Search

Although stimuli-responsive polymers have appeared as promising materials for drug administration, the effectiveness of these materials on various bioinspired stimuli is still open for dispute. In this work, the stimuli-responsive behaviors of radical-containing nonamphiphilic copolymers were investigated using electron paramagnetic resonance (EPR) spectroscopy while altering the pH and reductant concentration. The concentration of radicals in the aqueous solution had a considerable impact on self-assembly. The nitroxide radicals (hydrophobic) were converted to hydroxylamine (hydrophilic) when ascorbic acid (Asc) was added as a reductant, and the assemblies were switched to dissolution with a rate control behavior. The behavior of morphological change was observed in the acidic pH due to the disproportionation reaction of nitroxide radicals forming cationic species. The acquired results give new insights into the colloidal nanoparticles of nonamphiphilic copolymers and encourage the development of comparable responsive nanoparticles with intriguing drug delivery applications.

Study of the segmental mobility of polymer chains is important when the polymer is used as drag reduction agents of crude oil. Electron spin resonance (ESR) spectroscopy can provide important information about segmental dynamics of polymer chains, which is related to their microenvironment. In this article, we 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 nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR) spectroscopy, Cyclic voltammetry (CV) and ESR techniques. Inter-polymer complexes (IPCs) of SL-PSMA-2 were prepared by employing polyethylene glycols (PEGs) of varying molecular weights. The results showed that the increased hydrophobic interactions of nitroxide radicals on the SL-PSMAs’ chains reduced the rotational mobility of spin labels and the random coil-to-globular transition of polymer chains occurred at higher pH value for SL-PSMAs, 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.

The structure of polyolefin has an important influence on its performance and application. Ethylene/1-hexene copolymerization is one of the important ways to control the structure of the polyolefin. However, research on the ethylene/1-hexene copolymerization catalyzed by nickel complexes with different steric ligands remains to be refined. Here, three α-diimine nickel catalysts are used to study the ligand effect on catalytic performance in the ethylene/1-hexene copolymerization. Reaction activity, molecular weight, phase-transition temperature and branching density of the resultant copolymer are measured to evaluate the catalytic performance. The results indicate that the steric ligands could exert great effect on the copolymerization. As for the chemical valence of Ni species, detailed EPR demonstrate that the presence of excess co-catalyst can reduce Ni(II) to the lower valence and affect the catalytic performance.