Physically branched and cross-linked polymeric structures were produced for the first time by rotaxane formation during reaction of a pendant group of a preformed macromolecule. The rotaxane structure is believed to form from a hydrogen-bonded bimolecular complex of 5-(hydroxymethyl)-1,3-phenylene-1’,3’-phenylene-32-crown-10 (16) by esterification of the hydroxy group of one macrocycle through the cavity of the second in its reaction with poly(methacryloyl chloride) (12). For esters formed in model reactions of 12 with methanol and with 5-(hydroxymethyl)-1,3-phenylene-16-crown-5 (14), which is too small to be threaded, the degrees of polymerization were identical; however, the polymer from reaction of 12 and 16 under the same conditions had a significantly higher degree of polymerization and polydispersity, i.e., was highly branched via rotaxane formation. Increasing the concentration in the reaction of 12 with 16 led to the formation of a gel fraction along with a high molecular weight sol fraction; the gel represents a novel network structure based on mechanical interlocking via rotaxane structures. 2D NOESY NMR experiments clearly demonstrated the rotaxane structure as manifest in the through-space correlation of the benzylic protons of the "thread" with the intra-annular protons of the "bead".