Separation and Characterization of Polystyrene Synthesized by Atom Transfer Radical Polymerization

Abstract

Atom transfer radical polymerization (ATRP) is widely used method to obtain polymers with relatively narrow molecular weight distribution (MWD) and various chain-end functionalities (CEF). However, unavoidable termination reaction between free radicals results imperfect CEF while chains are growing in ATRP. Thus inevitable by-product, dead chain, is always remained in as-synthesized polymer via ATRP. High performance liquid chromatography (HPLC) and matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) are suitable methods to separate and characterize the polymer synthesized by ATRP. These analyses are important to obtain the high purity product and chase the origin of the impurities. In this dissertation, several polymers prepared by ATRP are separated and characterized by HPLC and MALDI-MS. In chapter 1, background knowledges of ATRP, HPLC and MALDI-MS are briefly introduced for separation and characterization of polymer synthesized by ATRP. In chapter 2, living and dead chains of polystyrene synthesized by ATRP were separated and characterized by HPLC, NMR, and MALDI-MS. The bromine end group in the living chains was first converted to an azide group and subsequently reacted with propargyl alcohol via copper catalyzed azide-alkyne cycloaddition (CuAAC) click reaction to introduce a polar end group. While the living chains bearing a polar end group are fully resolved from the unmodified dead chains by HPLC separation using a bare silica stationary phase, the elution peak of the living chains split into two. The peak split is attributed to the diastereomeric structure of the chain end by NMR and MALDI-MS analyses. Individual MWD of the living and dead chain are characterized by SEC and MALDI-MS. The MWD of the living chains is close to a Poisson distribution. In chapter 3, cyclic polystyrene synthesized by ATRP and CuAAC click reaction is characterized by HPLC and MALDI-MS. Cyclic polystyrenes are fully isolated from linear precursors in liquid chromatography at critical condition (LCCC) of linear polystyrene. In highly diluted synthesis condition, linear precursors and coupling impurities are hardly shown. Nevertheless, very little amount of cyclic/linear dimers can be separated and identified in interaction chromatography (IC) mode. In addition, self-click reaction of linear precursors at room temperature in powder state is revealed. In chapter 4, various topological polymers such as 4-arm star, figure-eight shaped and cage-shaped are synthesized by ATRP and characterized by HPLC, NMR, MALDI-MS. Although hydrodynamic sizes of the products with the same molar mass were somewhat different depending upon their shape, but the difference was not large enough to be fully resolved by size exclusion chromatography (SEC). On the contrary, IC can separate them due to dissimilar interactions of different topological polymers with the stationary phase. Pure products were fractionated by IC and byproducts were identified by MALDI-MS. Two-dimensional liquid chromatography (2D-LC) mapping of the products was obtained by coupling NPLC separation to SEC. In chapter 5, general protocol for separation of characterization of PS by ATRP is summarized. Future researches and perspectives for characterization of polymer by ATRP are also shown.