The ethylene-styrene copolymerization with two constrained geometry catalysts (CGCs), CGC1 and CGC2 was investigated by the density functional theory. Structures and energetics of reactants, pi complexes, transition states, and products during insertion of ethylene and/or styrene monomers into the catalytic reactive site of the CGCs are analyzed using the software DMOL3. The general gradient approximation correction was applied to the total energy calculation after being calculated at the local density approximation level. The relativistic effective core potential is used for titanium while all other electrons are treated explicitly. Structures at stationary points on the energy profile were generated by minimizing the energy with respect to all the degrees of freedom except for the reaction coordinate which is defined as the distance between the C-alpha carbon and the nearest carbon of inserting monomer. The CGC2 with an electron-rich ligand shows a weaker coordination with monomers, resulting in longer Ti-Cp, Ti-Ph, and Ti-C-gamma distances and higher energy states. The electron-rich styrene monomer forms a stronger pi complex than ethylene, whereas the insertion of styrene through the complex into the product suffers a higher energy barrier than the insertion of ethylene. All of the transition states show a typical four-membered ring structure and are stabilized by an alpha- or phenyl-agostic interaction. The gamma- or phenyl-agostic interaction stabilizes the product. From analysis of the relative energy, it is found that both CGCs yield E-S copolymers with sequences of ethylene blocks separated by styrene. More interestingly, CGC1 shows higher activity for all insertion processes than CGC2, while CGC2 is more effective than CGC1 in producing E-S copolymers with higher composition of styrene. (C) 2003 American Institute of Physics.