Xylylenes, vapor deposition polymerization

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. 

Recent Advances in the Vapor Deposition Polymerization of -Xylylenes... 

The process for preparing linear poly-p-xylylenes by pyrolytic polymerization of di-p-xylylenes has been extended to include the formation of p-xylylene copolymers. Pyrolysis of mono-substituted di-p-xylylenes or of mixtures of substituted di-p-xylylenes results in formation of two or more p-xylylene species. Copolymerization is effected by deposition polymerization on surfaces at a temperature below the threshold condensation temperature of at least two of the reactive intermediates. Random copolymers are produced. Molecular weight of polymers produced by this process can be controlled by deposition temperature and by addition of mercaptans. Unique capabilities of vapor deposition polymerization include the encapsulation of particulate materials, the ability to replicate very fine structural details, and the ability of the monomers to penetrate crevices and deposit polymer in otherwise difficultly accessible structural configurations. 

Particle Encapsulation. A unique capability of the p-xylylene vapor deposition process has been uncovered in the area of encapsulation (12) of particulate solids. The particles or granules to be encapsulated are placed in a container which in turn is placed in the deposition chamber, and the nozzle from the pyrolysis tube is inserted in the mouth of the bottle. During the run the monomers pass from the pyrolysis zone through a nozzle into the bottle and polymerize on the surface of the tumbling particles or granules. Polymer is also formed on the inner surface of the bottle which is rotated at 50-150 r.p.m. Relatively simple equipment (Figure 4) has been used to study this phenomenon. 

Metal-containing polymers can be synthesized by the vapor deposition polymerization of various monomer systems including organometallic compounds and metal-monomer co-condensates. Such co-condensates are produced by simultaneous or layer-by-layer deposition of metal and monomer vapors on substrate plates at low temperatures (usually, 77 K). Polymerization can proceed in different ways. Some metal-monomer systems polymerize during cocondensation (Ge and Sn with acetylene [11], Mg with CN-substituted p-xylylene [12]), most probably due to heat released at condensation. In references 13-16, co-condensates of metals (Pd, Ag, Au, etc.) and vinyl monomers... 

B. Ratier, Y. S. Jeong, A. Moliton, and P. Audebert. Vapor deposition polymerization and reactive ion beam etching of poly(p-xylylene) films for waveguide applications. Opt. Mater., 12(2-3) 229-233, June 1999. 

Nanocomposites from lead and poly(p-xylylene) with a sulfide content of 1-10 vol% sulfide could be synthesized by vapor deposition polymerization [102]. 

Poly(p-xylylene)-cadmium sulfide nanocomposites were synthesized using the vapor deposition polymerization in an inhomogeneous electric field [103]. There, a gradient of concentration and size of the cadmium sulfide nanoparticles along the direction of the applied electric field emerge. 

Silver nanoparticles have been embedded in a poly(/ -xylylene) matrix by low temperature vapor deposition polymerization [105]. 

The methods of the preparation of parylene nanofibers by oblique angle vapor deposition polymerization have been detailed [114]. Monomer vapors produced by the pyrolysis of chemically functionalized / -xylylene precursors are directed in an oblique angle toward a surface to initiate a structured polymer growth. 

Pu H, Jiang F, Wang Y, Yan B. Soft magnetic composite particles of reduced iron coated with poly(/7-xylylene) via chemical vapor deposition polymerization. Colloids Surf Part A 2010 361(l-3) 62-5. 

Morozov PV, Khnykov AY, Grigor ev El, Zav yalov SA, Klimenko VG, Chvalun SN, et al. Structure and optical properties of poly-/ -xylylene-lead sulfide nanocomposites derived by vapor deposition polymerization. Nano-technol Russ 2012 7(l-2) 41-6. 

Streltsov DR, Mailyan KA, Gusev AV, Ryzhikov lA, Erina NA, Su C, et al. Electrical properties, structure, and surface morphology of poly(p-xylylene)-silver nanocomposites synthesized by low-temperature vapor deposition polymerization. Appl Phys A 2012. 

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]... 

Parylene-C, the trade name of the film formed from the Union Carbide Corp. brand of dichloro-p-xylylene, is a vapor deposited film formed by the reaction shown in Figure 1. The solid dimer(I) is vaporized and pyrolyzed at 650° C to 750 C to the reactive olefinic monomer, chloro-p-xylylene(II), which polymerizes on cool surfaces in the low pressure deposition chamber to form the crystalline linear polymer poly(chloro-p-xylylene) (III) (5). 

The most common conformal coatings are derived from polyurethanes, acrylics, and epoxies the more special formulations for high-temperature performance are based on silicones, diallyl-phthalate esters, and polyimides. An example of a vapor deposited conformal coating is Parylene. It is obtained by vapor deposition of p-xylylene, which is formed as a transient by dehydrogenation of p-xylene at high temperature, and polymerization on the surface of the object to be coated. Because p-xylylene monomer is not stable, it is advantageous to work with the cyclic dimer, di-p-xylylene (paracyclophane), which, upon heating under reduced pressure, will produce the transient monomer which converts to the polymer at low temperatures. 

Organometallic derivatives of poly-p-xylylene (PPX) with Ge and Sn atoms covalently bonded to polymer chains have been synthesized by a vapor deposition technique using bridged [2,2]-paracyclophanes with corresponding organometallic groups [32, 33]. Pyrolysis of these cyclophanes, along with polymerization of the /7-xylylene monomer mixture, is shown in Scheme 2.2. 

Coating by vapor deposition has been used to form microcapsules, the coating being done on solid particles. The latter maybe droplets of frozen liquid or liquid encapsulated by some other process and the vapor deposition maybe caused by pyrolysis of di-p-xylylene in vacuuo [see Poly(p-xylylene), Chapter 5] the reactive p-xylylene radical formed by the pyrolysis polymerizes on the solid particles to be encapsulated, forming a thin coating of poly(p-xylylene). 

This siloxane-substituted polyCp-xylylene) exhibits an enhanced solubility. Therefore, the characterization with common techniques is possible, as well as processing electrospinning to get nanofibers. High molecular weights at moderate polydispersities were established by Gel permeation Chromatography (GPC) characterization which is unusual in the case of a vapor phase deposition polymerization technique. 

The design of biologically active surfaces offers a new dimension to the development of advanced materials for biomedical and microfluidic applications. Our approach to creating these surfaces is via the fabrication of vapor-based reactive polymer coatings, which provide sophisticated functional groups for the immobilization of biological ligands. Chemical vapor deposition (CVD) polymerization has been used to prepare a wide spectrum of hinctionalized poly(p-xylylenes). The applicability of a few of these reactive surfaces as stable platforms for biomimetic modifications is discussed in this review. 

In the recent past, chemical vapor deposition (CVD) polymerization of substituted [2,2]paracyclophanes has been instrumental in creating a wide array of functionalized poly(p-xylylenes) with a diverse class of functional groups, such as amines (15,16), esters (17-19) and alcohols (7,2(9,27), which facilitate... 

Figure 5, Chemical vapor deposition (CVD) polymerization to render various types of functionalpoly(p-xylylenes) within confined microgeometries.[51] (Reproducedfrom reference 51 with permission. Copyright 2006 American...

Paralene [para-xylene] Also called Gorham and also spelled parylene. A process for coating articles with poly-p-xylene. The vapor of di-p-xylylene is pyrolyzed at 550°C, yielding p-xylyl free radicals, -CHj-CgH CH, which deposit and polymerize on cooled surfaces. Developed by W. F. Gorham at Union Carbide Corporation. 

As mentioned above, the new method of cryochemical synthesis of polymer nanocomposite films has been developed based on co-deposition of M/ SC and monomer vapors at temperature 80K and subsequent low-temperature solid-state polymerization of monomer matrix ([2] and works cited herein). It has been established that a number of monomers (acrylonitrile, formaldehyde, /i-xylylene and its derivatives) polymerize in solid state in absence of thermal movement of molecules owing to own specific supra-molecular structure. When reaction is initiated by y- or UV-radiation the formation of a polymer matrix occurs even at the temperatures close to temperature of liquid helium [66-69]. 

Scheme 1. Preparation of p-xylylene monomers and their polymerization after deposition together with metal vapors...

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