Separation and characterization of living and dead chains of PS synthesized by atom transfer radical polymerization

MasterAtom transfer radical polymerization (ATRP) is widely used in these days to make polymers with narrow molecular weight distribution (MWD) and various chain-end functionality. Until now the MWD of ATRP synthesized polymer has been characterized by size exclusion chromatography (SEC), therefore, the MWD of ATRP polymers cannot be said to be addressed precisely yet. For analysis of a polymer with narrow MWD, SEC has a limit in resolution due to its serious, inherent band broadening. In addition, SEC separates polymers according to their hydrodynamic volume and cannot distinguish the dead chains from the living chains. The dead chains, spontaneously terminated during ATRP, do not have a halogen end group and cannot grow anymore while living chains contain the terminal halogen and it can grow further. In this thesis, the living chains and dead chains of ATRP synthesized polystyrene (PS) are separated to characterize precise MWD of each by high performance liquid chromatography (HPLC) and matrix assisted laser desorption ionization mass spectrometry (MALDI-MS). In the chapter 1, the focus is to understand how the MWD obtained by SEC/standard calibration and the MWD obtained by SEC/light scattering (LS) detection are affected by the SEC band broadening. The standard calibration method to obtain the MWD is widely used, however, this method measures the MWD broader than real value due to the SEC band broadening. When the MWD is obtained by LS detection, the SEC band broadening can be corrected in part by directly measuring MW of polymers in the effluent and MWD closer to the real MWD can be obtained. Therefore, a polymer prepared by anionic polymerization (AP) shows a relatively broader MWD when measured by the standard calibration method while a very narrow MWD is observed by LS detection. In this chapter, the MWD of ATRP PS is studied by the two MWD determination methods in SEC. Both methods yielded MWD of ATRP PS broader than AP PS. When ATRP PS is fractionated by temperature gradient interaction chromatography (TGIC), the MWD of the fractions gets narrower in the SEC analysis and the change of MWD is observable despite the band broadening in SEC. Therefore, SEC can be used to measure the MWD change of ATRP PS after narrowing down the MWD, for example, by removing the dead chains. In the chapter 2, to explore the MWD of living chains in ATRP, the focus is on the separation of the living chains from dead chains with HPLC. When a polymer is synthesized by ATRP, living chains with Br end and dead chains without Br end are formed. To separate the living chains from dead chains with HPLC, it is necessary to substitute the terminal Br in the living chain to a more strongly interacting functional group with the HPLC stationary phase. Three different methods to substitute the Br end to a hydroxyl group are tried. The first one is the substitution of Br with ethanolamine and the second one is the substitution to a hydroxyl group by Ag catalysis in acetone solvent. The third one is the substitution of Br to an azide group and to a hydroxyl group by a click reaction with propagyl alcohol. When the samples after the modification of the terminal Br to a hydroxyl group are separated by HPLC, dead chains without hydroxyl group interacts with the bare silica stationary phase weakly, therefore elutes out earlier than the living chains with a hydroxyl group that interacts with the stationary phase more strongly, therefore elutes out later. The living chains with a hydroxyl group which elutes out later always elute as two split peaks and the polymers in the two split peaks have the same molecular weight as measured by MALDI-TOF MS. To characterize the two split peaks further, they were fractionated by HPLC and analyzed by nuclear magnetic resonance (NMR). The two split HPLC peaks have different NMR peak shapes at the similar resonance frequency region and the split HPLC peak can be attributed to the diastereomeric structure of the chain end