A computational approach the free radical polymerization of acrylates and methacrylates

Abstract

Free radical polymerization (FRP) is one of the most favorable chemical reactions employed in both industry- and laboratory-scale chemical productions, because it can convert a wide variety of vinyl monomers into high molecular weight polymeric materials without extensive purification of commercially available monomers and solvents. On the other hand, the poor control of some of the key elements of macromolecular structures such as molecular weight (MW), polydispersity, end functionality, chain architecture, and composition are some important limitations for FRP. If the behavior of monomers during the FRP is well identified and understood, these limitations can be adjusted to moderate levels. The mechanism of the free radical polymerization process basically consists of four different types of reaction families involving free radicals: (1) initiation, (2) propagation, (3) chain transfer reactions, (4) termination reactions. Most of the reactions studied in this thesis are propagation reactions. However in some cases, also the termination reactions and chain transfer reactions have been modeled as they also influence the overall rate of polymerization. In the first part of this work the reactivity of a series of acrylates and alkyl - hydroxymethacrylates are modeled in order to understand the effect of the pendant group size, the polarity of a pendant group, and the nature of the pendant group (linear vs cyclic) on their polymerizability. Generally the rate constants for propagation kp mimic the qualitative polymerization trend of the monomers modeled and can be used with confidence in predicting the polymerizability behavior of acrylates. In the second part of this thesis, attention was focused on the polymerization behavior of alpha substituted acrylates. It is well known that the introduction of a heteroatom in the -position of acrylates can influence the reaction kinetics. In addition to investigation of this effect the chain length dependency of the propagation rate constant was examined by modeling monomeric, dimeric, trimeric, and tetrameric radical additions to a monomer. In the third part of this thesis, we focused on the influence of the solvent on the tacticity of free radical polymerization of methyl methacrylate by considering the propagation rate constants for the syndiotactic and isotactic free radical polymerization of MMA in vacuum, in methanol (CH3OH) and in 1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-ol ((CF3)3COH). Solvent effect has been explored by using explicit solvent molecules and a polarizable continuum model (IEF-PCM) with a dielectric constant specific to the solvent. In a forth part of this thesis the effect of solvent on the propagation rate coefficient of acrylic acid (AA) and methacrylic acid (MAA) has been elucidated. Both for MAA and AA it was experimentally found that the propagation rate constant of non-ionized monomers increases by more than one order of magnitude in going from the bulk to a highly dilute system. The reactivity of these two monomers have been explained in the bulk medium by using quantum chemical tools. In addition to the main part of the thesis concerning acrylates and methacrylates, some research was performed in collaboration with other researchers from the Bogaziçi University on chain transfer to poly ethylene. These results are taken up in part five of the thesis. This work brings a new perspective to the modeling of free radical polymerization reactions and provides a deeper insight into the factors that affect the polymerizability of various monomers. However, the scope of exploring the FRP reactions is very large and highly useful examples are not limited to the ones discussed in this study. Lots of DFT methodologies have been tested against the experimental results in order to assess the level of theory for modeling the FRP. In all cases, the MPWB1K/6-311+G(3df,2p)//B3LYP/6- 31+G(d) methodology is found to reproduce the experimental trend the best as a cost effective method. Future work will undoubtedly uncover many other important aspects of the FRP reactions.