The in situ synthesis of ethylene-co-norbornene copolymers/multi-walled carbon nanotubes (MWNTs) nanocomposites was achieved by rare-earth half-sandwich scandium precursor [Sc(eta(5)-C5Me4SiMe3)(eta(1)-CH2SiMe3)(2)(THF)] (1) activated by [Ph3C][B(C6F5)(4)], through a non-PFT (Polymerization Filling Technique) approach. MWNTs nanocomposites with low aluminum residue were obtained with excellent yields even though small amounts of triisobutylaluminium were needed as scavenger to prevent catalyst poisoning by MWNT impurities. MWNT bundles were disaggregated and highly coated with Poly(ethylene-co-norbornene) [P(E-co-N)] as revealed by transmission electron microscopy. Interestingly, P(E-co-N) copolymers showed T-g over 130 degrees C as well as norbornene content over 50 mol %; both values were higher than those obtained by the cationic active species in 1/[Ph3C][B(C6F5)(4)]. A series of copolymerization reactions by 1/[Ph3C][B(C6F5)(4)]/(AlBu3)-Bu-i without MWNTs produced copolymers with the same unexpected features. The NMR analysis revealed the presence of rac-ENNE and rac-ENNNE sequences. Thus, (AlBu3)-Bu-i changed the stereoirregular alternating copolymer microstructure produced by 1/[Ph3C][B(C6F5)(4)]. We conclude that (AlBu3)-Bu-i is not only a scavenger for CNT impurities, but it reacts with the THF ligand to give coordinatively unsaturated active species. Finally, P(E-co-N)/MWNT masterbatches were mixed with commercial TOPAS to produce cyclic olefin copolymer nanocomposites with excellent dispersion of filler. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5709-5719, 2009