Silanol reactivity

With exposure to moisture, hydrolysis of the cross-linking agent leads to the silanol reactive cure site, so such materials must be compounded and stored free of moisture.66... 

Obviously, as discussed earlier in the silica production process, silica surface can also be modified by other chemical species added during and after its preparation. In any case, this modifier only changes silanol reactivity by enhancing (Cardona-Martinez and Dumesic, 1990 Uytterhoeven and Fripiat, 1962) or decreasing its acidity. 

Several side reactions or post-cuting reactions are possible. Disproportionation reactions involving terminal hydride groups have been reported (169). Excess SiH may undergo hydrolysis and further reaction between silanols can occur (170—172). Isomerization of a terminal olefin to a less reactive internal olefin has been noted (169). Viaylsilane/hydride interchange reactions have been observed (165). 

Unfortunately, because self-condensation of silanols on the same silicone can occur almost spontaneously, the reaction of disdanol or trisilanol compounds with telechelic sdanol polymers to form a three-dimensional network is not feasible. Instead, the telechelic polymers react with cross-linkers containing reactive groups such as alkoxysdanes, acyloxysdanes, silicon hydrides, or methylethyloximesilanes, as in the reactions in equations 18—21 (155). 

The acetoxy-based moisture sensitive diacetoxymethylsiloxy-endblocked-PDMS (II) reactive polymer is prepared in situ in the packaging cartridges by mixing silanol-endblocked-PDMS (I) with... 

Upon application of the silicone by extrusion, moisture in the atmosphere comes into contact with the silicone surface. The hydrolysis of an acetoxy siloxy group of the diacetoxymethylsiloxy-endblocked-PDMS reactive polymer (II) proceeds and leads to a silanol-endblocked polymer as shown in Scheme 7, where OAc represents the acetoxy (CH3COO-) group. 

Two-part room temperature condensation cure. Silicone can be formulated into two-part systems [3,12,14,33] that prevent the reactive groups from coming into contact before they are needed. The reactions in these systems are based on the condensation of a silanol group with an alkoxy silane group, catalyzed by organo-tin compounds (Scheme 9). 

While structurally equivalent, they usually differ from a reactivity point of view. Firstly, the organometallic reagent, after reaction with the surface, readily extrudes extra ligands upon reaction with surface silanols, while they can give a stable adduct in solution. 

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. 

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