Polysiloxanes
The silicon atom is below carbon in the periodic table with a similar electronic arrangement, which in silicon is: 1s2 2s2 2p6 3s2 3p2. The larger atomic radius, however, makes the silicon–silicon single bond much less energetic. Because of this, silanes (SinH2n +2) are much less stable than alkanes. The opposite, however, is true of silicon–oxygen bonds that are more energetic (about 22 kCal/mole) than the carbon oxygen bonds. Polysiloxanes, therefore, have recurring Si–O linkages in the backbones. The starting materials can be prepared through hydrolyses of alkyl or arylsilicone halides . Organosilicone halides, in turn, are made commercially by heating alkyl or aryl halides with silicon at 250–289C. Copper catalyzes this reaction:
The same materials can also be formed by the Grignard reaction:
Alkyl silanes can also be prepared by additions of trichlorosilanes to ethylene or acetylene:
Trichlorosilane reacts with aromatic compounds in the presence of boron trichloride:
The siloxane linkages can result from hydrolysis of the halides. The products of hydrolyses, silanols, are unstable and condense:
The above shown reaction is one possible route to siloxane polymers. As a general method, however, this approach is not very satisfactory, because ring formations accompany the reactions. Some rings that form from hydrolyses of trichlorosilanes are structurally complex [161]. They may even possess three-dimensional structures [162, 163]. High molecular weight polymers, however, form readily by ring opening polymerizations. Such polymerizations can be applied to the simple rings that form from dihalides or complex ones from trihalides. Ring opening polymerizations, carried out on purified (by distillation) cyclic intermediates, are catalyzed by either acids or bases [164], leading to linear siloxane polymers:
Acid-catalyzed polymerizations yield lower molecular weight polymers that are mostly oils. The molecular weights of these oils can be controlled by additions of hexamethyldisiloxane during the polymerization reactions. When catalyzed by bases, high molecular weight elastic polymers form.
Another method of forming polydialkyldisiloxanes is by reacting difunctional oligomers with cyclic or ganisilicons:
Molecular weights of polydimethyl siloxanes can reach 700,000 or higher. Within the range of molecular weights between 4,000 and 25,000, the materials are fluids of various viscosities. Most common commercial liquid polydimethyl siloxanes are prepared from dimethyl dichloro siloxane. Many elastomers are also based on dimethyl siloxane. Special polymers are prepared with other substituents.
