using parylene N, unsubstituted poly(para-xylylene). Parylene dimer is heated until it sublimes. The dimer vapor passes through a high temperature pyrolysis zone where it cracks and becomes monomer vapor, i.e., monomer is created in vacuum. The monomer polymerizes and deposits in the deposition chamber, which is usually at room temperature. Parylene polymerization completed in a vacuum is a process involving no solvents, no curing, and no liquid phase. Its use essentially eliminates concern about the operator s health and safety, air pollution, and waste disposal. 

Parylene polymerization proceeds with well-defined chemical species, whereas plasma polymerization proceeds via variety of not-well-defined chemical species, which are created in the luminous gas phase. The reactive species for parylene polymerization is para-xylylene, which has features of (1) difunctional (e.g., diradicals), (2) reactive but relatively stable, and (3) highly selective reactivity (see Fig. 2.1). The exact nature of reactive species involved in glow discharge polymerization is not well known however, (1) they are not exclusively bifunctional, (2) they are highly reactive, and (3) consequently they have very low selectivity. The difference in the stability or the selectivity of reactive species is reflected in the distinctively different characters of polymer depositions of these two processes. 

In contrast to the above situations, parylene polymer deposition has very poor adhesion to a smooth surface substrate but can penetrate deep into small cavities. para-Xylylene prefers to react with another para-xylylene or its derivatives. Although it has the feature of difunctional free radical, it is rather stable and does not initiate polymerization of other monomers for conventional free radical polymerization. In spite of numerous attempts, the polymerization of various vinyl monomers initiated by para-xylylene or copolymerization of vinyl monomers with /7ura-xylylene has been elusive. 

The growth step of vacuum deposition polymerization can be conceived as polyrecombination of free radicals. Because para-xylylene (monomer) has the feature of diradical, the recombination does not terminate the propagation reaction (free radical living polymer) and leads to the presence of free spins in the final product (film). 

Parylene C, or monochloro-substituted poly(para-xylylene), is a polymer that has excellent bulk mechanical properties as well as excellent barrier properties for... 

The reactive species in Parylene C deposition that interacts with the substrate surface is para-xylylene, in which two free radicals exist in the para position of a benzene ring. Para-xylylene is relatively stable and reacts only with other free radicals or with other para-xylylene units. In order to create a good adhesion of Parylene C film to a smooth-surface substrate, it is necessary to create free radicals on the substrate surface. With the aid of plasma interface modification, it is possible to achieve strong adhesion of Parylene coatings to such smooth surfaces. Strong adhesion of Parylene C coating to bare 7075-T6 (an aluminum alloy) panels was achieved with the application of plasma polymers [16]. 

Soluble polyimide prepared by reaction of (6F) with l.S-diaminonaphthalene-Soluble polyimide prepared by reaction of (6F) 4th 1,4-diaminobenaene. Poly(para-xylylene) Unkm Carbide Reg. trademark. 

Soluble polyimide prepared by reaction of 4,4 -hexafluoroisopropylidene diphthalic anhydride (6F) with 3,5-diaminobenzoic acid. b Soluble polyimide prepared by reaction of (6F) with 1,5-diaminonaphthalene. c Soluble polyimide prepared by reaction of (6 F) with 1,4-diaminobenzene. d Poly(para-xylylene) Union Carbide Reg. trademark. e Dupont 100 CA-43. f Union Carbide Sulfone 47. 

Further persuasive evidence for this interpretation came from a systematic investigation of well-designed dinuclear model systems in which the central methylene group (with respect to complex 1) has been replaced by propylene (2) or para-xylylene (3) groups (Scheme 3). These spaced precursors 2 and 3 (although comparable to rac-1 both electronically and sterically) are only poor hydroformylation catalysts as compared to rac-1. 

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