Study On Synthesis Of Polymers Containing Nitroxide Radicals And Their Properties

Ubiquity of free radicals in polymer systems has made electron paramagnetic resonance(EPR)spectroscopy a useful tool in polymer science.EPR is being applied to polymer systems in multifarious ways especially in the form of spin labeling.In spin labeling,a stable radical is attached to the polymer chain covalently and the rotational motion of the spin label is studied with EPR spectroscopy.The spin label can detect the change in the microenvironment including change in polarity or proticity etc.The rotation of the spin label can also be used to study the change in the segmental motion of polymer chains.ESR study of polymers spin labeled with nitroxide radicals(spin labels)is an extensive field of research to gather information of the microscopic behavior of the polymer chains.This microscopic behavior could be very helpful to modulate the macroscopic response of the polymers.Majority of the scientific studies have dealt with the macroscopic behavior of stimuli responsive polymers.However,microscopic behavior plays a significant role to determine the properties of polymers.Thus,study of relationship between macroscopic and molecular-level behavior of stimuli responsive polymers is important in order to capitalize on their technological promise.In this dissertation,a number of pH/temperature responsive polymers containing nitroxide radicals were synthesized and their segmental dynamics in solution were studied with CW X-band ESR spectroscopy.Further,nitroxide based polymers were synthesized by increasing concentration of the nitroxide radicals on the polymer chains to understand the effect of radical concentration on the segmental dynamics of polymer chains.1)Poly(acrylic acid),PAA,is a well-studied pH responsive polymer.Recent studies of PAA especially with fluorescent spectroscopy have shown that the polymer exhibits a molecular weight dependent chain conformation.In our work,molecular weight dependent chain conformation of PAA was studied with ESR spectroscopy.PAAs of different molecular weights were spin labeled with 4-amino-2,2,6,6-tetra methyl piperidine-1-oxyl(4-amino TEMPO).The spin labeled PAAs(SL-PAAs)were characterized by FT-IR,UV-Vis,TGA,DSC and EPR techniques.Polyelectrolyte complex(PEC)coacervates of SL-PAAs were prepared by employing poly(diallyldimethylammonium chloride)(PDADMAC)to determine the effect of molecular weight of PAAs on the extent of interactions in polyelectrolyte complexes.The results showed that by increasing the molecular weight of SL-PAAs,the spin-spin interactions between SLs increased as indicated by exchange broadened ESR spectra.The SL-PAA-4 with highest molecular weight and nitroxide radical concentration provided a clear evidence of conformational transition of polymer chains at different pH values.In the case of PECs,the ESR spectra showed that for the higher molecular weight SL-PAAs,the rotational mobility of the spin labels(SLs)was reduced,which could be related to the different complexation behavior of SL-PAAs with PDADMAC depending on the molar mass of SL-PAAs.2)Rotational motion of SLs depends on their microenvironment and is modulated by change in polarity,proticity,hydrophilicity or hydrophobicity etc.In our research,we further employed an amphiphilic polymer to study the effect of hydrophilic/hydrophobic balance of the polymers on the segmental motion of polymer chains.Poly(styrene-co-maleic acid)(PSMA)was spin labeled with 4-amino TEMPO radicals by increasing the concentration of radical moiety on the polymer chains.The PSMA and spin labeled-PSMAs(SL-PSMAs)were characterized by NMR,FT-IR,TEM,TGA,CV and ESR techniques.The SL-PSMAs were studied with EPR spectroscopy at different pH values.Inter-polymer complexes(IPCs)of one of the SL-PSMAs,SL-PSMA-2,were prepared by employing polyethylene glycols(PEGs)of varying molecular weights.The results showed that the increased hydrophobic interactions of SLs on the SL-PSMAs'chains reduced the rotational mobility of spin labels at low pH values,which showed a slow motion component in the ESR spectra of SL-PSMAs.Further,by increasing the molecular weight of PEGs in IPCs the complexation was increased,which also reduced the rotational motion of spin labels due to interpolymer hydrogen bonding causing a slow motion component in the ESR spectra.3)Ethylene glycol based polymers are temperature responsive polymers and their biological properties are well-known.Nevertheless,in-depth analysis at the microscopic level and the effect of their different formulations including molecular weight on their properties have not been discussed in detail.Thus,we synthesized poly(oligo ethylene glycol methacrylate-co-2,2,6,6-tetra methyl-4-piperidyl methacrylate)s,OEG-TMPMs.The synthesized OEG-TMPMs were characterized by NMR,GPC,DSC and TGA techniques.The TMPM moiety in OEG-TMPMs was oxidized to 2,2,6,6-tetramethypiperidine-l-oxyl methacrylate(TMP)by employing meta-chloroperbenzoic acid.These radical-containing OEG-TMPs were analyzed with FT-IR,CV and ESR spectroscopy.The results showed that the change in temperature of OEG-TMP-2 reduced hydration of OEG-TMP-2 chains at elevated temperatures due to shorter hydrophilic chains,which reduced rotational mobility of nitroxide radicals.The reduced mobility due to dehydration and inter-chain interactions affected the shape of ESR spectra showing slow-motion dynamics as compared to OEG-TMP-3 with longer hydrophilic side chains.4)Poly(2-diethyl amino ethyl methacrylate)(PDEAEMA)is a pH sensitive polymer,while a few recent studies have shown it to possess temperature responsiveness as well.In our research,poly(2-diethyl amino ethyl methacrylate-co-2,2,6,6-tetra methyl-4-piperidyl methacrylate)s,DEA-TMPMs,were synthesized.The TMPM moieties in DEA-TMPMs were subsequently oxidized with hydrogen peroxide to obtain poly(2-diethyl amino ethyl methacrylate-co-2,2,6,6-tetramethylpiperidine-l-oxyl methacrylate),DEA-TMPs,having nitroxide radicals on the polymer chains.DEA-TMPMs were characterized by NMR,GPC,DSC and TGA techniques,whereas DEA-TMPs were analyzed by FT-IR,CV and EPR techniques.The results showed that at low pH,the rotational mobility of nitroxide radicals was reduced as ESR spectra mostly consisted of slow-motion component.This slow-motion component in the ESR spectra remained unchanged below neutral pH.These results indicated a sort of interaction among nitroxide radicals and protonated amine groups of DEAEMA monomers at low pH.