silicones

Silicone is the rubbery, water-shrugging stuff in bathroom sealant, soft baking molds, medical tubing, and waterproofing sprays. People often confuse it with silicon the element or with silica the rock, but silicones are something else again: synthetic polymers with a backbone unlike anything in ordinary plastics.

Silicones (more precisely polysiloxanes) have a backbone of alternating silicon and oxygen atoms, -Si-O-Si-O-, the same strong Si-O linkage that makes the silicates so stable, but with organic groups (usually methyl, CH3) attached to each silicon to cap off the remaining bonds. A typical repeating unit is -O-Si(CH3)2-, giving polydimethylsiloxane (PDMS). This hybrid design is the whole point: the inorganic Si-O-Si backbone is thermally stable, chemically inert, flexible, and resistant to water and UV light, while the organic methyl groups make the material oily, water-repelling, and rubbery rather than glassy. By changing chain length and the degree of crosslinking you span fluids (silicone oils), gels, and elastomers (silicone rubber).

Silicones matter because they bridge the inorganic robustness of silicate-like Si-O bonding with the processability of organic polymers. They are used as high-temperature lubricants and heat-transfer fluids, water-repellent coatings, sealants and caulks, electrical insulation, and, because they are unusually biocompatible, in implants, prostheses, and medical devices. They are a clean example of how main-group inorganic chemistry, the durable Si-O backbone, underpins materials we think of as everyday plastics.