The present invention relates to a process for preparing polymers by means of ring-opening metathetic polymerization, consisting of the following steps: a) providing a reaction mixture consisting of a cycloalkene or a cycloalkene mixture, b) adding a ruthenium compound of the general formula RuX1X2L1aL2b (I) where a=0 or 1, b=1 or 2, such that a+b=2; X1, X2=independently selected anionic ligands; L1=uncharged π-binding ligand; L2=N-heterocyclic carbene of the formula IIa, IIb or IIc; c) adding a chain transfer agent in a molar ratio in the range from 1:5 to 1:1000 based on the cycloalkene or the sum total of the cycloalkenes in the cycloalkene mixture, d) converting at a temperature within a range from 0° C. to 250° C.