Networks with well-defined structures were prepared by end linking vinyl-terminated poly(dimethylsiloxane) chains with tri- and tetrafunctional silanes. Modulus and extent of reaction were followed continuously on several systems. If trapped entanglements are accounted for, the modulus rise can be modeled quantitatively with branching theory. The effect of entanglements is demonstrated by changing molecular weight of the chains and by dilution. Comparison of data between tri- and tetrafunctional cross-linkers indicates clearly the importance of junction mobility. The C2Mooney-Rivlin constant is small. Sol fraction and swelling data are also in reasonable agreement with predictions from branching theory.