Synthesis Of Functional Block And Telechelic Polymers And Studies On The Mechanism Of Anionic Polymerization

This work is focused on the synthesis of styrenic telechelic polymer and block copolymer with pendent epoxy groups, the derivation of the kinetics of anionic bulk polymerization and the explanation of some experimental phenomena using quantum-chemical computation tools during anionic bulk polymerization of styrene.The end-epoxy functionalized telechelic polystyrene (PS-ep) has been synthesized by end-capping polystyryl precursors using diepoxides (1,4-butanediol diglycidyl ether (BDE) and neopentyl glycol diglycidyl ether (NGDE)). Polystyryl precursors were prepared at0℃using cyclohexane, a small amount of tetrahydrofuran (THF) and n-butyllithium (n-BuLi) as solvent, additives and initiator, respectively. The effects of termination reaction temperature, terminating reagents, normal/reverse addition and dilution of terminating reagent on the content of end-epoxy group in the products were investigated. Results from GPC show the existence of unexpected polystyrene dimers, the major byproduct from both oxirane rings opening reaction, whenever either normal or reverse additions were adopted. Under the same reaction conditions, the dimeric byproduct in the crude PS-ep product prepared using NGDE as terminating reagent is slightly lower compared to that using BDE. The lowest dimeric byproduct content in the product (c.a.20%) was obtained at0℃by reverse addition and molar ratio of NGDE and PSLi being3:1. The oxirane ring cannot be found from FT-IR or1H-NMR spectra owing to its very low concentration in the product. The concentration of epoxy group in PS-ep was determined by end-epoxy group titration (HCl-dioxane argentometry). The content of dimeric byproduct was reduced by increasing the reaction temperature and terminating reagent under normal addition; however, it is always higher than that from reverse addition. Under reverse addition, increasing the amount of terminating reagent resulted in lower dimeric byproduct content. It was also found that diluting the terminating reagents by adding THF, toluene and end-capping the polystyryl precursors by diphenylethylene (DPE) did not significantly reduce the dimer content.PS-PGMA was prepared using cyclohexane and n-BuLi as solvent and initiator, respectively, by adding GMA into polystryl precursors under ambient temperature. The effects of GMA dosage and GMA polymerization temperature have been investigated. It is shown that the oxirane ring opening reaction occurs when more GMA momoners were introduced (e.g. more than10GMA units per polystyrene chain). In particular, if the molar ratio of GMA to n-BuLi is over20, cross-linking takes place during GMA polymerization. The oxirane ring opening reaction of GMA was enhanced by increasing the GMA polymerization temperature. PS-nGMA block copolymers with narrow molecular weight distribution (≈1) have been synthesized in toluene by reducing the GMA polymerization temperature and end-capping the polystyryl precursors with DPE in the presence of LiCl. GPC, FT-IR,1H-NMR,13C-NMR, TLC and chemical titration results show that the oxirane were retained without ring opening reaction during GMA polymerization. It was also found that the molecular weight distribution broadened if the polymerization temperature of GMA was lower than-65℃and greater than-40℃. The reactivity of oxirane ring may increase with temperature and hence ring opening side reaction occurs. At very low temperature (≤-65℃), the solubility of PGMA segments becomes poor in toluene and poorer if larger PGMA segments are introduced. It was also found that the viscosity of PS-PGMA due to the poor solubility of GMA segments may be reduced by introducing THF in the system. PS-PGMA-PS was not successfully prepared using1,4-Bis(Bromomethyl)benzene (2-BMB) to link PS-PGMA anions. Only the mixture of PS-PGMA and PS-PGMA-PS was obtained.The kinetics of anionic bulk polymerization of styrene have been developed under the non-steady state assumption and assuming that all the living species (dimeric, hexameic and very large aggregates of polystyryllithium) can intiate the polymerization of styrene. The dependences of monomer conversion and various moments of molecular weight distribution on the time have been established in terms of model parameters (i.e. equilibrium contants between different aggregates and propogation rate constant of each aggregate). These established relationships were compared with the experimental data to estimate the model parameters. It has been shown that the propogation rate constants of dimeric and hexameric aggregates and equilibrium constants between them are almost of the same order of magnitude. In this context, the steady state assumption is not suitable for the anionic bulk polymerization of styrene. It is also interesting to find that the equilibrium constant between very large aggregates and hexameric aggregates is significantly greater (c.a.1000times) than that between dimeric and hexameric aggregates. The propogation rate constant of each aggregate is of the same order of magnitude with the apparent propogation rate constant in anionic solution polymerization of styrene reported in the literature. Among the propogation rate constants of the three aggregates, the propogation rate constant of very large aggregate is the greatest and that of the dimeric aggregate is the lowest.Quantum-chemical density functional theory (DFT) calculations at B3LYP/TZVP//B3LYP/SVP level of theory were performed to find the most stable aggregates of n-BuLi and PSLi living species during anionic bulk polymerization of styrene. It has been found that the hexamer of n-BuLi is the most stable aggregates in both gas and solution. In addition, dimeric PSLi is the most stable aggregates according to the DFT calculations using both HStLi and H(St)2Li as the models of PSLi. It was also found that the Li atom of monomeric PSLi chain-end is "wrapped" between the two benzene rings adjacent to the Li atom. To interpret the effect of P-ligands on the structure of n-BuLi and PSLi aggregates during the anionic polymerization of styrene, DFT calculations on the possible geometrical structures and relative stabilities of the living species and initiators in the presence of P-ligands were carried out. It was shown that the presence of P-ligands facilitated the deaggregation of the PSLi dimers and n-BuLi hexamers to form more stable mixed aggregates of P-ligands+monomeric n-BuLi and P-ligands+monomeric PSLi. It has also been demonstrated that the higher is the number of P-ligands coordinated with n-BuLi (or PSLi), the higher are their stabilities to bind n-BuLi (or PSLi) and their energies required to disaggregate n-BuLi (or PSLi) from the mixed aggregates.