Poly(-xylylene)s

Abstract

This chapter describes poly(p-xylylene)s (PPX)s in detail. The pyrolysis of p-xylene produces the p-xylyl radical. This radical disproportionates into a more stable p-quinodimethane diradical. Monomers for PPX are briefly summarized. In light-emitting devices, copolymers can be used to tune the wavelength of the emitted light. Graphene field effect transistors with a parylene back gate and an exposed graphene top surface have been reported.The polymers are obtained in the usual way by vapor phase deposition and condensation polymerization. By a modification of the monomers by suitable side chains, tractable PPX types can be obtained in liquid state. A poly(p-xylylene) type containing phenyl side groups has been synthesized by electrochemical polymerization. The properties and applications of Parylenes have been summarized.

Keywords

Cyclophanes; Parylene; Neutralization; Cladding; Encapsulation; Matrigel carrier

The discovery of poly(-xylylene)s (PPX)s is attributed to Szwarc in around 1947 [1]. He found that the pyrolysis of -xylene produces the -xylyl radical which disproportionates into a more stable -quinodimethane 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. Superficially, Szwarc was interested in the bond strength of aromatic hydrogens [2], however, he wrote a review on the topic [3], although he probably became more famous for living polymers.

The deposition process was then improved and commercialized by Gorham at Union Carbide [4,5]. Gorham used cyclophanes to increase the yield of polymers. In 1968 the licence was transferred to Para Tech Coating, Inc. which developed the process further. Meanwhile, parylene is a trademark used by several companies. The history of parylene is given on the internet [6].

Basically, PPX can be regarded as a special case of poly(-phenylene alkylene) polymers. However, polymers with other alkyl spacers have not been investigated very much and have been regarded as a forgotten class of polymers [7]. We emphasize that PPX contains the phenylene group and the ethylene group in the backbone, i.e., Φ—CH2—CH2—, which is identical to CH2—Φ—CH2—. A few papers deal with the phenylene group and the methylene group as a repeating unit of the backbone, namely Φ—CH2—, which is an incomplete notation. The latter polymer is referred to as poly(-phenylene methylene). Rarely, PPX polymers are abbreviated as poly(-phenylene ethylene) (PPE). However, the acronym PPE is used prevalently for poly(phenylene ether).

2.1 Monomers

Monomers for PPX are summarized in Table 2.1 and in Figure 2.1. Common precursor monomers belong to the class of cyclophanes. The chemistry of cyclophanes has been reviewed recently [24].

[2.2]Paracyclophane can be prepared by the Hofmann elimination of -methylbenzyltrimethylammonium hydroxide in the presence of 2-imidazolidinone as co-solvent and crown ethers, dimethyl sulfoxide (DMSO) and other compounds as reaction promoters [8,9]. Yields greater than 70% are reported.

The synthesis of ,α,α′α′-tetrafluoro--xylylene (TFPX) is relatively costly, time-consuming, and not suitable for commercial products [16]. TFPX can be obtained by mixing the chlorinated analog with potassium fluoride, and allowing it to react for 12 h at 260–280 °C. To prevent gelation, the reaction is conducted in a solvent such as sulfolane. A quaternary phosphonium salt is added as a phase transfer catalyst. A lower temperature of 160 °C is applied. However, the reaction time increases up to h.

Octafluoro[2.2]paracyclophane is prepared by refluxing tetrahydrofuran (THF) and hexamethylphosphoramide or DMSO solution from 1,4-bis (bromodifluoromethyl) benzene. Trimethylsilyltributyltin is used as a reducing agent [25]. Cesium fluoride as a catalyst gives superior yields (40%) in comparison to potassium fluoride. To favor the formation of rings, the reaction must be accomplished in highly diluted systems.

Octafluoro[2.2]paracyclophane and dodeca-fluoro[2.2]paracyclophane can be alternatively prepared by the treatment of 1,4-bis (halodifluoromethyl) benzene with 2/Al in ,N-dimethylformamide at room temperature via a cyclocoupling reaction [26].

Amino[2.2]paracyclophane and diamino[2.2]paracyclophane can be accessed by the nitration with trifluoromethanesulfonic acid/nitric acid and reduction of the nitro group with the strategy of incorporating triiron dodecacarbonyl. 33(CO)12 is a powerful reductant even under mild reaction conditions [12]. Crown ethers are used as a phase transfer catalyst. The reaction scheme is shown in Figure 2.2.

Functionalized monomers are used as anchor groups for surface modification in medical applications. Besides cyclophanes, ethers of xylene are suitable for the formation of PPX by chemical vapor deposition (CVD). Suitable ethers are 1,4-bis (phenoxymethyl) benzene and 1,4-Bis[(phenylmethoxy) methyl]benzene [22]. These ethers are more readily available than cyclophanes.

2.1.1 Chlorine-Substituted Monomers

The effect of an aromatic chlorine-hydrogen substitution on the structural and dielectric properties of poly(-xylylene) has been studied [27].

The chlorination of the aromatic rings stabilizes the crystalline structure of the materials, increases the d-spacing, decreases the crystallinity, and increases the value of the dielectric parameters. Also, the permittivity and the conductivity are increased. The increase of the dielectric properties and the changes of the morphologies structure are associated to the change in the intermolecular interaction due to the chlorine replacement [27].

2.2 Sulfide Modified Polymers

Poly(-xylylene tetrasulfide) has been synthesized from 1,4-bis (chloromethyl)-benzene and sodium tetrasulfide by an interfacial polycondensation technique [28]. As phase transfer catalysts, methyl tributyl ammonium chloride, tetrabutyl ammonium bromide, and benzyl triethyl ammonium chloride were used. A molecular weight of kDa was obtained.

2.3 Polymerization and Fabrication

A wide variety of PPXs have been synthesized, however only a few materials are commercially sold.

2.3.1 Chemical Vapor Deposition

One of the earliest methods for the preparation of polymeric -xylylene coatings is a high-temperature pyrolysis of -xylene at 800–1000 °C and subatmospheric pressures, followed by cooling the pyrolysis vapors to a polymerization temperature by condensing the vapors on a cold surface [2,29]. During pyrolysis, reactive radicals are formed that polymerize when cooled down again.

The early preparation methods of PPX described in the patent literature in fact followed the method by Szwarc. A process for preparing PPX has been disclosed wherein the vapors of -xylene were pyrolyzed in the presence...