The size of the cooperatively rearranging region (CRR) at T-g has been measured for syndiotactic poly(methyl methaerylate) confined between the layers of an organophilic montmorillonite (OMMT) according to the method developed by Donth. The results show that the size of the CRR is constrained by the layers and that it starts to decrease for interlamella spacings up to 6 times larger than the size of the CRR at T-g. This shrinkage of the CRR with the confinement distance is discussed in term of cooperativity in confined systems according to the nature of the interfacial interactions. Contrasting with T-g depression reported in previous studies for s-PMAU thin films, for which stronger interactions were at stake, a positive shift of T-g is recorded for the lowest distances. This behavior is ascribed to conformational rearrangements effects since weak interactions between the s-PMMA and the organophilic modified clay are expected.