Siloxane equilibration reactions

Despite the importance and synthetic utility of these siloxane equilibration reactions, relatively little has been reported with respect to the detailed kinetics and mechanisms involved, especially in the presence of functionalized end blockers. A major focus of our efforts (3, 4, 6-8) is the investigation of various aspects of siloxane equilibration reactions to establish the exact nature of the active polymerization species and the effect of various reaction parameters on the preparation of well-defined diftmctionalized siloxane oligomers. 

Siloxanolate Catalysts. The initial step for the study of the kinetics of base-catalyzed siloxane equilibration reactions was the preparation of a number of well-defined siloxanolate catalysts. The catalysts were prepared separately, prior to the equilibration reactions, so that a homogeneous moisture-free system with a known concentration of active centers might be obtained. The catalysts studied included potassium, tetramethylammonium, and tetrabutylphosphonium siloxanolate. 

The catalytic solutions were characterized by potentiometric titration of the siloxanolate groups with alcoholic HCl. Titration with acid generally yielded two end points The first end point was attributed to the siloxanolate group, and the second was attributed to the carbonate species. Previous workers (12) have also reported two end points for the titration of tetra-methylammonium hydroxide (TMAH) with HCl, with phenolphthalein and methyl orange as indicators. They attributed the end points to hydroxide and carbonate components, because TMAH readily absorbs carbon dioxide from the air. The carbonate species was reasonably assumed to have no catalytic effect on the siloxane equilibration reactions. 

In many cases, these cyclic siloxanes have to be removed from the system by distillation or fractionation, in order to obtain pure products. On the other hand, cyclic siloxanes where n = 3 and n = 4 are the two most important monomers used in the commercial production of various siloxane polymers or oligomers via the so-called equilibration or redistribution reactions which will be discussed in detail in Sect. 2.4. Therefore, in modern silicone technology, aqueous hydrolysis of chloro-silanes is usually employed for the preparation of cyclic siloxane monomers 122> more than for the direct synthesis of the (Si—X) functional oligomers. Equilibration reactions are the method of choice for the synthesis of functionally terminated siloxane oligomers. 

The synthesis of cyclic polydimethylsiloxane was first achieved through ring-chain equilibration of siloxane oligomers in the presence of potassium silanolate, as shown in Fig. 51 [163-165]. Cyclics recovered from ring-chain equilibration reactions have been fractionated by preparative GPC, yielding... 

So interpretation of the B-H vibration bands confirmed the different reactions of diborane with the hydroxylic surface groups put forward by Shapiro and Weiss.35,36,49 In addition the infrared analysis of the modified surface revealed (1) the reaction of B2H6 with siloxanes, explaining the low hydrolysis ratios found under certain reaction conditions and (2) the equilibration reaction existing between B2H6 in the gas phase and the monodentate groups formed through reaction of BH3 with a surface hydroxyl. 

Equilibration Reaction Kinetics. This study investigated the effect of various reaction parameters on the synthesis and equilibration reaction kinetics in the preparation of well-defined difunctionalized siloxane oligo-... 

This illustrative study of ring-chain equilibria in a paraffin-siloxane system demonstrates that the conformational characteristics of chain molecules can be investigated provided suitable labile groups are incorporated into the molecular structures. It should prove within the capabilities of chemists to synthesise a wide variety of molecules suitable for study by the equilibrium cyclic concentration method using the dimethylsiloxane (or other siloxane) linkage as a labile linkage, incorporated into the molecular structure, throu whicJi to effect ring-cJiain equilibration reactions. 

The usual way of preparing such aminopropyl-terminated siloxanes is the synthesis via a base-catalyzed equilibration reaction [1, 2]. A more convenient way is the termination reaction of silanol-terminated siloxanes with special organofimctional silanes [3,4]. 

The reaction of chlorosilanes produces hydrolyzates consisting of cyclics and linears with hydroxy and/or chlorine ends, depending upon conditions. Silanol functional linears are easily obtained and can be end-capped by silylation. More pertinent to this discussion, however, is the nature of end blocks that result from siloxane redistribution reactions. Conversion of cyclosiloxanes to equilibrium ring-chain distributions affords chains with ends arising from the catalyst i.e., if KOH is used, the chains have ends bearing silanol and K silanolate functionality. Neutralization with CO2 and HjO then converts the silanolates to silanols. Alternatively, the equilibrates, as well as the above hydrolyzates, can be silylated to convert the silanols and silanolates to other kinds of ends. 

Various reactions, both of polymerization and of polymer degradation, can produce cyclic polymer molecules. A well-known process is the ring-chain equilibration reaction, which may be used to produce cyclic siloxanes and o er cyclic polymers. The linear chain reacts intramolecularly and yields a cyclic and a linear chain. In the initial stages, the molar fraction of cyclics increases at the expense of the linear chains. After some time, equilibrium conditions are achieved and the molar fraction of cyclics remains constant. In some cases, all the sites in the macromolecular backbone are equivalent and no peculiar bond exists which is preferentially attacked. This case is referred to as thermodynamically controlled cyclization. 

The hydrophobic part of the molecule is the trisiloxane group, which does not exhibit the pronounced oleophobicity typical of siloxanes because of its lack of dimethylsiloxane groups. It is obtained by an equilibration reaction of... 

This mixture can be condensed and equilibrated by a number of different catalysts, or the cyclic siloxanes can be removed and polymerized separately. Catalysts for these equilibration reactions can be basic, such as the hydroxides of lithium, sodium, potassium, and cesium, and potassium amide." Some acidic catalyst are sulfuric and ethylsulfuric, chlorosulfonic phosphoric, py rophos-... 

Elimination of the multidentate interaction of a counterion with the siloxane chain is crucial. Otherwise, as mentioned before, the equilibration reactions would make the precision polymerization impossible. Specific initiator-solvent systems used for this purpose may be divided into three groups (1) basic solvent and a hard counterion, which interacts with solvent stronger than with siloxane, for example, lithium/THF (2) bulky and soft counterions, for example, Me4N BtuP, and phosphazenium cations, which weakly interact with nucleophiles (3) basic promoters strongly interacting with counterions, such as HMPT, DMSO, DMF, cryptands, and crown ethers. ... 

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