Silanols polycondensation

The first mechanistic studies of silanol polycondensation on the monomer level were performed in the 1950s (73—75). The condensation of dimethyl sil oxanediol in dioxane exhibits second-order kinetics with respect to diol and first-order kinetics with respect to acid. The proposed mechanism involves the protonation of the silanol group and subsequent nucleophilic substitution at the siHcone (eqs. 10 and 11). 

Linear combination of atomic orbitals (LCAO) method, 16 736 Linear condensation, in silanol polycondensation, 22 557-558 Linear congruential generator (LCG), 26 1002-1003 Linear copolymers, 7 610t Linear density, 19 742 of fibers, 11 166, 182 Linear dielectrics, 11 91 Linear elastic fracture mechanics (LEFM), 1 509-510 16 184 20 350 Linear ethoxylates, 23 537 Linear ethylene copolymers, 20 179-180 Linear-flow reactor (LFR) polymerization process, 23 394, 395, 396 Linear free energy relationship (LFER) methods, 16 753, 754 Linear higher a-olefins, 20 429 Linear internal olefins (LIOs), 17 724 Linear ion traps, 15 662 Linear kinetics, 9 612 Linear low density polyethylene (LLDPE), 10 596 17 724-725 20 179-211 24 267, 268. See also LLDPE entries a-olefin content in, 20 185-186 analytical and test methods for,... 

Silanol polycondensation is catalyzed by strong acids or bases, as well as by species as mild as amines or carboxylic acids combined with their quaternary ammonium salts (67). With strong acids or bases, the process is highly reversible, and the equilibrium can be represented as follows ... 

Fig. 12, Silanol polycondensation. A hydrosgrl-terminated PDMS polymer chain reacts with an ethyl silicate (PDES) in the presence of an organotin catalyst. The reaction results in a crosslinked network and releases ethanol as a condensation product.

The silanols formed above are unstable and under dehydration. On polycondensation, they give polysiloxanes (or silicones) which are characterized by their three-dimensional branched-chain structure. Various organic groups introduced within the polysiloxane chain impart certain characteristics and properties to these resins. 

Polycondensation reactions (eqs. 3 and 4), continue to occur within the gel network as long as neighboring silanols are close enough to react. This increases the connectivity of the network and its fractal dimension. Syneresis is the spontaneous shrinkage of the gel and resulting expulsion of Hquid from the pores. Coarsening is the irreversible decrease in surface area through dissolution and reprecipitation processes. 

This formation of smaller polyhedra when larger substituents are present is not surprising when the stability of intermediate silanols formed in these reactions toward polycondensation is taken into account. The methylsilanols condense... 

The fabrication of colloidal silica and optical glasses by the sol-gel process has attracted a great deal of attention (8). The process relies on the hydrolytic polycondensation reactions of alkoxysilanes, usually (EtO)4Si, in which the reactive silanols (EtO)4 Si(OH)n (n = 1-4) are formed. These then undergo acid- or base-catalyzed condensation with both water and alcohol formation, as shown in Scheme 2. 

Polycondensation of silanols to form a hydrated silica gel layer. 

Chojnowski, J. The Behaviour of the Silanol Function in Polycondensation Systems. In Silicon-Containing Polymers Jones, R. G., Ed. Special Publication 166 - Royal Society of Chemistry Cambridge, 1995 pp 59-72. 

In addition, polymers containing an odd number of silicon atoms in the siloxanylene segment can be prepared by this approach. However, reaction conditions employed in condensation of bis-silanols, III, with diaminosilanes (41) or siloxazanes (42) led to the cleavage of the carbonate group. Consequently, the recently reported silanol-acetoxysilane polycondensation reaction (43) was investigated for the polycondensation of bis-silanols, III, with diacetoxysilanes or diacetoxydisiloxanes (reaction 3). 

It was found that the reaction conditions which were optimized for the synthesis of poly(arylene siloxanylenes) (43) could be employed for the synthesis of siloxane-modified poly-(arylene carbonates). 2,4,6-Trimethylpyridine (collidine) was selected as the most suitable of all catalysts investigated (43) for the synthesis of the siloxane modified poly(arylene carbonates). Properties of polymers prepared by this method are given in Table I. In comparision to the phosgene-catalyzed homo-polycondensation of bis-silanols, III, the inherent viscosities... 

Polycondensation of Bis-silanols, III, with Diacetoxy-silanes, Diacetoxydisiloxanes or Diacetoxytrisiloxanes. A mixture of 12.00 mmole of a bis-silanol, III, 12.00 mmole of a diacetoxysilane, diacetoxydisiloxane or diacetoxytrisiloxane,... 

Various functional silanes (e.g., R2SiCl2 or RSiCl3) can be used as starting materials for the preparation of polysiloxanes. The silanes are first hydrolyzed to the corresponding silanols, which are very unstable and easily undergo polycondensation with the elimination of water and the formation of -Si-O-Si- link-... 

The preferentially employed approach for the fabrication of inorganic (silica) monolithic materials is acid-catalyzed sol-gel process, which comprises hydrolysis of alkoxysilanes as well as silanol condensation under release of alcohol or water [84-86], whereas the most commonly used alkoxy-silane precursors are TMOS and tetraethoxysilane (TEOS). Beside these classical silanes, mixtures of polyethoxysiloxane, methyltriethoxysilane, aminopropyltriehtoxysilane, A-octyltriethoxysilane with TMOS and TEOS have been employed for monolith fabrication in various ratios [87]. Comparable to free radical polymerization of vinyl compounds (see Section 1.2.1.5), polycondensation reactions of silanes are exothermic, and the growing polymer species becomes insoluble and precipitates... 

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