Platinum-catalyzed polymerization

Polyaddition of organosilicon dihydrides, mainly dihydro(poly)siloxanes to dialkenyl-substituted organic compounds also known as hydrosilylation copolymerization, leads to polycarbosiloxanes with functionalized organic segments (359). Platinum-catalyzed polymerization hydrosilylation of a,allyl-substituted bisphenols, imides, or amides leads to the synthesis of block copolymers that are useful thermoplastic elastomers (Scheme 40). 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

Acetyl ligands, in niobium complexes, C-H BDEs, 1, 298 Achiral phosphines, on polymer-supported peptides, 12, 698 Acid halides, indium compound reactions, 9, 683 Acidity, one-electron oxidized metal hydrides, 1, 294 Acid leaching, in organometallic stability studies, 12, 612 Acid-platinum rf-monoalkynes, interactions, 8, 641 Acrylate, polymerization with aluminum catalysts, 3, 280 Acrylic monomers, lanthanide-catalyzed polymerization,... 

As shown in Scheme 45, DMSB can undergo polymerization or dimerization in the presence of similar platinum catalysts in good to excellent yields. The course of the reaction is apparently linked to the presence or absence of phosphine ligands platinum complexes that include added phosphines lead to dimerization, whereas polymerization usually occurred under ligandless conditions. Divinyltetramethyldisiloxane also serves as a ligand for the platinum-catalyzed dimerization of SCBs and disilacyclobutanes as reported by Chu and Frye <1993JOM(446)183>. 

Cydodextrins can catalyze certain chemical reactions. Styrenesulfonate polymerization was accelerated in the presence of cyclodextrin, and the polymers had a higher molecular weight.109 Platinum-catalyzed hydroxylation reactions were accelerated in the presence of cyclodextrin.110... 

Ordinarily, a catalyst is not limited to a single reaction. Thus finely divided platinum catalyzes the hydrogenating reactions just as well as the oxidizing reactions HCl catalyzes hydrolytic reactions just as well as polymerizations as to nickel, the experiments of Sabatier and Senderens have shown that it... 

Finally, the platinum-catalyzed hydrosilylation polymerization was also used very recently by Cassidy et al. [82,83] to prepare fluorine containing silicon-organic hybrid polymers in supercritical carbon dioxide (SCCO2) (cf. Scheme 32). 

The platinum-catalyzed or thermal ring opening polymerization of the substituted silacyclobutane monomers yields linear polycarbosilanes. The platinum-catalyzed hydrosilations of the AB (allyl) and AB2 (diallyl) monomers yield linear and hyperbranched polycarbosilanes, respectively. A wide range of random copolymers are readily available from polymerization of mixtures of the silacyclobutane or AB and AB2 monomers. The preparation of random copolymers is often of interest for achieving desired physical properties. 

There are numerous applications where cure reaction by-products are not acceptable as they may contaminate other sensitive areas of the devices, and where fast deep section cure is required. These constraints have directed the choice to the platinum-catalyzed hydrosilylation addition cure system. The reaction involves the addition of a hydrido silane (SiH) to an alkenyl organic group (-CH=CH2), typically the vinyl or hexenyl groups. The product of the reaction is the ethylenic bridge (-CH2-CH2-). The alkenyl groups are usually placed at the end of the polymer base chains in structures such as The SiH groups are usually placed in a combed structure within the polymeric or copolymeric chains called cross-linker M-D -M or M-D -Dy-M. (The M, D, etc. nomenclature is explained in Silicones structures.) Typical values of n cover a wide range from 100 to 1000, while the values of x and y vary from 3 to 100. 

Many other difiinctional organosilicon monomers containing both alkenyl groups and Si—H bonds in the molecule have also been successfully used in the platinum-catalyzed hydrosilylation polymerization (353-355) (Scheme 35). 

Block polymers containing polydimethylsiloxane soft blocks have been the subject of considerable recent synthetic activity. In particular, polydimethylsiloxane-b-polystyrene polymers have received considerable attention as thermoplastic elastomers. For the most part, anionic polymerization methods have provided the most successful routes to the preparation of these block polymers. Among the most notable papers in this field are those of Dean, Saam et al, Juliano, and Bajaj and coworkers. Recently Chaumont and his coworkers have prepared polydimethylsiloxane-b-polystyrene polymers by the platinum catalyzed condensation polymerization of a,o)-vinyl terminated polystyrene oligomers with a,o)-hydrogen terminated polydimethyl-siloxanes. 

The platinum catalyzed hydrosilylation reaction, equation 1, is commonly employed as a curing method in polymeric systems where the oligomers are functionalized with silyl hydride and vinyl groups, as exemplified in equation 2. 

Uenishi K, Imae I, Shirakawa E, Kawakami Y (2002) Synthesis of steieoregular and optically active poly[ methyl(l-naphthyl)silylene (o-phenylene)methylene] by platinum-catalyzed ring-opening polymerization. Macromolecules 35 2455-2460... 

Temple, K., J le, R, Sheridan, J.B., and Manners, 1. (2001) The nature of the active catalyst in late transition metal-mediated ring-opening polymerization (ROP) reactions Mechanistic studies of the platinum-catalyzed ROP of silicon-bridged [l]ferrocenophanes. Journal the American Chemical Society, 123,1355. 

It was initially proposed that the transition-metal catalyzed polymerization proceeded via a homogenous mechanism." However, Manners proposed that the ROP of [l]silaferrocenophanes followed a heterogeneous catalytic cycle. Scheme 52 shows the proposed mechanism in which Pt(l,5-cod)2 is tiiought to initially form a [2]platinasilaferrocenophane (176) via oxidative addition of die zero-valence Pt complex wifli elimination of a 1,5-cod ligand. Elimination of a second 1,5-cod ligand then leads to the production of platinum colloids, which are believed to be the active catalyst. Subsequent oxidative addition and reductive elimination (or a-bond metathesis) at the colloid surface forms the polymeric material (175). 

Enzyme and acid-catalyzed polymerizations Aizawa and Wang have reported that the copper-containing enzyme bilirubin oxidase (BOX) catalyzes the oxidative polymerization of pyrrole to give thin films of polypyrrole on substrates such as glass, plastic or platinum plates. The BOX was first adsorbed onto the matrix support from an aqueous acetate buffer solution (pH 5.5), followed by incubation with the pyrrole monomer (0.2 M) in acetate buffer (pH 6) for several hours at room temperature. The deposited polypyrrole film was reported to have similar properties to PPy made by conventional chemical or electrochemical methods. 

Anhydrous, monomeric formaldehyde is not available commercially. The pure, dry gas is relatively stable at 80—100°C but slowly polymerizes at lower temperatures. Traces of polar impurities such as acids, alkahes, and water greatly accelerate the polymerization. When Hquid formaldehyde is warmed to room temperature in a sealed ampul, it polymerizes rapidly with evolution of heat (63 kj /mol or 15.05 kcal/mol). Uncatalyzed decomposition is very slow below 300°C extrapolation of kinetic data (32) to 400°C indicates that the rate of decomposition is ca 0.44%/min at 101 kPa (1 atm). The main products ate CO and H2. Metals such as platinum (33), copper (34), and chromia and alumina (35) also catalyze the formation of methanol, methyl formate, formic acid, carbon dioxide, and methane. Trace levels of formaldehyde found in urban atmospheres are readily photo-oxidized to carbon dioxide the half-life ranges from 35—50 minutes (36). 

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)... 

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