This thesis describes studies into the ring opening metathesis polymerisation (ROMP) of some monocyclic alkenes, the living polymerisation of cyclopentene and the preparation of novel block copolymers and novel polyethylene architectures using having poly(1 -pentenylene) as the precursor polymer. Chapter 1 reviews general aspects of ring opening metathesis polymerisation of relevance to die themes of this diesis. Chapter 2 describes the ROMP of cw,cw-cycloocta-l,5-diene, cyclopentene, cycloheptene, cyclooctene, cyclodecene and cyclododecene using a series of well- defined initiators of general formula M(N-2,6-i-Pr(_2)-C(_6)H(_3))(CHR)(OR')(_2) where M=Mo or W, the characterisation of the resulting polymers using (^1)H-, (^13)c-NMR and IR spectroscopy, differential scanning calorimetry and gel permeation chromatography and an analysis of their (^13)C-NMR spectroscopic data in terms of the γ-gauche effect. Chapter 3 reports on a (^1)H-NMR study of the synthesis of poly(bis(trifluoromethyl)norbomadiene)-block-poly(l-octenylene) and the synthesis and characterisation of poly(l-pentenylene)-block-polyacetylene-block-poly(1-pentenylene). Chapter 4 describes the synthesis of narrow molecular weight distribution linear and three-branched star polyethylenes via ROMP of cyclopentene initiated by a well-defined tungsten based initiator at low temperature and subsequent capping with mono-, di- and trifunctional aldehydes. Chapter 5 presents an investigation on routes to a well-defined difunctional ROMP initiator which could potentially be used for the synthesis of polyethylene networks. Finally, Chapter 6 summarises the conclusions and outlines some suggestions for future work.