Metal-free isotactic-specific radical polymerization of N-isopropylacrylamide with pyridine N-oxide derivatives: The effect of methyl substituents of pyridine N-oxide on the isotactic specificity and the proposed mechanism for the isotactic-specific radical polymerization

The radical polymerizations of N-isopropylacrylamide (NIPAAm) in chloroform at low temperatures in the presence of pyridine N-oxide (PNO) derivatives were investigated. It was found that the methylation at meta-positions of PNO improved the isotactic specificity induced by PNO, whereas the methylation at ortho-positions prevented the induction of the isotactic specificity. NMR analysis revealed that NIPAAm and PNO derivatives formed predominantly 2:1 complex through a hydrogen-bonding interaction. Furthermore, the induction of the isotactic specificity was attributed to the conformationally limited propagating radicals. Based on these findings, the mechanism of the isotactic-specific radical polymerization was discussed.     

Metal-free isotactic-specific radical polymerization of N-alkylacrylamides with 3,5-dimethylpyridine N-oxide: The effect of the N-substituent and solvent on the isotactic specificity

Radical polymerization of N-methylacrylamide (NMAAm), N-n-propylacrylamide, N-isopropylacrylamide (NIPAAm) and N-benzylacrylamide was investigated in CHCl₃, CH₂Cl₂ and CH₃CN, in the presence of 3,5-dimethylpyridine N-oxide (35DMPNO) to examine the effects of the N-substituent and the solvent on the isotactic specificity induced by 35DMPNO. With addition of 35DMPNO to radical polymerization of N-alkylacrylamides in CHCl₃, isotactic specificity was significantly induced in NIPAAm polymerization but only slightly induced in NMAAm polymerization. Furthermore, mixed solvents of CH₃CN and halomethanes such as CHCl₃ and CH₂Cl₂ enhanced the ability of 35DMPNO to induce isotactic specificity, and poly(NIPAAm) with 74% meso dyad was obtained.     

Direct synthesis of syndiotactic-rich poly(N-isopropylacrylamide) via radical polymerization of hydrogen-bond-complexed monomer

Radical polymerization of N-isopropylacrylamide (NIPAAm) in toluene was investigated in the presence of hexamethylphosphoramide (HMPA). We succeeded in directly preparing syndiotactic-rich poly(NIPAAm), the syndiotacticity of which (r=70%) is the highest among those of radically-prepared poly(NIPAAm)s so far reported, by lowering polymerization temperature to −60 °C in the presence of a two-fold amount of HMPA. The NMR analysis revealed that the induced syndiotactic-specificity was ascribed to 1:1 complex formation between NIPAAm and HMPA. Furthermore, thermodynamic analysis described that the induced syndiotactic-specificity was enthalpically achieved.     

Effect of a combination of hexamethylphosphoramide and alkyl alcohol on the stereospecificity of radical polymerization of N-isopropylacrylamide

Radical polymerization of N-isopropylacrylamide (NIPAAm) was investigated at low temperatures in the presence of both hexamethylphosphoramide (HMPA) and alkyl alcohols. Although HMPA and alkyl alcohols separately induced syndiotactic specificity in NIPAAm polymerization in toluene at low temperatures, a combination of HMPA and less bulky alkyl alcohols, such as methanol and ethanol, was found to induce isotactic specificity at −80 °C. NMR analysis of mixtures of NIPAAm, ethanol and HMPA suggested the formation of a 1:1:1 complex through O-H•••O=C and N-H•••O=P hydrogen bonding. It is believed that the steric effect of HMPA enhanced by cooperative hydrogen bonding was responsible for the combined effect of HMPA and alkyl alcohols in inducing isotactic specificity.     

Effect of polymerization conditions on the syndiotactic-specificity in radical polymerization of N-isopropylacrylamide and fractionation of the obtained polymer according to the stereoregularity

Radical polymerization of N-isopropylacrylamide (NIPAAm) was examined in the presence of hexamethylphosphoramide (HMPA). The addition of an excess amount of HMPA induced syndiotactic-specificity that gradually enhanced as the feed monomer was consumed. The syndiotacticity of the obtained poly(NIPAAm)s was improved by increasing the [HMPA]₀/[NIPAAm]₀ ratio to five and prolonging the polymerization time to 96 h (racemo=72%). It was also revealed that more stereoregulated poly(NIPAAm) could be fractionated by reprecipitating the resulting polymers from hexane-THF mixture. This result suggested that more stereoregulated poly(NIPAAm) showed a lower solubility than less stereoregulated poly(NIPAAm)s. Furthermore, unusual hysteresis was observed in transmittance analysis of an aqueous solution of the fractionated syndiotactic poly(NIPAAm).     

Heterotactic poly(N-isopropylacrylamide) prepared via radical polymerization in the presence of fluorinated alcohols

Radical polymerization of N-isopropylacrylamide in toluene at −40 °C in the presence of fourfold amounts of fluorinated alcohols was investigated. The ¹³C NMR analysis of the obtained polymers suggested that the addition of fluorinated alcohols induced heterotactic specificity in radical polymerization of NIPAAm, although syndiotactic poly(NIPAAm)s were obtained by adding alkyl alcohols as we have previously reported. To the best of our knowledge, this is the first synthesis of heterotactic poly(NIPAAm).     

Hydrogen-bond-assisted stereocontrol in the radical polymerization of N-isopropylacrylamide with bidentate Lewis base

Radical polymerization of N-isopropylacrylamide (NIPAAm) in toluene was investigated in the presence of bidentate Lewis base such as diphosphonates. Isotacticity of the obtained poly(NIPAAm)s slightly increased at −80 °C, whereas syndiotactic-rich poly(NIPAAm)s were obtained at −40-0 °C. This result corresponded to the results observed in the presence of primary alkyl phosphates. NMR analysis revealed that NIPAAm monomer and tetraisopropyl methylenebisphosphonate formed mono-binding hydrogen-bond-assisted complex at 0 °C, but a chelate complex at −80 °C. Thus, it was concluded that the stereospecificity in NIPAAm polymerization strongly depended on the complexation mode of the added bidentate Lewis base.     

Preparation of hydrogel nanoparticles by atom transfer radical polymerization of N-isopropylacrylamide in aqueous media using PEG macro-initiator

PEG-b-PNIPAM block copolymers are synthesized by the atom transfer radical polymerization of NIPAM using PEG macro-initiator. When the polymerization temperature is 25 °C, the block copolymer is soluble in water, whereas the block copolymer is phase-separated to form micelles during polymerization as the polymerization temperature is raised to 50 °C, the temperature above the LCST of PEG-b-PNIPAM. To prepare stable hydrogel nanoparticles in water at room temperature, a small amount of N,N′-ethylenebisacrylamide is added as a cross-linker to the reaction system, where the size of nanoparticles is controlled by the composition of mixed solvent.     

Poly(N-isopropylacrylamide) hydrogels with improved shrinking kinetics by RAFT polymerization

Functional poly(N-isopropylacrylamide) (PNIPAM) hydrogels were prepared by reversible addition fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) in the presence of N,N-methylenebisacylamide (BIS) as a cross-linker and 4-cyanopentanoic acid dithiobenzoate as chain transfer reagent (CTA). The swelling behaviors were investigated and the hydrogels by RAFT polymerization (RAFT gels) showed accelerated shrinking kinetics and higher swelling ratio comparing with conventional hydrogel (CG). It could be attributed to the presence of dangling chains mainly caused by CTA, which could retard the crosslinking reaction rate greatly. Another CTA, 3-(trithiocarbonyl) propanoic acid, was adopted to further investigate the effect of CTA. It showed the similar effect except the different accelerated degree to the shrinking kinetics. Furthermore, the living character of the RAFT process was used to polymerize a new batch of monomer (NIPAM) from functional RAFT gels to introduce grafted structure. The PNIPAM-g-PNIPAM hydrogels indicted further accelerated shrinking kinetics than functional backbone hydrogels.     

pH-Controlled association of PEG-containing terpolymers of N-isopropylacrylamide and 1-vinylimidazole

Temperature- and pH-controlled association of terpolymers of N-isopropylacrylamide (NIPA) with 1-vinylimidazole (VI) and polyethylene glycol (PEG) has been investigated by light scattering and atomic force microscopy (AFM) in situ. The polymers contained 0-15 mol% VI and 0-2 mol% PEG. The phase transition temperatures (T_p) have been in the range of 32-45 °C and exhibited significant dependence on the pH of solution in the pH range between 5 and 8. The T_p of the polymers increased with increasing VI content and with decreasing pH, confirming major effect of VI ionization status on T_p. The presence of PEG grafts in the polymer structure had augmenting effect on the magnitude of pH-responsiveness and on the pH-independent colloidal stability of the polymer particles formed above T_p. Incorporation of VI into the polymer structure had similar, but pH-dependent effect on colloidal stabilization of the polymer particles. The size of the particles formed after the phase transition is driven by the association of the collapsed NIPA segments in the globule conformation and it decreased with decreasing pH. The phase transition temperature of the polymers could be adjusted to increase from temperatures below, to temperatures above body temperature upon decreasing pH from 7 to 6, suggesting that such polymers could provide a material platform for a variety of biomedical applications. AFM analysis in situ showed a fully reversible formation of particles in the solutions of the polymers above their T_p.     

Photosensitive copolymer of N-isopropylacrylamide and methacryloyl derivative of spyrobenzopyran

The copolymer of N-isopropylacrylamide (NIPAM) and methacryloyl derivative of spirobenzopyran (MSBP) with a molecular weight of 21?000 g/mol and the average molar MSBP content of 1.9% was prepared by free radical polymerization. The copolymer displayed its phase transition in water in the temperature range of 30-50 °C. UV irradiation of its aqueous solution caused photoinduced transformation of MSBP units into their coloured merocyanine forms, while the cloud point of the irradiated copolymer shifted by ca. 10 °C to lower temperatures. During a long-term exposure to daylight (20 days) the copolymer gradually elapsed to its colourless spiropyran form, the process being ca. 100-times slower than that for monomeric MSBP. Due to the slow reverse isomerization of its merocyanine form and low solubility in water at room temperature the UV irradiated copolymer could be quantitatively separated from aqueous solution by centrifugation.     

Swelling and mechanical behavior of poly(N-isopropylacrylamide)/Na-montmorillonite layered silicates composite gels

Clay-polymer hydrogel composites have been synthesized based on poly(N-isopropylacrylamide) (PNIPAM) gels containing 0.25-4 wt% of the expandable smectic clay Na-montmorillonite layered silicates (Na-MLS). The morphology of the composite gels has been studied using a polarized optical microscope. The size of Na-MLS aggregates increases with Na-MLS concentration. The swelling ratio of the Na-MLS/PNIPAM composite in water is increased at the low Na-MLS concentration but decreases as the concentration increases. Correspondingly, the shear modulus of the gel is found to exhibit a distinct minimum against clay concentration. For Na-MLS concentrations ranging from 2.0 to 3.2 wt%, the composite gels have larger swelling ratio and stronger mechanical strength than those for a pure PNIPAM. The presence of Na-MLS does not affect the value of the lower critical solution temperature (LCST) of the PNIPAM. However, the gel volume change at the LCST is first increased and then decreased upon the increase of the Na-MLS. No pH induced phase transition is observed for the Na-MLS/PNIPAM composites. The experimental results can be explained by considering that Na-MLS is physically entrapped inside rather than chemically bonded into the gel.     

Super porous organic-inorganic poly(N-isopropylacrylamide)-based hydrogel with a very fast temperature response

Porous organic-inorganic (O-I) hydrogels showing a very fast temperature response, including very fast reswelling were prepared: only 6 s are needed for 72% deswelling (gel collapse) as well as for 72% reswelling. Both deswelling and reswelling are practically complete in 14 s. The gels were prepared from N-isopropylacrylamide (NIPA), N,N′-methylenebisacrylamide (BAA) and tetramethoxysilane (TMOS) by simultaneous radical polymerization and hydrolytic polycondensation of TMOS. The syntheses were carried out at temperatures below the lower critical solution temperature (LCST) of poly(NIPA) in two steps: during the first stage the temperature was held at T = +15 °C and during the second the temperature was lowered below the freezing point of the reaction mixture, T = −18 °C. The ice crystals, which grew during the second stage, served as the pore-forming agent. The best samples were obtained if the second stage was started shortly before the gel point of the reaction mixture. The introduction of the inorganic phase (silica) is necessary for the ability of fast reswelling and also results in a strong improvement of the hydrogels' mechanical properties, while the maximum swelling degree remains nearly unaffected.     

On the conformation of the cellulose solvent N-methylmorpholine-N-oxide (NMMO) in solution

The N-oxide group of the cellulose solvent N-methylmorpholine-N-oxide showed a strong preference for the axial position compared with N-methyl as determined by NMR experiments and computational studies. In solvents with negligible solvent-solute interaction, about 95% of the NMMO molecules obtained a typical chair conformation with an axial N-O (1) while 5% had an equatorial N-O (2) at room temperature (25 °C). Other conformations (boat and twist) are energetically largely disfavored. N-Benzylmorpholine-N-oxide was prepared as reference compound possessing 100% axial N-O. Aprotic solvents of increasing polarity slightly shifted the conformation equilibrium towards the more polar conformer 2. The effect of protic solvents, capable of forming H-bonds, was more pronounced, with water increasing the percentage of 2 to 25% of the total population. Addition of sugar model compounds reversed this effect, so that formation of 2 was suppressed and exclusively 1 was found, indicating a strong interaction between the latter conformer and the carbohydrate.     

Synthesis and characterization of pH- and temperature-sensitive hydrogel of N-isopropylacrylamide/cyclodextrin based copolymer

A reactive β-Cyclodextrin (β-CD) based monomer carrying vinyl carboxylic acid functional groups was synthesized via reaction of β-CD with maleic anhydride (MAH) in N,N-dimethylformamide (DMF) at 80 °C. By copolymerization of the monomer with N-isopropylacrylamide (NIPA), a novel hydrogel, poly(NIPA-co-MAH-β-CD) with pH and temperature sensitivities plus molecular inclusion function, was obtained using free radical polymerization in aqueous solution. The hydrogel's composition was determined by element analysis and infrared spectroscopy. Equilibrium swelling ratio (ESR) of hydrogels was tested under different environment of pH, temperature and ionic strength. The results indicated that ESR of hydrogels presents marked variations following the change of experimental conditions used.     

Dynamic swelling of hydrogels based on random terpolymers of N-isopropylacrylamide, methacrylic acid and poly(ethylene glycol) macromonomer

A series of pH-responsive hydrogels based on N-isopropylacrylamide (N-iPAAm), methacrylic acid (MAA) and poly(ethylene glycol) monomethyl ether monomethacrylate macromonomer (PEGMEMA), P(N-iPAAm-co-MAA-co-PEGMEMA) random terpolymers, were synthesized and their swelling behaviour studied as a function of both monomer composition and previous swelling treatment. The swelling kinetic curves were followed using gravimetric, photographic and magnetic resonance imaging (MRI) techniques, which provide spatial and temporal resolution. The swelling behaviour was non-Fickian at pH 7, being this fact more relevant when the samples were pre-soaked in pH 2 solution. Low pH promotes hydrogen bond arrangements that disrupt at pH 7, where sigmoidal swelling curves were observed. The sigmoidal shape of the curves increases as well as the swelling time with increasing N-iPAAm/PEGMEMA ratio. This indicates that hydrogen bond arrangements between MAA and N-iPAAm are stronger that those formed by MAA and PEGMEMA. The influence of the polymer composition on the hydrogen bond arrangements was also studied from the swelling kinetics curves at different pH media, observing that the swelling rate, the swelling curve shape and the whole amount of water absorbed were clearly dependent on this parameter.     

Synthesis of poly(N-isopropylacrylamide) copolymer containing anhydride and imide comonomers - A theoretical study on reversal of LCST

The stimuli sensitive copolymer NIPAM-co-MI was prepared by copolymerizing NIPAM (N-isopropylacrylamide) with varying concentrations of maleimide (MI). The copolymer showed the same ratio of the monomeric components as that of the initial monomer feed ratio, with the two components arranged in the chain in a purely random sequence. Interestingly, the lower critical solution temperature (LCST) of NIPAM-co-MI was found to decrease with increase in MI loading in the copolymer. This behavior was in drastic contrast to the LCST behavior of a similar copolymer NIPAM-co-MA of NIPAM and maleic anhydride (MA) where the LCST showed an increase with increase in the MA concentration. A theoretical interpretation of the contrasting LCST behavior of both NIPAM-co-MI and NIPAM-co-MA was obtained by quantum mechanical (QM) modeling on small structural units of the polymers as well as molecular dynamic (MD) simulations at LCST and above the LCST on 50-unit oligomer model of the polymers. The QM models showed that the MI based polymer is more inclined towards bend structure, higher hydration, and higher intramolecular hydrogen bond formation between its monomer units when compared to those of the MA based polymer. The results of the large scale MD simulation was in complete support of the QM results as it showed the formation of a more folded and highly hydrated NIPAM-co-MI than NIPAM-co-MA.     

Thermosensitive poly(N-isopropylacrylamide) nanocapsules with controlled permeability

In this paper, novel thermosensitive poly(N-isopropylacrylamide) (PNIPAM) nanocapsules with temperature-tunable diameter and permeability are reported. Firstly, the core-shell composite microparticles were synthesized by precipitation polymerization with isothiocyanate fluorescein (FITC) entrapped SiO₂ as core and cross-linked PNIPAM as shell. Then, the SiO₂ core was etched by hydrofluoric acid at certain condition and the pre-trapped FITC molecules remained within the inner cavity. The FITC release profile and TEM studies clearly indicate that the release behavior of FITC could be controlled effectively by the external temperature. Above the LCST of PNIPAM (32 °C), the dehydrated PNIPAM shell inhibited the release of FITC from the internal cavity while below its LCST, the fluorophore could permeate the swollen shell easily.     

Temperature dependence of density of polymer gels: Effects of ionizable groups in copoly(N-isopropylacrylamide/acrylic acid or sodium acrylate)-water systems

Effects of ionizable groups in hydrogels of copolymer networks on the volumetric contraction-expansion process were investigated. Polymer networks used were: copoly[N-isopropylacrylamide (NIPA)(1 − x)/acrylic acid (HAc) or sodium acrylate (NaAc)(x)] with mole fraction of minor component (x) assuming 0.0114 and 0.0457. From the temperature (T) dependence of total volume of gels, densities of the polymer and solvent (water) components, and stoichiometry, we evaluated (1) the volume of gels occupied by a single mean polymeric residue and associated water molecules (expressed in units of nm³), mean v_sp(gel), and (2) number of water molecules per single mean polymeric residue, mean N_s(gel), from near 273 K to 323 K. These quantities (1) and (2) listed above showed how acid and salt forms affect differently on volumetric changes of gels over 50 K. We developed an approach to evaluate volumetric changes of gels solely caused by a single polymeric residue of a minor component (x < 0.05) plus associated water by applying thermodynamic first-order perturbation theory. They are specific v_sp(gel)(T) for a single HAc or NaAc polymeric residue plus associated water and the corresponding specific N_s(gel)(T). Specific v_sp(gel)(HAc or NaAc)(T) and the corresponding specific N_s(gel(T)) revealed specific characteristics in thermal behavior near their respective transition temperatures from the swollen to shrunken states. We found these thermal changes shown at the nano-scale match very well with specific changes in the molality(T) of both ionizable groups. In fact, these are directly triggered by varying contents of water in gels. Based on the understanding of dissociative equilibrium attained by ionizable groups, we successfully replaced Na⁺ in hydrogels of copoly[NIPA(1 − x)/NaAc(x)] (x = 0.0457) by hydrogen ions. Absence of Na⁺ in treated hydrogels was experimentally verified by ²³Na NMR and Na atomic absorption flame photometry. Discontinuity in the volumetric contraction-expansion process from the swollen to shrunken states and vice versa was not observed in contradiction to the previous reports [Hirotsu S, Hirokawa Y, Tanaka T. J Chem Phys 1987;87:1392-5. Matsuo SE, Tanaka T. J Chem Phys 1988;89:1695-703.] obtained by the conventional swelling experiments.     

Synthesis and characterization of thermoresponsive N-isopropylacrylamide/methacrylated pullulan hydrogels

Thermosensitive hydrogels were prepared by free radical polymerization starting from a methacrylated pullulan derivative (acting as the cross-linker) and using N-isopropylacrylamide (NIPAAM) as the monomer. Several hydrogels were obtained by changing the monomer to cross-linker ratio. A significant thermosensitivity was observed only when the molar amount of NIPAAM incorporated in the network was at least eight times higher that of methacrylate groups on pullulan. The hydrogel with high amount of NIPAAM deswells more than 80% after the T-jump. The lower critical solution temperature of thermosensitive hydrogels decreases with increasing amount of NIPAAM. The mechanical properties of the hydrogels are strongly affected by the percentage of incorporated NIPAAM and by the temperature.