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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, bulk- and surface-initiated controlled radical polymerizations (CRP) in implicit solvents are simulated using a stochastic Monte Carlo algorithm implementing the bond-fluctuation scheme.  We study the effect of system properties such as bulk vs. surface initiation, surface density of initiators and solvent quality on the molecular weight and molecular weight distributions of the resulting polymers.  We implement variations in the solvent conditions by means of truncated inter- and intra-molecular potentials between bonded polymer beads and we achieve variations in the surface density of initiators by changing the linear dimensions of the lattice while keeping the total volume of the lattice constant.

In order to determine the effect of geometry and steric hindrance on the polymerization we monitor the sizes of the growing polymers, the number of reactive species located near the reactive polymer chain-ends and, for surface-initiated systems, the concentration profiles of monomers, polymers and chain-ends as a function of distance from the substrate.  Our results indicate that confinement during polymerization hinders the ability of chain-ends and monomers to approach each other, which has detrimental consequences on the capacity of CRP to yield nearly monodisperse polymers. In addition, surface-initiated polymerizations do not yield polymer brushes with equal tethering densities as those of the precursor initiators because the growing radicals may not all be simultaneously activated and they do not possess equal access to the monomers due to chain crowding. Confinement of polymers to impenetrable substrates and decreases in the solvent quality therefore result in decreases in the rate of polymerization and increases in PDI.

Complementing our previous results, one of our most recent findings relates to the efficiency of surface bound initiator molecules. In an ideal system, one would want all initiator molecules to activate simultaneously so that the propagation reaction commences instantaneously for all growing polymers. In reality, not all initiators activate simultaneously (or activated at all). There are multiple reasons for this behavior. Some of them are associated with the chemical nature of the initiator and the environmental trigger that activates it (typical examples include azo-based initiators for free radical polymerizations) or with steric hindrance of the initiator molecules in confined systems. Since the properties, particularly the PDI, of the polymeric tethers depend crucially on the initiator efficacy, it is important to monitor the fraction of active initiators that participate in the reaction. Although this is a challenge experimentally, computer simulations can handle monitoring this parameter very easily. For nearly all values of grafting density of initiators (σ), complete (i.e., 100%) initiation efficiency is achieved when a monomer conversion of 20% is reached. The sole exceptions are systems with very high grafting density, σ ³ 0.44 in good solvents and σ ³ 0.25 for poor solvents. In these instances, the initiator efficiency plateaus indicating that as we approach the experimentally relevant initiator grafting densities we can expect less of the initiators to take part in the polymerization reaction. In poor solvent conditions and at the highest σ, the initiator efficiency reaches a plateau value of 0.8, which might have implications for polymerization of methyl methacrylate in water/alcohol mixtures if large concentration gradients develop in the experimental system.