The entrapment of linear polyisobutene (PIB) in semi-IPN architecture is shown to be as efficient as it is in cross-linkable telechelic PIB based full IPN architectures as far as the suppression of cold flow is concerned. Indeed, homogeneous linear PIB/cross-linked polycyclohexylmethacrylate (PCHMA) semi-IPNs containing from 20 to 70 wt% PIB and synthesized without solvent show no cold flow and higher mechanical properties than those of linear PIB or 50 wt% PIB containing blend. In addition, the particular barrier properties toward gas and water are preserved. Those properties arise from the phase co-continuity morphology of the semi-IPN materials which moreover compares with that of corresponding IPNs. A systematic study of the synthesis conditions (nature of the initiator, temperature, cross-linking density) showed that the reacting mixture viscosity is an important parameter that controls the phase separation degree in the final material. (C) 2010 Elsevier Ltd. All rights reserved.