Disiloxanes structure

The Si-O-Si bond angle and the Si-0 bond length were changed in 10° and in 0.03 A steps, and after a partial optimization of the disiloxane structure at the B3LYP/6-31G(d) level of theory a subsequent NMR calculation (GIAO-MP2/A) [6, 7] was performed. The combined effect of both... 

Table 3. General structure of a,ca-organofunctionally-terminated disiloxanes...

Detailed procedures for the synthesis ofa,o>-organofunctionally terminated siloxane oligomers with well defined structures have been given 50,66-67). Tables 6 and 7 provide the data on the synthesis and characteristics of aminopropyl and hydroxybutyl terminated polydimethylsiloxane oligomers prepared via anionic and cationic ringopening polymerization of octamethylcyclotetrasiloxane (D in the presence of appropriate disiloxanes, respectively. 

Synthesis of hydrolytically stable siloxane-urethanes by the melt reaction of organo-hydroxy terminated siloxane oligomers with various diisocyanates have been reported i97,i98) -yhg polymers obtained by this route are reported to be soluble in cresol and displayed rubber-like properties. However the molecular weights obtained were not very high. A later report56) described the use of hydroxybutyl terminated disiloxanes in the synthesis of poly(urethane-siloxanes). No data on the characterization of the copolymers have been given. However, from our independent kinetic and synthetic studies on the same system 199), unfortunately, it is clear that these types of materials do not result in well defined multiphase copolymers. The use of low molecular weight hydroxypropyl-terminated siloxanes in the synthesis of siloxane-urethane type structures has also been reported 198). 

A variety of simple disiloxanes of general formula (H0R2Si)20 (where R = Me, Et, Prn, Pr1, c-C5H9, or Ph) 48, a thienyl compound 49, and the cyclic species 50 have all been structurally investigated. The simplest... 

The hydrogen bonding in the acyclic disiloxanes may take one of several forms, depending on the relative positions of the molecules along the two sides of a double chain, as shown in Fig. lOa-c, while the cyclic compound adopts a more complicated structure, shown in Fig. lOd. 

For equilibration processes, one must synthesize both oligomers and what are termed dimers, or disiloxanes. Our primary interest is in the utilization of these functional oligomers for the synthesis of both linear block or segmented copolymers, and also surface modified, oughened networks such as the epoxy and imide systems (3-27). The generalized structure of the oligomers of interest is shown in Scheme 1. 

The structural comparison (Fig. 3) with both the twofold tris(trimethylsilyl)methyl substituted acetylene and 1,4-benzene derivatives (Fig. 1) as well as with the literature data [6a] for hexa-kis(rm.butyl)disilane [6b] containing a SiSi bond elongated to 270 pm ( ), for the linear ( ) hexa-kis(rm.butyl)disiloxane [6c] or for di(tris(trimethylsilyl)silyl)zinc [6d] is based advantageously on a model in which the two substituent half-shells are separated along their central C3 axes by spacers of different lengths. 

Structures of immobilized rhodium complexes on the sihca support have been proposed on the basis of the data obtained from C, P and Si MAS-NMR. NMR spectra of the rhodium-modified solid materials confirmed that trimethylsiloxide ligand was removed from the rhodium coordination sphere during the immobilization process. Formation of a new covalent bond between the rhodium organo-metallic moiety and the silica support occurs, probably with evolution of trimethylsilanol, which is rapidly converted into disiloxane (Me3Si)20. The presence of this molecule in the solution obtained after the silica surface modification process was confirmed by GCMS analysis. 

Disilathiane has been the subject of several recent studies.1 7 Interest has been centered on the importance of (p-d)7r-bonding in determining both the structure and the base strength relative to the disiloxanes and carbon analogues.2 6 The utility of disilathianes has been demonstrated by their conversion to a wide variety of silyl derivatives by exchange reactions or protolyses.2,7... 

West and coworkers28 used /(SiSi) to discuss the structure of long-debated 1,3- cyclo-disiloxanes (12a) for which a few alternative structures were proposed. One such structure retains the a bond between two silicon atoms (12b). In a structure such 12a as 12b, the /(SiSi) should be close to the standard values of 80-90 Hz while for 12a 2/(Si—O—Si) will be observed with values of about 4 ppm. The values observed for a variety of 1,1,3,3-tetraaryl-l,3-cyclodisiloxanes are in the range of 3.85-4.02 Hz, which support structure 12a. There is another proposal for the structure of 1,3-cyclodisiloxanes (dibridged ir complex) and the authors suggested this structure is also consistent with the observed coupling constants. 

Similarly, the bis(allyl)disiloxane 372 when photolyzed underwent [2 + 2] cycloaddition to yield the compound 373 as the sole product, the disiloxane linkage acting as a tether to assist in the orientation of the carbon-carbon double bonds involved in the cycloaddition193. The product had the a -trans structure (equation 43). 

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