Polymerization on surfaces

In the preceeding section we discussed physisorbed polymers. Now we concentrate on chemisorbed polymer layers (review Ref. [424], see also Section 6.7). Chemisorbed polymers on solid surfaces have the advantage of forming thick flexible layers up to several 100 nm thickness. Due to the flexibility of the polymer chains the layer is relativley homogeneous. Additionally, the large variety of the monomers suitable for surface polymerization leads to a large variety in the surface properties. Also, the mechanical flexibility can be manipulated by the polymer chain density. A high density leads to polymer brushes. 

In grafting from this limitation is overcome by polymerizing the polymer directly on the surface. The polymerization is carried out step-by-step after initiation by eligible chemical groups which are called initiators I. The scheme for a grafting-from polymerization is shown in Fig. 10.11. It is also called surface-initiated polymerization or SIP. 

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Under favorable conditions, low molecular weight organics may polymerize on surface of adsorbent (dialkenes, 1-alkenes, alkynes, conjugated double-bond systems, and epoxides are especially susceptible to this behavior). 

Figure 3. The effect of degree of polymerization on surface coverage (fractional site occupancy) at various polymer concentrations. The solid lines represent the present model and the symbols correspond to the theory of Scheutjens and Fleer. The parameter values are the same as in Figure 2.

The best developed example of a material produced by VDP is poly(p-xylylene) designated as Parylene-N by the Union Carbide Corporation. Poly(/i-xylylene) was discovered by Szwarc12 in 1957 and then commercialized by Gorham at Union Carbide.13,14 (Scheme 1). Gorham has reported that di-p-xylylene is quantitatively cleaved by vacuum vapor-phase pyrolysis at 600°C to form two molecules of the reactive intermediate /i-xylylene, which subsequently polymerizes on the cold substrate. In a system maintained at less than 1 Torr, p-xylylene spontaneously polymerizes on surfaces below 30°C to form... 

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. 

Y.Z. Bayindir, M. Yildis, F. Bayindir, The effect of soft-start polymerization on surface hardness of two packable composites. Dent. Mater. J. 22 (2003) 610-616. 

Direct Oxidation of Propylene to Propylene Oxide. Comparison of ethylene (qv) and propylene gas-phase oxidation on supported silver and silver—gold catalysts shows propylene oxide formation to be 17 times slower than ethylene oxide (qv) formation and the CO2 formation in the propylene system to be six times faster, accounting for the lower selectivity to propylene oxide than for ethylene oxide. Increasing gold content in the catalyst results in increasing acrolein selectivity (198). In propylene oxidation a polymer forms on the catalyst surface that is oxidized to CO2 (199—201). Studies of propylene oxide oxidation to CO2 on a silver catalyst showed a rate oscillation, presumably owing to polymerization on the catalyst surface upon subsequent oxidation (202). 

Patterns of ordered molecular islands surrounded by disordered molecules are common in Langmuir layers, where even in zero surface pressure molecules self-organize at the air—water interface. The difference between the two systems is that in SAMs of trichlorosilanes the island is comprised of polymerized surfactants, and therefore the mobihty of individual molecules is restricted. This lack of mobihty is probably the principal reason why SAMs of alkyltrichlorosilanes are less ordered than, for example, fatty acids on AgO, or thiols on gold. The coupling of polymerization and surface anchoring is a primary source of the reproducibihty problems. Small differences in water content and in surface Si—OH group concentration may result in a significant difference in monolayer quahty. Alkyl silanes remain, however, ideal materials for surface modification and functionalization apphcations, eg, as adhesion promoters (166—168) and boundary lubricants (169—171). 

These siUca-supported catalysts demonstrate the close connections between catalysis in solutions and catalysis on surfaces, but they are not industrial catalysts. However, siUca is used as a support for chromium complexes, formed either from chromocene or chromium salts, that are industrial catalysts for polymerization of a-olefins (64,65). Supported chromium complex catalysts are used on an enormous scale in the manufacture of linear polyethylene in the Unipol and Phillips processes (see Olefin polymers). The exact stmctures of the surface species are still not known, but it is evident that there is a close analogy linking soluble and supported metal complex catalysts for olefin polymerization. 

The observation of the spectrum for styrene polymerized on the surface of silane-treated silica and of the difference spectrum of polystyrene adsorbed on the surface of silica have revealed that there are absorption bands of atactic polystyrene at 1602, 1493, 1453, 756, and 698 cm. The absorption bands at 1411 and 1010 cm are related to vinyl trimethoxy silane, and C of the difference spectrum is below the base line. This indicates that the vinyl groups of silane react with styrene to form a copolymer. 

Bryk MT (1981) Polymerization on the solid surface of the inorganic substances, Naukova Dumka, Kiev, USSR... 

Bruk MA, Pavlov SA (1990) Polymerization on solid surfaces (in Russian), Khimiya, Moscow, USSR... 

Theoretical calculations were also conducted on the influence of/-functional initiators on DB in SCVCP [72]. In the semi-batch system, DB is only sHghtly affected by the presence of polyinitiator and is mostly governed by the comonomer content. The calculations are also applied to polymerizations from surface-bound initiators (see later). 

Corrosion of steel during oil well acidizing or acid pickling treatments can be controlled effectively and economically with organic corrosion inhibitors. These additives interact with the steel surface to form an adherent barrier, the nature of which depends on the additives physicochemical properties. Work to date has established that acetylenic alcohols chemisorb and subsequently polymerize on steel surfaces (1-5"). a,/MJnsaturated aldehydes and a-alkenyl-phenones appear to behave in a similar manner (6j7"). The nature of Current address Amoco Production Company, Tulsa, OK... 

This chapter draws a comprehensive picture of what has been done in the field of dendrimers with polymeric cores putting emphasis first on synthetic issues and then on experiments investigating the aggregation behavior of these intruiging macromolecules both in the solid state and on surfaces. Additionally, experiments will be described which show that some of these dendrimers can be considered cylindrical molecular objects. The macromolecules treated in this chapter may be considered as either dendrimers with polymeric core or alternatively dendronized polymers (or polymers with appendent dendrons) depending on whether one sees them from the vantage point of an organic or macromolecular chemist. 

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