P XYLYLENE POLYMERS

Figure 2. Mesh coated with poly(chloro-p-xylylene). Polymer is transparent inner rings. 16.5 X...

J. Lahann, D. Klee, and H. HOcker. CVD-polymerization of a functionalized poly(p-xylylene). a generally applicable method for the immobilization of drugs on medical implants Materialwiss. Werkstofflech., 30 763-766,1999. S. Y. Park, J. Blackwell, S. N. Chvalun, A. A. Nikolaev, K. A. MaUyan, A. V. Pebalk, and I. E. Kardash. The structure of poly(cyano-p-xylylene). Polymer, 41(8) 2937-2945, April 2000. 

Poly(p-xylylene) polymer and copolymers will become thermally crosslinkable, if they contain acyclohexenyl moiety [50]. Suchpolymers can be prepared via the Gilch route [51] as shown in Figure 2.7. 

Kahouli A, Sylvestre A, Pairis S, Laithier JF. Effect of CIH aromatic substitution on structural and dielectric properties of poly(p-xylylene). Polymer 2012 53(14) 3001-7. 

Figure 4c also describes the spontaneous polymerization of para-xylylene diradicals on the surface of solid particles dispersed in a gas phase that contains this reactive monomer (12) (see Xylylene Polymers). The poly(p-xylylene) polymer produced forms a continuous capsule shell that is highly impermeable to the transport of many penetrants, including water. This is an expensive encapsulation process, but it has produced capsules with impressive barrier properties. It is a type B encapsulation process, but is included here for the sake of completeness. 

Parylene is the trade name for a number of chemical vapor deposited poly (p-xylylene) polymers. Due to its ehemieal inertness, conformal coating, and excellent barrier properties, parylene has been used in a wide range of applieations, particularly as protective coatings for biomedical devices and microelectronics. 

PX forms j xylylene when heated above 1200°C. The stmctuie of J-xylylene is represented by a i)-quinoid stmcture or as a i)-ben2enoid brtadical. Condensation yields poly(p-xylylene) (19—22) (see Xylene polymers). 

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. 

Although the first two materials discussed in this chapter, the polyphenylenes and poly-p-xylylenes, have remained in the exotic category, most of the other materials have become important engineering materials. In many cases the basic patents have recently expired, leading to several manufacturers now producing a polymer where a few years ago there was only one supplier. Whilst such competition has led in some cases to overcapacity, it has also led to the introduction of new improved variants and materials more able to compete with older established plastics materials. 

This polymer first appeared commercially in 1965 (Parylene N Union Carbide). It is prepared by a sequence of reactions initiated by the pyrolysis of p-xylene at 950°C in the presence of steam to give the cyclic dimer. This, when pyrolysed at 550°C, yields monomeric p-xylylene. When the vapour of the monomer condenses on a cool surface it polymerises and the polymer may be stripped off as a free film. This is claimed to have a service life of 10 years at 220°C, and the main interest in it is as a dielectric film. A monochloro-substituted polymer (Parylene C) is also available. With both Parylene materials the polymers have molecular weights of the order of 500 000. 

Hertler16 was the first to report the preparation ofpoly(tetrafluoro-p-xylylene) by a multistep synthesis as shown in Scheme 2. Pyrolysis (330°C, 0.025 Torr) of dibromotetrafluoro-p-xylene (B CgFL,) over zinc led to deposition of the polymer film in a cold trap. 

To simplify the synthetic effort required to deposit such films, attempts were made to deposit films by pyrolyzing tetrafluoro-p-xylene (F4C8H6). Under similar reaction conditions, a polymer film was deposited that was different from poly(tetrafluoro-p-xylylene) as the FTIR spectrum indicates that it contains more hydrogen and less fluorine. Presumably HF is preferentially eliminated rather than H2. 

Attempts were made not only to find an alternative way to replace dimer and to deposit high-quality poly(tetrafluoro-p-xylylene) film, but also to eliminate the dibromide as the precursor because of the difficulty of synthesis. Therefore, the deposition of poly(tetrafluoro-p-xylylene) film by using hexafluoro-p-xylene as the precursor instead of dibromotetrafluoro-p-xylene was tried. However, no polymer film was deposited on the wafer. Effort was expanded and other metal reagents such as nickel or copper were used to react with l,4-bis(trifluoromethyl)-benzene to generate a,a,a, a -tetrafluoro-p-xylylene to deposit poly(tetrafluoro-p-xylylene) film. However, the result showed that no film was deposited, which was not unexpected, because a C—X bond that is weaker than C—F bonding might be necessary to initiate the formation of the desired intermediate. 

Accidently, using hexafluoro-p-xylene with the contaminated copper wire obtained from the precursor method experiments, a polymer film was deposited on the silicon substrates. Obviously, some dibromotetrafluoro-p-xylene from the precursor method that adhered to, or reacted with, the metal could somehow initiate this VDP process. However, a complete explanation of these results is not yet available. As an extension of this discovery, commercially available 1,4-bis(trifluoromethyl)benzene in conjunction with a catalyst/initiator has proved to be a potential alternative by which to deposit poly(tetrafluoro-p-xylylene) film successfully.23... 

Although Parylene-N possesses an outstanding combination of physical, electrical, and chemical properties, the benzylic C—H bonds present are potential sites for thermal and oxidative degradation. It is well known that replacing a C— bond with a C—F bond not only enhances the thermal stability of the resulting polymer, but also reduces the dielectric constant. Because incorporation of fluorine is known to impart thermal and oxidative stability, it became of interest to prepare poly(a,a,a, a -tetrafluoro- p -xylylene), Parylene-F Joesten reported that the decomposition temperature of poly(tetrafluoro-j9-xylylene) is ca. 530°C. Thus, it seemed that the fluorinated analog would satisfy many of the exacting requirements for utility as an on-chip dielectric medium. 

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... 

The photoconductivity of poly-p-xylylenes were investigated under various conditions [218-221]. The fundamental edge absorption of the polymers is at 300 nm. The photocurrents in the visual range of the spectra depended on the electrode nature. So it was interpreted as photoinjection of the charges from electrodes and separation of them at a Schottky type barrier. Suppression of hole injection for the plasma-treated polymer is related to the existence of an oxidized surface layer. 

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 ... 

In the commercial Gorham process. PX is generated by the thermal cleavage of its stable dimer, cyc/o-di-p-xylylene (DPX). a [2.2]paracyclo-phane (3). In many instances, substituents attached to the paracyclophane framework are carried through the process unchanged, ultimately becoming substituents of the polymer in the coating. 

The p-xylylene species plays a central role in the coating process itself as well as in the making of the dimers which are used as feed-stocks for the coating process. Polymers and dimers have both been made from precursor p-xylene compounds (4) featuring a variety of X and Y leaving groups. 

Purification. Unsubsliluled di-p-xylylene (DPXN) is readily purified by recrystallization front xylene. It is a colorless, highly crystalline solid. The principal impurity is polymer, which is insoluble in the recrystallization solvent and easily removed by hot filtration. In purifying DPXC and DPXD, care is taken not to disturb the homologue composition, so that product uniformity is maintained. 

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