Rhodium-mediated Cl polymerization of glycidyl 2-diazoacetate resulted in the novel functional poly methylene poly(glycidyl 2-ylidene-acetate), the first functional polymethylene suitable for a subsequent controlled post polymerization modification at moderate reaction conditions via nucleophilic ring-opening of the epoxide. Nucleophilic ring opening reactions with water, various amines, and thiols were examined. Even bulky nucleophiles such as alpha-aminodiphenylmethane reacted quantitatively, resulting in densely packed functional polymers. The obtained results were compared with previously published results on post-polymerization modification of poly(glycidyl methacrylate). Contradictory to the expectations, epoxide ring-opening with primary amines led to soluble products when the Cl polymer poly(glycidyl 2-ylidene-acetate) was employed, while the same reaction with the C2 polymer poly(glycidyl methacrylate) resulted in gelation.