Isoxazolino- and isoxazolidino-norbornenes, prepared by cycloaddition to norbornadiene of nitrile oxides and nitrones, respectively, undergo ring opening metathesis polymerisation (ROMP) initiated by the Grubbs ruthenium alkylidenes 1 and 2. The structures of the resulting polymeric isoxazolines were established from their NMR spectra by comparison to polynorbornene and to model isoxazolines prepared by nitrile oxide cycloaddition to cyclopentene. The average degree of polymerisation was regulated using an acyclic chain transfer agent (hex-1-ene) affording products with av DP = 4-157, and with narrow molecular weight distributions (PDI = 1.45-1.65). Oligomeric analogues (n = 4,16) with saturated backbones were prepared by reduction with p-toluenesulfonyl hydrazide. (c) 2006 Elsevier Ltd. All rights reserved.