The synthesis and characterization of copolymers from ethylene and cyclopentene are reported. Polymerization of ethylene anti cyclopentene using a living bis(phenoxyimine)titanium catalyst yields copolymers with cis-1,2 cyclopentane units, high molecular weights, and low polydispersities. The effects of cyclopentene concentration, ethylene pressure, and reaction temperature on cyclopentene incorporation were determined; under the optimal reaction conditions 47 mal % cyclopentene was incorporated to give a nearly perfectly alternating copolymer. The glass-transition temperatures of the polymers, ranging from -27.3degreesC (27 mol % cyclopentene) to 10.1degreesC (47 mol % cyclopentene), were found to be linearly dependent on the mole percent cyclopentene content. Synthesis of atactic and isotactic model poly(ethylene-alt-(cis-1,2-cyclopentane))s using ring-opening metathesis polymerization allowed the assignment of the fine structure of C-13 NMR spectra of atactic polymers made from ethylene and cyclopentene. Finally, polymers containing blocks with varying ratios of ethylene/cyclopentene were synthesis. By repetitively raising and lowering the ethylene pressure of a cyclopentene/ethylene polymerization, polymers with multiple blocks were synthesized.