Disiloxane equilibration

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. 

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. 

Siloxane-containing block copolymers are often prepared by step-growth or condensation polymerization of preformed diftmctionalized siloxane oligomers with other diftmctionalized monomers or oligomers. Our current work (3, 4, 6-8) on siloxane chemistry includes the preparation of a number of functionalized oligomers, with emphasis on equilibration processes with the commonly available cyclic tetramer, octamethylcyclotetrasiloxane (D4), in the presence of a functionalized disiloxane or end blocker. 

Significant differences were observed in the rate of incorporation of D4 and l,3-bis(3-aminopropyl)disiloxane for similar concentrations of potassium, tet-ramethylammonium, and tetrabutylphosphonium siloxanolate catalysts. The rate differences affected the reaction times that were required to obtain a completely equilibrated reaction mixture with the desired molecular weight. The potassium catalyst required excessively long reaction times or high concentrations before sufficient incorporation of the aminopropyldisiloxane was realized. The tetramethylammonium and tetrabutylphosphonium catalysts were much more efficient for the preparation of controlled-molecular-weight aminopropyl-terminated polysiloxane oligomers. 

Figure 5. Viscosity V5. time of reaction for equilibration of 1 mol of hexamethyU disiloxane and 1 mol of with 4% HzSO at room temperature ( 3).

The piperazine capped disiloxane as discussed in 6 above could easily be equilibrated under exactly identical conditions to those discussed for the primary amine system earlier, that is, the disiloxane, cyclic tetramer, and 0.5 weight percent of the siloxanolate catalyst were heated to 80°C for 44 hours. The work-up procedure was identical, that is, the catalyst was deactivated at 150°C for 3 hours. The excess cyclics were then stripped under vacuum (0.5 torr, lOO C) and the material was further characterized by amine endgroup titration. 

The oligomers utilized in this study to date have been prepared by the equilibration polymerization of a cyclic tetramer in the presence of functional disiloxanes utilizing a... 

The equilibration of the aminopropyl terminated disiloxane was conducted in a very similar manner to the epoxy terminated disiloxane and is shown in Scheme 5. The reaction in this case could not be followed by our GPC unit since we noticed significant absorption of the oligomers onto the columns. 

A very similar equilibration was conducted as shown in Scheme 6 for the synthesis of secondary amine terminated siloxane oligomers. The amine terminated disiloxane here is a secondary... 

Silicone oils. These are formed when the crude hydrolysis product mentioned above is made to undergo further polymerization using acid or alkali as catalyst. In this equilibration process the hydroxy-ended oligomers condense with elimination of water, and the cyclic oligomers open and also produce long chain polymers. The average chain length is determined by the addition of exactly controlled amounts of hexamethyl-disiloxane ... 

---