Siloxanes equilibration

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. 

Although the hydroxide salts of these bases are effective as catalysts for siloxane equilibrations, the solid hydroxides of both potassium and tetramethylammonium are quite hygroscopic and generally contain significant amounts of water. Also, tetrabutylphosphonium hydroxide (TBPH) is avail-... 

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. 

Zheng P, McCarthy TJ (2012) A Surprise from 1954 siloxane equilibration is a simple, robust, and obvious polymer self-healing mechanism. J Am Chem Soc 134 2024... 

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. 

General route for the Preparation of a,oo-Organofunctionally-Tenninated Siloxane Oligomers by Equilibration (Redistribution) Reactions8... 

Poly(carboxyalkyl methyl siloxane) and octamethyl cyclo tetrasiloxane are mixed in a monomer ratio of 1 6, hexa methyl disiloxane is added to bring the polymerization degree of the ensuing silicone to about 60, and 6 volume percent of concentrated sulfuric acid are used as equilibration catalyst. 

As it is well known, the (Si-0) bond in organosiloxanes may be considered to be polar or partially ( 50%) ionic.(12) Therefore, it can be cleaved by the attack of strong acids or bases. This is the main rationale behind the "equilibration" route to the synthesis of a wide variety of functionally terminated siloxane oligomers(12-14) from cyclic siloxanes and a.ordifunctional disiloxanes as shown in Scheme 3. 

In their earlier studies on the synthesis of a, ajrhydroxyl terminated siloxane oligomers Marvel and co-workers (21) have utilized sulfuric acid as the catalyst in the equilibration of cyclic tetramer (D ) and l,3-bis(4-hydroxylbutyl)tetramethyldisilox-ane (DSX). However the molecular weights of the oligomers obtained were several times higher than the expected values. They have... 

Base catalyzed equilibrations of hydroxyl terminated siloxane oligomers can also be achieved, however these systems can sometimes be complicated by the attack of base to the (OH) end groups to form [(CH2)x 0- X+] type species, which may also cleave the siloxane bonds. This generally results in the loss of terminal functionality in the oligomers produced, which is of course not desirable. 

As an example, gas flow from a cola beverage was analyzed and calibration of benzalde-hyde was performed as described in Support Protocol 1. A 100-pm-thick polydimethyl-siloxane (PDMS) SPME fiber was used to extract the gas flow from the cola beverage. Since equilibrium with the flow concentration was determined to occur within 5 min for the slowest-equilibrating compound, the fiber was exposed to the gas flow for 5 min and then desorbed in the injection port of a GC/MS. A 0.01 %... 

Another class of functional siloxane polymer that has received some attention are the fluorosiloxane materials, especially 3,3,3-trifluoropropylmethylsiloxanes. The use of conventional equilibration catalysts to produce these materials gives products which favour the cyclosiloxane in the bulk. Clarson and coworkers41 report the use of specific condensation catalysts such as stannous octanoate, potassium carbonate and barium hydroxide to prepare hydroxy terminated fluorosilicone polymers. 

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. 

---