AbstractNew synthetic techniques are now available for the preparation of “model” polymer networks of known and carefully controlled molecular structure. The preparation of such networks and their utilization in elucidating molecular aspects of rubberlike elasticity are illustrated using highly elastomeric, unfilled networks of polydimethylsiloxane. In one such preparative technique, hydroxyl‐terminated chains are end‐linked using either a tetrafunctional or trifunctional silicate, in either the undiluted state or in solution. Because of the selectivity and completeness of the reaction, the molecular weight and molecular weight distribution of the network chains are to good approximation those of the chains prior to their end‐linking into the network structure. The experimentally determined stress‐strain isotherms and degrees of equilibrium swelling of the resulting networks are then used to evaluate the molecular theories of rubberlike elasticity, particularly with regard to the dependence of the elastic modulus and ultimate properties on the degree of cross linking and the cross‐link functionality. Another preparative technique employs random and block copolymers containing reactive vinyl groups in known amount and known distribution along the chains. Selectively cross‐linking through these sites then yields model networks ranging from random to highly non‐random in chain length distribution. These networks are particularly useful for characterizing the effects of very short chains on the average degree of cross‐linking or average network chain length as obtained from stress‐strain and swelling equ