Room temperature atom transfer radical polymerizations of N-isopropylacrylamide (NIPAM) carried out in 2-propanol (i-PrOH) and tert-butyl alcohol (t-BuOH) resulted in PNIPAMs with polydispersities between 1.1 and 1.2 and degrees of polymerization of up to 300. Methyl 2-chloropropionate (MCP), copper(I) chloride, and tris[2-(dimethylamino)ethyl]amine (Me6TREN) were used as initiator, catalyst, and ligand in a 1:1:1 ratio. Conversions were as high as 91 and 79%, respectively, without the need for excess catalyst as was required in previous studies. Aqueous solutions of these narrow-disperse PNIPAMs showed a strong decrease of the phase transition temperature with increasing molecular weight, as measured by turbidimetry and differential scanning calorimetry. In low molecular weight samples, containing significant oligomeric fractions, the slightly hydrophobic methyl propionate end group becomes significant and further decreases the onset temperature of the phase transition.

Four series of narrow-disperse poly(N-isopropylacrylamide) (PNIPAM) with well-controlled molecular weights and with end groups of varying hydrophobicity were synthesized by room temperature atom transfer radical polymerization in 2-propanol using the corresponding chloropropionate and chloropropionamide initiators. The thermal phase transitions of aqueous solutions of these PNIPAMs were studied by turbidimetry and high-sensitivity differential scanning calorimetry (HS-DSC) and showed an inverse molecular weight (MW) dependence of their cloud points. The magnitude of the MW dependence decreases when using more hydrophobic end groups. The choice of end group further affected the shape of the cloud point curves and the enthalpy of the phase transition. Above the cloud point, narrow-disperse PNIPAM sedimented more rapidly than polydisperse PNIPAM produced by conventional free radical polymerization, especially at concentrations above 1%. Thus, multiple HS-DSC scans of PNIPAM prepared by ATRP typically gave repeatable results only at lower concentrations. IntroductionAqueous solutions of poly(N-isopropylacrylamide) (PNIPAM) exhibit a reversible thermal phase separation above a critical temperature, known as the lower critical solution temperature (LCST). 1,2 On the molecular level, this involves a change from solvated random coils below the LCST to tightly packed globular particles above the LCST. 3-5 This thermoresponsiveness has led to applications in bioengineering 6-9 and nanotechnology 10-13 and promises exciting future applications in the area of biosensors and membranes. Much effort has also been invested in better understanding the phase transition behavior and the parameters affecting the phase transition temperature. Most often, this involved studying the cloud point of dilute aqueous PNIPAM solutions, rather than the actual LCST, i.e., the minimum of the two-phase curve in the PNIPAM-water phase diagram.The molecular weight (MW) dependence of the cloud point of such polymers has been an active yet controversial topic. The cloud points of PNIPAM and related thermoresponsive polymers have been reported to be inversely dependent, 14-20 directly dependent, 21,22 or independent 5,23,24 on the molecular weight. However, most of these studies involved conventionally prepared, polydisperse polymers, which may have precluded precise examination of MW effects. In addition, different initiators, terminators, or chain-transfer agents led to different polymer end groups, which can in turn affect the cloud points. 22,24-28 Hydrophobic end groups decrease cloud points while hydrophilic end groups tend to increase them, with the magnitude of the effect depending on the nature of the end group. Hydrophobic groups act by increasing the degree of ordering of solvating water while hydrophilic ones tend to decrease the ordering of solvating water. These effects are believed to be greater for hydrophobic/hydrophilic groups located at chain ends rather than midchain. 25 End group effects are most pronounced for low MW polymers bu...

Nanoparticles bearing a strongly bound polymer coating were formed by the thermal decomposition of iron pentacarbonyl in the presence of ammonia and polymeric dispersants. The dispersants consist of polyisobutylene, polyethylene, or polystyrene chains functionalized with tetraethylenepentamine, a short polyethyleneimine chain. Polystyrene-based dispersants were prepared with both graft and block copolymer architectures. Inorganic-organic core-shell nanoparticles were formed with all three types of dispersants. In addition, more complex particles were observed in the case of the polystyrene-based dispersants in 1-methylnaphthalene. The core material was identified as metallic iron, while the particle shells are formed from the polymeric dispersant which binds to the core. High-resolution TEM revealed evidence for crystallization within the polymer shell, possibly facilitated by chain alignment upon binding. The nanocomposites display room-temperature magnetic behavior ranging from superparamagnetic to ferromagnetic. The saturation magnetization and coercivity were found to depend on the diameter of the iron core.

2-Hydroxyethyl methacrylate (HEMA)-based (co)polymers showed soluble to insoluble (S–I) thermoresponses, as measured by turbidimetry, when heated in aqueous solutions of appropriate ionic strength and/or pH. Surprisingly, many of the polymers showed a second insoluble to soluble (I–S) thermoresponse at higher temperatures, probably as a result of breaking of polymer–polymer H-bonding that allowed redissolution of the polymer chains. The thermoresponse of the charged copolymers was very sensitive to the polymer concentration, pH, and ionic strength likely due to the role of charge screening in the chain collapse and aggregation necessary to observe cloud points. Urea, a commonly used “H-bond breaker”, raised the cloud point (and decreased the clearing point for systems that showed I–S transitions); however, subsequent cooling or heating runs in the presence of urea often showed dramatically different thermoresponses as a result of urea hydrolysis, leading to a pH change and/or polymer hydrolysis at high temperature. Marked hysteresis or changes from run to run were seen for some systems because of polymer precipitation or slow rehydration of phase-separated material or, in some cases, slow hydrolysis of HEMA units.