We use Monte Carlo simulation implementing the bond fluctuation model formalism in the canonical (NVT) ensemble to study living polymerization initiated concurrently in bulk and on flat substrates. Our results reveal that the molecular weights and molecular weight distributions of both classes of polymers depend on the grafting density of the surface-bound polymers (sigma) and the fraction of polymers on the surface (eta) relative to that in bulk. In general, polymer grafts on the surface possess lower molecular weight and higher polydispersity index compared to their bulk counterparts. The difference between the molecular weight of the two populations of polymers decreases with decreasing sigma and increasing eta. Our work provides evidence that the common practice of using the molecular weight of bulk-initiated polymers in estimating the grafting density of polymeric anchors on flat substrates is not generally valid.