The prime role of the siloxane backbone is to present the available methyl groups to their best advantage and it does this by virtue of its unique exibility. In most hydrocarbons, the bond angles are very rigid, and steric packing considerations often prevent the available methyls from adopting lowest surface energy orientations. In silicones, the Si-O bond length is signicantly longer and the Si-O-Si bond angle atter than comparable C-C and C-O bonds resulting in a very low barrier to rotation and making the polymer chains very exible. This exibility makes many orientations possible and provides “free space” to accommodate different sized substituents or to allow easy diffusion of gaseous molecules: a property useful in the formation of “breathable” lms. Coupled with the low intermolecular forces between methyl groups, this exibility also has a profound effect on the bulk as well as the surface properties of silicones, seen in the small variation of physical parameters with temperature and molecular weight, the low freezing and pour points of uids, the high compressibility, and the retention of liquid nature to unusually high molecular weights. It also makes a number of structural and compositional variations possible, resulting in many families of silicones, including linear and cyclic structures, a wide range of molecular weights, and varying degrees of branching or cross-linking.