Surface-catalyzed polymerization

This conceptual link extends to surfaces that are not so obviously similar in stmcture to molecular species. For example, the early Ziegler catalysts for polymerization of propylene were a-TiCl. Today, supported Ti complexes are used instead (26,57). These catalysts are selective for stereospecific polymerization, giving high yields of isotactic polypropylene from propylene. The catalytic sites are beheved to be located at the edges of TiCl crystals. The surface stmctures have been inferred to incorporate anion vacancies that is, sites where CL ions are not present and where TL" ions are exposed (66). These cations exist in octahedral surroundings, The polymerization has been explained by a mechanism whereby the growing polymer chain and an adsorbed propylene bonded cis to it on the surface undergo an insertion reaction (67). In this respect, there is no essential difference between the explanation of the surface catalyzed polymerization and that catalyzed in solution. 

Adhesion occurs very rapidly (generally a matter of seconds) when the liquid monomer is spread in a thin layer between the surfaces to be bonded. Traces of bases (even as weak as alcohol or water) on the surfaces catalyze polymerization by an anionic mechanism. Adhesion arises in part from mechanical interlocking between polymer and surface and in part from strong secondary bond forces. 

Obviously, the first example of poly(substituted methylene) synthesis (PSMS) is polymerization of diazoalkanes and aryldiazomethanes, which was extensively studied in 1950-1970 [9-11] and whose importance as an alternative method for polyolefin synthesis was well recognized. In that period, in spite of the high explosiveness of the diazo compounds, many researchers were engaged in the polymerization and some unique and important characteristics for the process were revealed. Although some reviews of the polymerization were published [9-11], it is worthwhile to mention briefly some of the representative results, which are closely related to the contents of the later sections of this review. In addition, the unique character of surface-catalyzed polymerization of diazomethane has been utilized for surface modification recently, which is also described in this section. The efficient synthesis of nanometer-scale polymethylene thin films from diazomethane is quite remarkable, compared to the difficulty of polyethylene thin film formation because of the low solubility of the polymer. 

Scheme 7 Mechanism for Au surface-catalyzed polymerization of diazomethane...

A particularly important breakthrough was achieved by Bai and Jennings in 2005 in the surface-catalyzed polymerization of diazomethane by using ethyl diazoacetate (EDA) as a comonomer [30] (Scheme 8). Although surface-catalyzed homopolymerization of EDA did not proceed, EDA can be incorporated into the resulting polymer chain when an ether solution of a mixture of diazomethane and EDA was contacted with an Au-surface. The resulting surface bound copolymer, poly(methylene-co-ethoxycarbonylmethylene), has the same structure as that of random copolymer of ethylene and ethyl acrylate. Whereas the incorporation efficiency... 

The use of EDA as a comonomer in the surface-catalyzed polymerization not only contributed to the efficient formation of polymer films, but can also be utilized for the modification of surface properties by using the incorporated ester groups. In particular, pH-responsive membranes have been achieved by hydrolysis of the ester group [30-33] or introduction of amino group via substitution at the ester group [34],... 

Pyridine was found to polymerize on a Pt electrode from a solution of 1 M pyridine in 1 M LiC104/CH3CN at potentials above 0.8 V vs Ag/AgCl. A colorless film was formed, but it could be oxidized and reduced when placed in plain electrolyte solution. The infrared spectrum of the electrochemically formed poly(pyridine) film is shown in Figure 5. It displays a very intense, narrow band at 1500 cm indicative of C=C stretches that are perpendicular to the surface. 3,5 Lutidine also was polymerized on a platinum electrode under the same conditions, and its infrared spectrum is similar to that for the surface catalyzed poly(lutidine). The C=C stretching band for the poly(lutidine)... 

The first examples employed a lithium-terminated polymer chain, utilizing the primary product of the -butyl-lithium-catalyzed polymerization of a masked disilene (see Section 3.11.4.1.2) and a reactive siloxy alkylbromide anchor-derivatized quartz surface, affording the end-grafted polysilane 79, as shown in Scheme 27.191... 

There are several other active topics under examination in our laboratory, for example, surface modification of polymers under phase transfer catalyzed reactions and single electron transfer phase transfer catalyzed polymerizations. The limited space, however, precludes discussion here. 

A guide to the manner in which structural theory may be applied to a detailed consideration of the mechanism of a surface-catalyzed reaction is found in papers by Cossee (113), Arlman (114), and Arlman and Cossee (115) concerning the mechanism of the stereoregular heterogeneous catalyzed polymerization of propylene. Particular crystallographic sites are shown to be the active centers at which the reactants combine and ligand field theory is used to demonstrate a plausible relationship between the activation energy for the conversion of adsorbed reactants to the product and the properties of the transition metal complex which constitutes the reaction center. 

With no sufficient hydrogen present, the molecules get stuck on the surface. Owing to purely statistical reasons (Scheme I), this is more probable in an elongated position. Such molecules may combine with each other to give high molecular weight polymers ( coke ). Metal-catalyzed polymerization has actually been observed with lower molecular weight hydrocarbons (61). Such reactions are responsible for more rapid deactivation of the catalyst by trans isomers (Table III). 

The change in surface acidity with water content was also demonstrated by the ability of kaolinite to promote acid-catalyzed polymerization (236). Styrene, p-methylstyrene, and p-methoxymethylstyrene polymerized vigorously on kaolinite that was dried at 110°C. At 0.2% wt water content, p-methylstyrene and p-methoxystyrene polymerized and at 0.6% wt water content, only p-methoxystyrene polymerized. The polymerization results are consistent with lower acidity at higher water contents since the susceptibility of these monomers to acid-catalyzed polymerization is in the order p-methoxystyrene > p-methylstyrene > styrene. 

PVPyrH Poly-4-vinylpyridine (PVPyr) prepared by conventional persulfate catalyzed polymerization was dissolved in ethyl alcohol or methyl Cellosolve (2 to 3% solution) for application to polyester film substances. These films adsorb heparin from solutions of sodium heparin to produce anticoagulant surfaces (PVPyrH). Films were painted on, or spread by knife coating. 

Thomas and his coworkers have studied the gas-phase thermolysis and photolysis of cyclopropanone in detail" ". They have found that the thermal reaction of gaseous cyclopropanone follows a first-order rate to give only the polymerized product in what appears to be a surface-catalyzed process. The heat of formation of cyclopropanone was measured to be -f 3.8 kcal mol" and this value was used to predict an activation energy of 30.5 kcal mol" for its homogeneous decomposition via the oxyallyl biradical. 

Oxidation of wood by HNO3 at ambient temperature that results in the formation of carboxylic acid groups on the surface has been substantiated (157). These carboxylic groups are sufficiently acidic to catalyze polymerization of furfuryl alcohol. 

The PEC-1000 membrane of Toray Industries, Inc., has been described by Kurihara et al (21). This membrane was characterized as a thin-film composite type made by an acid catalyzed polymerization on the surface. Membrane performance reported for seawater tests was 99.9 percent TDS rejection at fluxes of 5.0 to 7.4 gfd (8.3 to 12.3 L/sq m/hr) when tested with 3.5 percent synthetic seawater at 800 psi (5516 kPascals). The membrane was stable in 1500-hour tests in spiral-wrap elements and exhibited stability in a temperature range of 25 to 55°C and in a pH range from 1 to 13. High organic rejections were reported for the PEC-1000 membrane rejection of dimethylformamide from a 10 percent solution was 95 percent and similar tests with dimethylsulfoxide showed 96 percent rejection. The composition and conditions for preparation of PEC-1000 membrane is not disclosed in Reference 21. Apparently it is a dip-cast membrane related to compositions described by Kurihara, Watanaba and Inoue in Reference 18. 

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