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Our goal is to investigate how substrate geometry and solution conditions affect the effectiveness of controlled radical polymerization (CRP) from substrates (“grafting from”) in producing polymers with controlled molecular weight and polydispersity index (PDI).  

In order to achieve this objective, Monte Carlo computer simulations were performed in which the degree of confinement, the kinetic reaction probabilities, and the solution conditions were controlled precisely.  Benchmark experiments of polymerization processes, in which terminations have been eliminated, reveal that variations in geometrical confinement can have profound effects on the final molecular weight and PDIs of the grown polymers.  Specifically, polymers grown in solution exhibit the smallest degree of confinement and possess the lowest PDIs.  In contrast, polymerizations initiated from surfaces with low initiator densities possess increased PDI relative to those grown in solution, the PDI of surface-grown polymers increases appreciably with increasing density of the initiator.  The increase in PDI is explained by the depletion of monomers in the near-surface region and by the inability of the monomer to access shorter chains due to the presence of longer neighboring chains (both parameters have been monitored during the simulations).  The effect of increased confinement on PDI is further enhanced when the solvent quality in the simulation is changed from good to poor.  Under poor solvent conditions the polymer chains tend to collapse into small globules, aggregate and adsorb to the surface in order to maximize polymer/polymer contacts and minimize their exposure to the solvent.  The tendency of the chain to form globules hinders further the accessibility of free monomer to the shorter growing chain ends.

The combined effects of geometrical confinement, solution conditions and a finite rate of termination were also explored.  In our simulation scheme, only a fraction of the polymers in the simulation box is allowed to propagate or terminate at a given time.  If this fraction is set to be small, a high probability of termination does not result in a large increase of the final PDI.  This is due to the improbability that two active chain ends will come close enough to one another in order to terminate.  As the fraction of active chain increases, however, the probability of termination plays a more important role in determining the final distribution of molecular weights.  This effect gets magnified when the chains are confined to the surface because the active growing ends have a higher probability of coming within reaction distance and therefore terminate more frequently than the chains in the bulk.  We have also determined that the combination of surface confinement, poor solvent conditions, and high termination rates all possess detrimental effects on the controlled nature of the polymerization reaction and, as a result, yield polymers with the highest PDI value observed in this study.

Despite these newfound limitations of CRP when applied to polymerizations in confined environments we have determined that it is possible, by judicious selections of the polymerization conditions, to achieve controlled reactions even in confined geometries.  First, the density of initiators on the surface should be maintained to the minimum required for a specific application.  This could be achieved, for example, by dilution of the initiator molecules on the surface with non-reactive molecules.  Second, the kinetics of the polymerization should be adjusted in order to keep the fraction of active chains as low as practically possible.  Reducing the fraction of active chains will, in fact, decrease PDI but it might also increase the time for polymerization to prohibitive levels.  Finally, experimenters should avoid performing polymerizations in poor solvent conditions due to the adverse effects of chain collapse on the controlled nature of the reaction.