The interfacial shear strength of polymer thin film coating on calcium-silicate-hydrate (C-S-H) or cement paste significantly depends on the time-point, from addition of water, when the polymer is coated. This is because the physico-chemical properties of C-S-H surface evolve over time as hydration progresses. The instant of coating thus determines the interfacial performance of a thin film coating in an organic-inorganic material system. In this work, we provide molecular insights into the interactions of poly(methyl methacrylate) (PMMA) functional groups with the pore solution calcium and bound substrate calcium present in hydrating C-S-H. The experimental observation reveals that PMMA thin film coated on second day of hydration exhibits larger interfacial shear strength than when coated on the 28th day. The possible reasons for this observations are: a) chemical crosslinking of two oxygen atoms from two different PMMA polymer chains by pore solution calcium ions and b) the physical interaction between double bond oxygen (O1) present in -COO functional group of PMMA with pore solution calcium. In this work, physical interaction between double bond oxygen (O1) present in -COO functional group of PMMA with pore solution calcium and substrate calcium in C-S-H is addressed.