Synthesis and Characterization of Cyclic polymer by using HPLC and its Intrinsic viscosity property

Abstract

Cyclic polymers exhibit unique topological effects on physical properties such as chain dimensions, viscoelastic properties and chain dynamics. Various properties of cyclic polymers have been predicted by theoretical studies and computer simulations and examined by experimental studies. Typically, cyclic polymers are prepared by end-linking telechelic linear precursors, but contamination with linear polymers due to incomplete ring-closure reactions or side reactions to the targeted cyclic polymer is usually unavoidable. The purity of the cyclic polymers strongly affects their physical properties, but the effect of the purity of the polymers on their physical properties is not understood well due to the difficulty in obtaining highly pure cyclic polymers. Since direct synthesis of pure cyclic polymers is difficult, to obtain cyclic polymers with high purity, post-synthesis purification is necessary. To fractionate cyclic polymers from a ring-closure reaction mixture, two methods have been employed most often: fractional precipitation and Size Exclusion Chromatography (SEC) fractionation. None of the methods has been fully successful in separating the cyclic polymers from the side products. The SEC retention times of a cyclic polymer and its linear precursor are not sufficiently different to provide complete resolution of the elution peaks that also suffers from serious band broadening of SEC. Ultracentrifugation sedimentation can be more successful than SEC in separating cyclic polymers from the linear precursor, but it is impractical even for a lab scale fractionation. In this thesis research, cyclic polystyrenes were prepared by end-linking telechelic linear precursors with wide range of molecular weights. All as-synthesized cyclic polymers were characterized and fractionated by HPLC (High Performance Liquid Chromatography) methods. Using the cyclic polymers of high purity, we compared the HPLC retention behaviors of cyclic vs. linear polymer and characterized the intrinsic viscosity of cyclic polymer in good solvent. In chapter 1, various methods to synthesize cyclic polymers are described and methods to characterize cyclic polymers are briefly reviewed. In chapter 2, the synthesis method and basic principle of the characterization methods used in this thesis are described in detail. In chapter 3, HPLC elution behaviors of cyclic and linear polymer are investigated by using a series of polystyrene precursors with Mw = 16.4 kg/mol, 33.1 kg/mol, 64.6 kg/mol, and 92.2 kg/mol. Their retention behavior in reversed phase liquid chromatography (RPLC) at the critical absorption point (CAP) was investigated using a C18-bonded silica stationary phase and a mixed solvent of CH2Cl2 and CH3CN. The CAP of cyclic PSs (C-PS) occurred at a higher temperature (TCAP) than linear PSs (L-PS). At TCAP of linear PSs, cyclic polymers elute later than their counterpart linear PS in the increasing order of MW like the elution sequence in interaction chromatography (IC) mode. At the TCAP of cyclic PSs, linear polymers elute earlier than cyclic polymers in the decreasing order as in SEC mode. The experiment results concur well with computer simulation results using the Random Walk (RW) model. In addition, the normal phase LC (NPLC) retention behavior at the CAP of each polymer was investigated using a bare silica stationary phase and a mixed solvent of THF and Hexane. While C-PS eluted later than L-PSs at the CAP of each polymer as in RPLC, the TCAP of C-PS was lower than that of L-PS in contrast to the RPLC result. Therefore the L-PSs eluted in the increasing order of Mw as in IC mode at the CAP of C-PS and C-PSs eluted in SEC mode at the CAP of L-PS. The inversion of TCAP of C-PS and L-PS in NPLC is not predicted in the computer simulation result and it is seemingly due to the contribution of the end-funtionality that is not reflected in the computer simulation. In chapter 4, each part of cyclic PS and linear PS (L-PSs) containing dimeric and high Mw PSs were fractionated using liquid chromatography at the critical condition (LCCC). Fractionated C-PSs with purity > 99% was used to measure intrinsic viscosities of C-PSs and L-PSs (from 16k to 370k) in tetrahydrofuran (THF), a good solvent for PS. Exponent of the Mark-Houwink-Sakurada (MHS) relationship was 0.67 for C-PSs and 0.70 for the L-PS. Therefore, the ratio of the intrinsic viscosity (g’-factor) of C-PS to L-PS in THF is a function of Mw (0.622 ≥ [η]C/[η]L ≥ 0.572). In chapter 5, we synthesized telechelic ABA triblock copolymer of polystyrene and polyisoprene as A and B block, respectively. The synthesis of AB cyclic copolymer is accomplished by coupling reaction of chain ends of the telechelic ABA triblock copolymer. HPLC was employed for the separation and characterization of as-synthesized cyclic AB diblock copolymer. And, we compared the morphology of linear triblock copolymer and cyclic diblock copolymer by T-SAXS experiments