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...

Highly crosslinked monodisperse poly(divinylbenzene) microspheres were produced by precipitation polymerization with acetonitrile as solvent. The radical initiators AIBN, BPO, and ADVN were used. The process does not require stabilizers of any type, and produces monodisperse particles with diameters between 2 and 5 μm, depending on the conditions. These microspheres do not swell or dissolve in any common solvent, and have clean, stabilizer‐free surfaces. The particle formation and growth mechanism is proposed to resemble that of dispersion polymerization, except that the particles are stabilized against coagulation by their rigid, crosslinked surfaces rather than by added stabilizers. Spherical particles were formed only at effective crosslinker/monomer or divinyl/monovinyl ratios larger than 1 : 2. © 1993 John Wiley & Sons, Inc.

The residual surface vinyl groups in poly(divinylbenzene) microspheres prepared by precipitation polymerization in acetonitrile were converted to hexyl groups by treatment with n-butyllithium and to ethyl groups by catalytic hydrogenation in the presence of Wilkinson's catalyst. These modified particles and unmodified particles were used as seeds in separate precipitation polymerizations of divinylbenzene in acetonitrile, under identical conditions. Only the unmodified seeds were able to capture the oligomers formed and grow without secondary initiation. Both the butylated and the hydrogenated samples showed extensive secondary initiation instead of seed particle growth. These results demonstrate that precipitation polymerization of divinylbenzene in near-ϑ solvents is an entropic precipitation, involving radical reactions between the macromonomer particles and newly formed oligomers. These results further imply that the growing particles are autostabilized by the transient solvent-swollen gel layer on their surfaces, formed by a recently captured oligomer.

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.

ABSTRACT:The precipitation polymerization of commercial divinylbenzene in acetonitrile containing up to 40 vol. % toluene or other cosolvents is shown to produce novel porous monodisperse poly(divinylbenzene) microspheres. These microspheres have diameters between 4 and 7 mm, total pore volumes of up to 0.52 cm 3 /g, and surface areas of up to 800 m 2 /g. As no surfactant nor stabilizer was used in the preparation of these particles, their surfaces are free of any such residues. The particles were slurry-packed into stainless steel columns for size exclusion chromatography evaluation, and the results show an exclusion limit at molecular weights of 500 g/mol.

Monodisperse cross-linked core-shell polymer microspheres having diameters in the micrometer-size range were prepared by semibatch and by two-step precipitation polymerization in the absence of any stabilizer. Commercial divinylbenzenes, containing 55% or 80% divinylbenzene, were used for the core, and several functional monomers including chloromethylstyrene, monovinyl, or divinyl methacrylic monomers were incorporated into the shell. Acetonitrile and toluene/acetonitrile mixtures were used as reaction media. Depending on the type of monomer and reaction medium used, the shell could be made nonporous or porous. The particle size distribution, surface morphology, internal texture, and porosity of the resulting core-shell particles were studied. The chromatographic properties of selected particles were evaluated.

Grafting of polystyrene from narrow disperse polymer particles by surface-initiated atom transfer radical polymerization was investigated. Poly(DVB80) particles prepared by precipitation polymerization were used as starting particles. Their residual surface vinyl groups were hydrochlorinated to form chloroethylbenzene initiating sites for subsequent ATRP of styrene using CuBr/2bipy as catalyst system. Polystyrene was found grafted not only from the particle surfaces but also from within a thin shell layer, leading to particles size increases from 2.96 to 3.07 µm. The surface layer of polystyrene improved colloidal stability and facilitated formation of colloidal arrays. Block copolymers of poly(styreneb-4-methylstyrene) were grown from the particles, and the living nature of surface-initiated ATRP is discussed.