Silicone sealants generally consist of the simple combination of polydimethylsiloxane, trior tetrafunctional silane cross-linker, filler(s), plasticizer and catalyst. Some properties of the sealant are a consequence of the siloxane polymer, silica filler and silane cross-linker. These produce a system with only the very stable SiOSi link in the entire network. To the cured sealant this imparts high temperature stability, low temperature flexibility, UV radialion resistance, resistance to chemical attack and unsurpassed weatherability. Other properlies are a function of polymer chain length, filler type and concentration, and plasticizer. These include strengths, extrusion rate and extrudability. A new technique of chain extension during the cure process, before cross-linking occurs, allows for a sealant to be made with the desirable characteristics imparted by very high molecular weight polymer and still maintain the soft paste type consistency desired without a plasticizer. -- AATA