Poly p-xylylene s

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  

The process was improved by using di-p-xylylene as an intermediate. This dimer converts to a polymer under milder conditions quantitatively. Both methylene bridges cleave to form p-xylylene, which is a reactive intermediate  

The molecular weights of these polymers were estimated to be as high as 500,000. The total process is sometimes called transport polymerization. Poly(p-xylylene) films are produced commercially. The value of Tm for this polymer, which is crystalline, is 400 °C and it carries the trade name of Parylene. Films of poly(p-xylylene) have only fair thermal stability and are brittle, but exhibit good chemical resistance and are very good electrical insulators. Pyrolysis of xylene in steam at 950 °C yields the dimer intermediate. The yield is reported to be 15%.  

It is possible to form substituted poly(p-xylylene)s by starting with substituted structures. Among the compounds reported were chlorinated and brominated compounds as well as some containing alkyl, cyano, acetyl, and carboxymethyl derivatives. When the di-p-xylylenes are unsymmetrically substituted, two homogeneous polymers form during pyrolysis, because the two condense with spontaneous polymerization at two different temperatures.  

Poly(p-xylylene)s can also be prepared by other reactions. Among them is the condensation of trimethyl(p-methylbenzyl) ammonium halide in the presence of a base  

7 Step-Growth Polymerization and Step-Growth Polymers 

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Zur kathodischen Elektro-polymerisation von l,4-Bis-[difluor-brom-mcthyl]-benzoi unter Debromicrurvg s.Lit.3. Zur Polymerisation von l,4-Bis-[brommethyl]-benzol zu Poly-p-xylylen s.Lit.4. 

Knapton D, Iyer P, Rowan SJ, Weder C. Synthesis and properties of metaUo-supramolecular poly(p-xylylene)s. Macromolecules 2006 39 4069-4075. 

The discovery of poly(p-xylylene)s (PPX)s is attributed to M. Szwarc at around 1947. He found that the p5n olysis of p-xylene produces thep-xylyl radical. This radical disproportionates into a more stable p-quinodimeth-ane diradical. The diradical is somehow stable in the gas phase, but not in the liquid phase. An insoluble polymer is formed with a softening point at 175°C. Superhcially, Szwarc was interested in the bond strength of aromatic hydrogens, however, he wrote a review on the topic, although he probably became more famous for living polymers. 

Schafer, F. Brink-Spalink, B. Smarsly, C. Schmidt, J. H. Wen-dorff, C. Witt, T. Kissel, and A. Greiner. Synthesis and properties of substituted poly(p-xylylene)s prepared by base-induced 1,6-dehydrohalogenation. Mflcromo/. Chem. Phys., 200 1942-1949, 1999. 

M. Ishaque, S. Agarwal, and A. Greiner. Synthesis and properties of novel poly(p-xylylene)s with aliphatic substituents, e-polymers 031/2002 [electronic] http //www.e-polymers.org/, 2002. 

The chemical structures of sulfonated poly(4-phenoxybenzoyl-l,4-phenyl-ene) (S-PPBP) (1), poly(p-xylylene) (S-PPX) (2), poly(phenylene sulfide) (S-PPS) (3), poly(phenylene oxide) (S-PPO) (4), poly(ether ether ketone) (S-PEEK) (5), poly(ether ether sulfone) (S-PEES) (6), arylsulfonated poly(ben-zimidazole) (S-PBl) (7) sulfonated polyphenylquinoxiiline (S-PPQ) (8) and sulfonated phenoxy polyperyleneimide (PSPPI) (9) are shown below. ACPs are sulfonated using common sulfonating agents [82-85]. In particular, PEEK can be sulfonated in concentrated sulfuric acid [50], chlorosulfonic acid [86], SO3 (either pure or as a mixture) [53,65,86,87], a mixture of methanesulfonic acid with concentrated sulfuric acid [88] and acetyl sulfate [89,90]. 

Supramolecular poly(p-xylylene)s based on 2,6-bis (I -methylbenzimidazolyl) pyridine end-capped telechelic oligomers with a p-xylylene core and different metal salts can be prepared in this way. The basic structure is shown in Figure 2.8. 

In contrast to other metallosupramolecular polymers, where the nature of the metal salt plays a critical role, only minor property differences were observed for the materials studied here. The properties of the supramolecular poly(p-xylylene)s appear to be primarily governed by the crystalline nature of the telechelic oligomer [52]. 

Mulpuri SV, Shin BG, Bognitzki M, Greiner A, Yoon DY. Thermally cross-linkable poly (p-xylylene)s for advanced low-dielectric applications. Macromol Chem Phys 2012 213 (7) 705-12. 

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