Synthesis of polymer brushes by surface-initiated controlled radical polymerization in the presence of free initiators. A Monte Carlo investigation

Simultaneous controlled radical polymerization from free and surface-grafted initiators is studied by means of computer simulations. We model ``truly living'' reactions by neglecting chain termination and chain transfer. We study the competition between bulk- (i=b) and surface-initiated (i=s) polymers by monitoring the rates of polymerization (d$<$N$>_{i}$/dt), the average molecular weight ($<$N$>_{i})$, and the polydispersity index (PDI$_{i})$ while varying the initiator grafting density ($\sigma )$, the fraction of surface based polymers ($\eta )$, and the initial number of free monomers (I$_{o})$. We find that d$<$N$>_{B}$/dt$>$d$<$N$>_{S}$/dt and that d$<$N$>_{S}$/dt decreases with increasing $\sigma $. The difference in polymerization rates between the bulk and surface polymerizations results in polymer populations with different $<$N$>$; this has implications for the experimental determination of $\sigma $ and $<$N$>_{S}$. Although $\eta $ affects $<$N$>_{S}$ and $<$N$>_{B}$, it does not influence the PDI. At a specific $<$N$>_{i}$, the PDI$_{i}$ has a value independent of $\eta $. In addition, PDI$_{S}>$PDI$_{B}$; this difference increases with increasing $\sigma $. We therefore conclude that polymerization conditions that yield bulk polymers with low PDI do not guarantee the same PDI for the surface-initiated polymers.