page_title: Parylene
section_title: From substituted p-xylenes
hierarchical_section_title: Parylene / Coating process / Monomer generation / From substituted p-xylenes
caption_attribution_description: precursor to poly(alpha,alpha'-dimethyl-p-xylylene)
context_page_description: Parylene is the common name of a polymer whose backbone consists of para-benzenediyl rings –C
  ₆H
  ₄– connected by 1,2-ethanediyl bridges –CH
  ₂–CH
  ₂–. It can be obtained by polymerization of para-xylylene H
  ₂C=C
  ₆H
  ₄=CH
  ₂.
  The name is also used for several polymers with the same backbone, where some hydrogen atoms are replaced by other functional groups. Some of these variants are designated in commerce by letter-number codes such as "parylene C" and "parylene AF-4". Some of these names are registered trademarks in some countries.
  Coatings of parylene are often applied to electronic circuits and other equipment as electrical insulation, moisture barriers, or protection against corrosion and chemical attack. They are also used to reduce friction, and in medicine to prevent adverse reactions to implanted devices. These coatings are typically applied by chemical vapor deposition in an atmosphere of the monomer para-xylylene.
  Parylene is considered a "green" polymer because its polymerization needs no initiator or other chemicals to terminate the chain; and the coatings can be applied at or near room temperature, without any solvent.
context_section_description: Another route to generation of the monomer is to use a para-xylene precursor with a suitable substituent on each methyl groups, whose elimination generates para-xylylene.  
  Selection of a leaving group may consider its toxicity (which excludes sulfur and amine-based reactions), how easily it leaves the precursor, and possible interference with the polymerization. The leaving group can either be trapped before the deposition chamber, or it can be highly volatile so that it does not condense in the latter.
  For example, the precursor α,α'-dibromo-α,α,α',α'-tetrafluoro-para-xylene (CF
  ₂Br)
  ₂(C
  ₆H
  ₄) yields parylene AF-4 with elimination of bromine.
  The advantage to this process is the low cost of synthesis for the precursor. The precursor is also a liquid and can be delivered by standard methods developed in the Semiconductor Industry, such as with a vaporizer, vaporizer with a bubbler, or a mass-flow controller. Originally the precursor was just thermally cracked, but  suitable catalysts lower the pyrolysis temperature, resulting in less char residue and a better coating. By either method an atomic bromine free-radical is given off from each methyl end, which can be  converted to hydrogen bromide HBr and removed from monomer flow. Special precautions are needed since bromine and  HBr are toxic and corrosive towards most metals and metal alloys, and bromine can damage viton O-rings.
  A similar synthesis for parylene N uses the precursor α,α'-dimethoxy-p-xylene.  The methoxy group H
  ₃CO– is the leaving group; while it condenses in the deposition chamber, it does not interfere with the deposition of the polymenr. This precursor is much less expensive than [2.2]para-cyclophane.  Moreover, being a liquid just above room temperature, this precursor can delivered reliably using a mass-flow controller; whereas the generation and delivery of the gaseous monomer of the Gorham process are difficult to measure and control.
  The same chemistry can generate parylene AM-2 can be generated from the precursor α,α'-dimethyl-α,α'-dimethoxy-p-xylene. 
  Another example of this approach is the synthesis of parylene AF-4 from α,α'-diphenoxy-α,α,α',α'-tetrafluoro-para-xylene.  In this case, the leaving group is phenoxy CH
  ₅O–, which can be condensed before the deposition chamber.
caption: α,α'-dimethyl-α,α'-dimethoxy-p-xylene.
exif: {"Image ImageWidth": "2899", "Image ImageLength": "653", "Image BitsPerSample": "8", "Image Compression": "PackBits", "Image PhotometricInterpretation": "3", "Image StripOffsets": "[8, 100, 192, 284, 376, 468, 560, 668, 856, 1061, 1294, 1613, 1985, 2411, 2841, 3261, 3644, 4019, 4399, 4780, ... ]", "Image Orientation": "Horizontal (normal)", "Image SamplesPerPixel": "1", "Image RowsPerStrip": "2", "Image StripByteCounts": "[92, 92, 92, 92, 92, 92, 108, 188, 205, 233, 319, 372, 426, 430, 420, 383, 375, 380, 381, 411, ... ]", "Image XResolution": "300", "Image YResolution": "300", "Image PlanarConfiguration": "1", "Image ResolutionUnit": "Pixels/Inch", "Image ColorMap": "[0, 32896, 0, 32896, 0, 32896, 0, 49344, 49344, 42662, 16448, 24672, 32896, 41120, 49344, 57568, 0, 8224, 16448, 24672, ... ]"}