Abstract : This thesis is concerned with the development of new methodologies that could affect the radical synthesis of small molecules and the controlled construction of polymers. A new synthetic approach allowing the synthesis of 1,2-dione derivatives, taking advantage of the radical chemistry of xanthates was established. These diones were then used in the preparation of new pyrazine structures, a family of compounds of potential biological interest. In the course of this work, a new reaction for the preparation of imidazoles and benzimidazoles was discovered. An other study involved the development of polymers on soluble support. These polymers are easily prepared by living radical polymerisation using the xanthate group technique (Madix). Their application overcomes the difficulties encountered with insoluble polymers used in solid supported synthesis which suffers from limitations due to the heterogeneous nature of the chemical transformation. One of the most important characteristic of such polymers is that they facilitate monitoring of the reaction process as there is no need to detach the substrate from the polymer. The last part of this thesis consisted in the identification, synthesis and screening of the properties of new modulating agents for controlled radical polymerisation. The homolytic dissociation of some dihydro-pyrazolone compounds was found capable of initiating a radical polymerisation and partially of controlling the polymer growth. The scope and the limitations of this system were examined in detail.