Abstract: A sealant made from a room temperature curing polydimethylsiloxane composition has a siloxaphobic surface layer at the air-sealant interface where this layer is a fluorocarbon compound and a drying oil oxidation product. Elastomeric compositions made from polydimethylsiloxanes having both low reactivity endgroups and high reactivity endgroups and a siloxaphobic agent of fluorocarbon alcohol or a reaction product of the fluorocarbon alcohol and a hydrolyzable silane, are particularly useful for sealing buildings because they can withstand the expansions and contractions. One siloxaphobic agent is the reaction product between a fluorocarbon alcohol and a methyltri(methylethylketoximo)silane and the formula of one of the reaction product compounds is ##STR1## wherein x has an average value of at least 6.

Abstract: A polydimethylsiloxane which has both low reactivity endgroups and high reactivity endgroups. These polydimethylsiloxanes are useful for making sealants with reduced modulus. An example of these polydimethylsiloxanes is one having low reactivity endgroups of the formulaXR.sub.2 SiO--where X is methoxy or methylethylketoximo, and R is methyl or vinyl and high reactivity endgroups having a formulaY.sub.b R.sub.(3-b) SiO--in which b is 2 or 3, R is methyl radical, and each Y is a hydrolyzable group selected from the group consisting of a ketoximo group and methoxy.