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Direct Polymer Coating via Polymerization of Gaseous Intermediates Chung J. Lee Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180
Direct coating of polymeric materials was made possible by gas-solid phase polymerization of reactive intermediates. The gaseous intermediates for polymerization were either produced by glow discharge of small organic compounds or by thermolysis of stable precursors. This process also provided opportunities to synthesize various uncommon, or nonexisting polymers with unique properties for direct coating.
A direct polymer coating has been made possible since the development of the plasma polymerization technique in 1940 (Havens et al., 1976). In this process, both the unconventional monomers, such as CH4, CHC13, cc14, and some silicone compounds (SiX4, X = halogens, alkyl groups) etc., or conventional monomers such as ethylene, tetrafluoroethylene, and benzene etc., have been used for the polymerization (Lee, 1977b). These small compounds were subjected to glow discharge in order to produce reactive intermediates for polymerization. The intermediates produced by the collision of high-energy electrons and gaseous molecules, however, will have chemical structures of various natures (radicals, cationic ions, and anion radicals etc.). The polymers produced from reactions of these intermediates had very complicated structures. The polymerization mechanism for a given system is usually unpredictable. The properties of the polymer coating are in general difficult to control for specific purposes. However, this process has become an important industrial technique in electronic coatings since the middle 1960's. Recent applications also have been found in other diverse fields, such as protective coatings, antireflection optical coatings, and selective permeable coatings (Havens et al., 1976). Lee (1975) has pointed out that carbenes and its analogues in the group 4A elements can be used as reactive intermediates for polymerizations. Later developments in transport polymerization techniques have made this process useful also for polymerizing diradicals (Lee, 1977b). Basically, the process involves a pyrolysis or photolysis of a stable precursor to give the intermediate for polymerization. However, in contrast with plasma polymerization, transport polymerization starts with structure well-defined intermediates and yields polymers of known structure. These intermediates include (Lee, 1977), for instance, carbenes (:CX2, X = halogens, H; :CHPh), silylenes (23x2,X = halogens, CH3, Ph; :Si-SiR3), germylenes (:GeR2), and diradicals such as benzynes, p-methylene benzyl diradicals, aSiR2-CH. etc. Also, from various combinations of different intermediates, or combinations of these intermediates with the conventional monomers, countless newer polymers with unique structure and properties could be prepared for direct coating. For instance, copolymers of :SiXY with ethylene and propylene are typical examples. Following are some transport polymerizations which have been well investigated (Lee, 197713). Transport polymerization was observed as early as 1887 during a transport reaction of Si with SiF4. The reaction gave :SiFZ. The :SiF2 can be polymerized a t temperatures as low 0019-7890/78/1217-0091$01.00/0
as -196 OC to give polymer thin films. When the polymeric thin film was pyrolyzed at temperatures above 300 "C, semiconducting polymeric film resulted. Si
+ SiF4
-(SiF2-lm
-
- 2:SiFz
Si,Fan+2
2~t-(SiF2-)~/,
+ -(SiF-)x
(n
(1)
14)
(2)
Transport polymerization of p -methylene benzyl diradical was first attempted by Szwarc in 1947. His pyrolysis of p xylene at high temperatures (above 900 "C) gave cross-linked polymeric films. Linear poly(p-xylylene) was first produced from a transport polymerization of [2,2]-paracylophane a t moderate temperature (600 "C). The polymeric film can be produced over a temperature range of about 300 "C (from -196'to 85 "C). Polymers of two distinct crystal structures have been obtained over this temperature range. From 0 to 500 "C, the polymer film has an LY structure which can be changed irreversibly to the fl structure at temperatures above 250 O C . The p form, or the high-temperature crystal, can also be obtained from crystallization during polymerization of p xylylene a t -196 "C (Iwamoto et al., 1975). In addition, the crystallinity, the melting temperature, and the modulus of these polymer films are all dependent on the temperatures at which the films were deposited. Dimethylsilylene can also be polymerized via a gas-solid phase reaction. Polydimethylsilylene has recently found its application in heat-resistant (above 1200 "C) materials by converting into polycarbosilane (Yajima, 1976). Polymeric film of polydiphenylsilylene has been produced by transport polymerization of octaphenylcyclotetrasilane (Lee, 1976). It was suspected that this polymer might possess unusual conducting properties due to the u-electron delocalization of its Si-Si backbone (Lee, 1976; 1977a). Another polymeric film, -(SN-),,has been also prepared by transport polymerization (Street and Greene, 1977). The epitaxial growth of solid S2N2 on the pre-drawn polytetrafluoroethylene substrate gave polymeric films with semiconducting properties. On the other hand, extended-chain -(SN-),single crystals can also be prepared directly from its gaseous S2N2 by transport polymerization of S4N4. The fibrillar, extended-chain polymer crystals produced by this latter method are superconducting material. Other polymeric films have been produced by this process, and reactions which can be used for transport polymerization have been recently reviewed (Lee, 1977b). 0 1978 American Chemical Society
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Literature Cited Havens, M. R., Biolsi, M. E., Mayhan, K. G., J. Vac. Sci. Techno/., 13, 575 (1976). Iwamoto, R., Bopp, R. C., Wunderllch, B., J. Polym. Sci. Polym. pvs. Ed., 13, 1925 (1975). Lee, C . J., Wunderlich, B., Appl. Polym. Symp., No. 26, 291 (1975a). Lee, C . J., Wunderlich, B., Bull. Am. Phys. SOC.Ser. ///., 20, 314 (1975b). Lee, C . J., Ph.D Thesis, Rensselaer Polytechnic Institute, 1976. . Ed., 15 (2), 355 Lee, C. J., Wunderlich, B., J. Polym. sci. P O / ~ phys. (1977a). Lee, C. J., J. Macromol. Sci.-Rev. Macromol. Chem., C16 (I), 79 (1977b).
Street, G. B., Greene, R. L., ISMJ. Res. Dev., 21 (2), 94 (1977). Yaiima’ ‘., “od. Res. Dev.r 157 219
Received f o r review July 8, 1977 Accepted November 28,1977 The author wishes to thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial financial support through grant No. PRF 8901-AC 3,6.
