The motion correlation along the backbone and between the side-chain and the backbone for the two PMMA chains of opposite tacticity, are investigated using molecular dynamics simulations. According to the coupling model (CM), the index of fragility, which can be considered as a measure of the cooperativity, can be related to the Kohlrausch-Williams-Watts (KWW) exponent. This exponent actually stems from the computation of the correlation time of different bonds inside the polymer chains. From suitably selected bonds, its behavior with temperature can thus unveil cooperativity along the backbone and between the side-chain and the backbone. Compared with experimental data and present theories, these simulation data show that at a relative temperature above the glass transition temperature, T-g, both configurations exhibit the same behavior for the backbone, but a different coupling is clearly observed between the side-chain and the backbone. Such a behavior tends to explain the difference in T(g)s between the two configurations in PMMA. (C) 2003 Elsevier Ltd. All rights reserved.