Poly -siloxane synthesis

Zhou, Y, Shimizu, K., Cha, J.N., Stucky G.D., and Morse, D.E. (1999) Efficient catalysis of poly-siloxane synthesis by silicatein a requires specific hydroxy and imidazole functionalities. Angew Chem. Int. Ed., 38, 779-782. 

Recently siloxane-imide copolymers have received specific attention due to various unique properties displayed by these materials which include fracture toughness, enhanced adhesion, improved dielectric properties, increased solubility, and excellent atomic oxygen resistance 1S3). The first report on the synthesis of poly(siloxane-imides) appeared in 1966, where PMDA (pyromellitic dianhydride) was reacted with an amine-terminated siloxane dimer and subsequently imidized 166>. Two years later, Greber 167) reported the synthesis of a series of poly(siloxane-imide) and poly(siloxane-ester-imide) copolymers using different siloxane backbones. However no physical characterization data were reported. 

Electrochemical doping of insulating polymers has been attempted for polyacetylene, polypyrrole, poly-A/-vinyl carbazole and phthalocyaninato-poly-siloxane. Significantly, Shirota et al. [91] claim to have achieved the first synthesis of electrically conducting poly(vinyl ferrocene) by the method of electrochemical deposition (ECD) [91]. This is based on the insolubilization of doped polymers from a solution of neutral polymers. A typical procedure applied [91] for polyvinyl ferrocene is to dissolve the polymer in dichlorometh-ane and oxidize it anodically with Ag/Ag+ reference electrode under selective conditions. The modified polymer [91] (Fig. 28) is a partially oxidized mixed valence salt containing ferrocene and ferrocenium ion pendant groups with C104 as the counter anion. 

An example for a synthesis of a poly(siloxane) network is shown in Fig. 37b. In a one-step reaction the mesogenic moieties as well as the crosslinking agent are coupled via an addition reaction to the reactive linear poly(methylhydrogensiloxane) backbone 92). Because of similar reactivity of the crosslinking agent and mesogenic molecules, a statistical, disordered addition to the backbone has to be expected. 

In 1999, Muzafarov and coworkers published a preliminary report describing the synthesis of a hyperbranched poly(siloxane) from triethoxysilanol via a rapid, ammonia-catalyzed condensation process (Scheme 28)192. The authors acknowledged the potential difficulties of this process (e.g. head-head condensation leading to crosslinked products) but presented 29Si NMR data to support their conclusion that hyperbranched polymer was formed. The fact that no gel formation was observed also suggested that the polymerization proceeded as expected. The polymer, a transparent, yellow liquid, was characterized with NMR and IR spectroscopy, and GPC. 

The traditional approach used in poly(imide-siloxane) synthesis is the reaction of aminopropyl-terminated dimethylsiloxane oligomers with aromatic dianhydrides and additional diamines (9-13). Typically, subambient temperatures and dipolar aprotic solvents are used. The resulting high-molecular-weight polyamic acid solution can be heated to effect imidization and solvent evaporation. This procedure is analogous to the synthetic method used to prepare conventional polyimides for films and coatings. 

The aim of this work was to examine the catalytic activity of rhodium siloxide complexes in the hydrosilylation of allyl glycidyl ether by triethoxysilane and hydro(poly)siloxanes, leading to optimization of procedures for the synthesis of epoxy-functional silanes and siloxanes. 

On the basis of the obtained results, the technologies of synthesis of epoxy-functional silane and (poly)siloxanes have been worked out and will be implemented in the very near future. 

A general strategy for the synthesis of dendritic poly(siloxanes) with discrete molecular weights greater than 15000 (third generation) was developed by Masamune and co-workers in 1991 (Figure 4).14 Synthesis... 

Figure 3. Rebrov s seminal synthesis of hyperbranched poly(siloxanes)...

The synthesis and structure of MQ resins have been described above. The poly(siloxane) network is traditionally derived by free-radical cross-linking of a high molecular weight PDMS polymer or gum using a peroxide catalyst, such as benzoyl peroxide or 2,4-dichlorobenzoyl peroxide. The curing reaction is performed immediately after the PSA has been coated onto a tape substrate, such as PET, PTFE, or Kapton. Uncured PSAs are supplied as a solution in an organic solvent. Some silicone PSAs also incorporate phenyl groups onto the gum portion of the adhesive to increase the use temperature. 

The poly(siloxane) polymers are usually prepared by the acid or base hydrolysis of appropriately substituted dichlorosilanes or dialkoxysilanes, or by the catalytic polymerization of small ring cyclic siloxanes [71-75]. The silanol-terminated polymers are suitable for use after fractionation or are thermally treated to increase molecular weight and in some cases endcapped by trimethylsilyl, alkoxy or acetyl groups [76,77]. Poly(siloxanes) synthesized in this way are limited to polymers that contain substituent groups that are able to survive the relatively harsh hydrolysis conditions, such as alkyl, phenyl, 3,3,3-trifluoropropyl groups. Hydrosilylation provides an alternative route to the synthesis of poly(siloxanes) with labile or complicated substituents (e.g. cyclodextrin, oligoethylene oxide, liquid crystal, amino acid ester, and alcohol) [78-81]. In this case... 

In conclusion, we have demonstrated the first example of Pd nanoparticles as a selective and recyclable catalyst for the alcoholysis of polyhydrosiloxane. Fair numbers of alcohols with diverse structures (primary, secondary, sterically bulky, and functionalized alcohols) were selectively and efficiently grafted onto the poly-siloxane backbone without any side reactions and under moderate reaction conditions. Additionally, active participation of Pd nanoclusters during the catalytic transformations was established by in situ EM analysis and controlled poisoning experiments. Moreover, a new approach for the synthesis and stabilization of Pd nanoclusters as a stable isolable powder and their redispersion in common solvents was presented. 

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