Synthesis of thermo‐responsive poly(N‐vinylcaprolactam)‐containing block copolymers by cobalt‐mediated radical polymerization

Thermo‐responsive block copolymers based on poly(N‐vinylcaprolactam) (PNVCL) have been prepared by cobalt‐mediated radical polymerization (CMRP) for the first time. The homopolymerization of NVCL was controlled by bis(acetylacetonato)cobalt(II) and a molecular weight as high as 46,000 g/mol could be reached with a low polydispersity. The polymerization of NVCL was also initiated from a poly(vinyl acetate)‐Co(acac)₂ (PVAc‐Co(acac)₂) macroinitiator to yield well‐defined PVAc‐b‐PNVCL block copolymers with a low polydispersity (M_w/M_n = 1.1) up to high molecular weights (M_n = 87,000 g/mol), which constitutes a significant improvement over other techniques. The amphiphilic PVAc‐b‐PNVCL copolymers were hydrolyzed into unprecedented double hydrophilic poly(vinyl alcohol)‐b‐PNVCL (PVOH‐b‐PNVCL) copolymers and their temperature‐dependent solution behavior was studied by turbidimetry and dynamic light scattering. Finally, the so‐called cobalt‐mediated radical coupling (CMRC) reaction was implemented to PVAc‐b‐PNVCL‐Co(acac)₂ precursors to yield novel PVAc‐b‐PNVCL‐b‐PVAc symmetrical triblock copolymers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Unexpected reactions associated with the xanthate‐mediated polymerization of N‐vinylpyrrolidone

The monomer N‐vinylpyrrolidone (NVP) undergoes side reactions in the presence of R group functional xanthates and impurities. The fate of the monomer NVP and a selection of six O‐ethyl xanthates during xanthate‐mediated polymerization were studied via NMR spectroscopy. A high number of by‐products were identified. Significant side reactions affecting NVP include the formation of an unsaturated dimer and hydration products in bulk or in solution in C₆D₆. In addition, the xanthate adjacent to a NVP unit was found to undergo elimination at moderate temperature (60–70 °C), resulting in unsaturated species and the formation of new xanthate species. The presence of the chlorinated compound α‐chlorophenyl acetic acid, ethyl ester, a precursor in the synthesis of the xanthate S‐(2‐ethyl phenylacetate) O‐ethyl xanthate, resulted in a dramatic increase in the rate of side reactions such as unsaturated dimer formation and a high ratio of unsaturated chain ends. The conditions for the occurrence of such side reactions are discussed in this article, with relevance to increasing the control over the polymerization kinetics, endgroup functionality, and control over the molar mass distribution. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6575–6593, 2008     

Fast and effective copper(0)‐mediated simultaneous chain‐ and step‐growth radical polymerization at ambient temperature

Vinyl‐conjugated monomer (methyl acrylate, MA) and allyl 2‐bromopropanoate (ABP)‐possessing unconjugated C?C and active C? Br bonds were polymerized via the Cu(0)‐mediated simultaneous chain‐ and step‐growth radical polymerization at ambient temperature using Cu(0) as catalyst, N,N,N′,N″,N″‐pentamethyldiethylenetriamine as ligand and dimethyl sulfoxide as solvent. The conversion was reached higher than 98% within 20 h. The obtained polymers showed block structure consisting of polyester and vinyl polymer moieties. The Cu(0)‐catalyzed simultaneous chain‐ and step‐growth radical polymerization mechanism was demonstrated by NMR, matrix‐assisted laser desorption ionization time‐of‐flight, and GPC analyses. Furthermore, the obtained copolymers of MA and ABP were further modified with poly(N‐isopropylamide) through radical thiol‐ene “click” chemistry from the terminal double bond. The thermoresponsive behavior of this block copolymer was investigated. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3907–3916     

Chain transfer to solvent in the radical polymerization of N‐isopropylacrylamide

Chain transfer to solvent has been investigated in the conventional radical polymerization and nitroxide‐mediated radical polymerization (NMP) of N‐isopropylacrylamide (NIPAM) in N,N‐dimethylformamide (DMF) at 120 °C. The extent of chain transfer to DMF can significantly impact the maximum attainable molecular weight in both systems. Based on a theoretical treatment, it has been shown that the same value of chain transfer to solvent constant, C_tr,S, in DMF at 120 °C (within experimental error) can account for experimental molecular weight data for both conventional radical polymerization and NMP under conditions where chain transfer to solvent is a significant end‐forming event. In NMP (and other controlled/living radical polymerization systems), chain transfer to solvent is manifested as the number‐average molecular weight (M_n) going through a maximum value with increasing monomer conversion. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis of poly(2‐ethylhexyl acrylate)/clay nanocomposite by in situ living radical polymerization

This investigation reports the preparation of tailor‐made poly(2‐ethylhexyl acrylate) (PEHA) prepared via in situ living radical polymerization in the presence of layered silicates and characterization of this polymer/clay nanocomposite. Being a low T_g (?65 °C) material, PEHA has very good film formation property for which it is used in paints, adhesives, and coating applications. 2‐Ethylhexyl acrylate was polymerized at 90 °C using CuBr and Cu(0) as catalyst in combination with N,N,N′,N″,N″‐pentamethyl diethylenetriamine (PMDETA) as ligand. A tremendous enhancement in reaction rate and polymerization data was achieved when acetone was added as additive to increase the efficiency of the catalyst system. PEHA/clay nanocomposite was prepared at 90 °C using CuBr as catalyst in combination with PMDETA as ligand. Different types of clay with same loading were also used to study the effect on reaction rate. The molecular weight (M_n) and polydispersity index of the prepared nanocomposites were characterized by size exclusion chromatography. The active end group of the polymer chain was analyzed by ¹H NMR analysis and by chain extension experiment. Polymer/clay interaction was studied by Fourier Transform Infrared spectrometry and wide‐angle X‐ray diffraction analyses. Distribution of clay in the polymer matrix was studied by the transmission electron microscopy. Thermogravimetric analysis showed that thermal stability of PEHA/clay nanocomposite increases on addition of nanoclay. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Single electron transfer‐living radical polymerization of methyl methacrylate catalyzed by ytterbium powder

Zerovalent ytterbium (Yb) powder is firstly used as a catalyst in single electron transfer‐living radical polymerization of methyl methacrylate initiated by carbon tetrachloride in N, N‐dimethylformamide (DMF) and dimethyl sulfoxide, respectively. Polymerization proceeds in a “living”/controlled way as evidenced by kinetic studies and chain extension results, producing well‐defined polymers with controlled degree of polymerization and narrow molecular weight distribution. The apparent activation energy of polymerization in DMF is accounted to be 36.2 kJ/mol, and the energy of equilibrium state is calculated to be 13.9 kJ/mol. An increase in the concentration of Yb(0) yields a higher monomer conversion. It is observed that polymerization rate experiments a rapid increase in the presence of more polar solvent water, and increasing in the content of H₂O results in an increase in the apparent rate constant of polymerization, and a decrease in the molecular weight distribution. The reaction rate and molecular weight increase along with the decrease of DMF content. The effect of Yb(0) powder content, different ligands and concentration of initiator on the polymerization is also investigated. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Spontaneous Cu(I)Br–PMDETA‐mediated polymerization of isobornyl methacrylate in heterogeneous aqueous medium at ambient temperature

Isobornyl methacrylate (IBMA), a bulky hydrophobic methacrylate, undergoes very fast polymerization, in bulk, with Cu(I)Br/N,N,N′,N″,N″‐pentamethyldiethylenetriamine (PMDETA)/ethyl‐2‐bromoisobutyrate system, at ambient temperature. IBMA also undergoes a spontaneous initiator‐free polymerization, at ambient temperature, with Cu(I)Br/PMDETA catalytic system in dimethyl sulfoxide–water mixtures. The rate of the polymerization is seen to increase with the water content up to 80 mol % of water. A possible intervention of air in initiation is proposed. The active Cu(0) formed by the disproportionation of Cu(I) species in aqueous medium probably plays a vital role for a possible air‐initiation of IBMA via single electron transfer‐living radical polymerization (SET‐LRP) mechanism. A high tolerance level to water under SET‐LRP conditions is demonstrated. The poly(IBMA) samples obtained exhibit low molecular weight distributions (1.1–1.3). Similar behavior was not observed with other common methacrylates such as methyl methacrylate, t‐butyl methacrylate, cyclohexyl methacrylate, and benzyl methacrylate. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Controlled/living heterogeneous radical polymerization in supercritical carbon dioxide

Supercritical carbon dioxide (scCO₂) is an inexpensive and environmentally friendly medium for radical polymerizations. ScCO₂ is suited for heterogeneous controlled/living radical polymerizations (CLRPs), since the monomer, initiator, and control reagents (nitroxide, etc.) are soluble, but the polymer formed is insoluble beyond a critical degree of polymerization (J_crit). The precipitated polymer can continue growing in (only) the particle phase giving living polymer of controlled well‐defined microstructure. The addition of a colloidal stabilizer gives a dispersion polymerization with well‐defined colloidal particles being formed. In recent years, nitroxide‐mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer (RAFT) polymerization have all been conducted as heterogeneous polymerizations in scCO₂. This Highlight reviews this recent body of work, and describes the unique characteristics of scCO₂ that allows composite particle formation of unique morphology to be achieved. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3711–3728, 2009     

Synthesis of low‐polydispersity poly(N‐ethylmethylacrylamide) by controlled radical polymerizations and their thermal phase transition behavior

Controlled radical polymerizations of N‐ethylmethylacrylamide (EMA) by atom transfer radical polymerization and reversible addition‐fragmentation chain transfer processes were investigated in detail for the first time, employing complementary characterization techniques including gel permeation chromatography, ¹H NMR spectroscopy, and matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry. In both cases, relatively good control of the polymerization of EMA was achieved, as revealed by the linear evolution of molecular weights with monomer conversions and the low polydispersity of poly(N‐ethylmethylacrylamide) (PEMA). The thermal phase transitions of well‐defined PEMA homopolymers with polydispersities less than 1.2 and degrees of polymerization up to 320 in aqueous solution were determined by temperature‐dependent turbidity measurements. The obtained cloud points (CPs) vary in the range of 58–68 °C, exhibiting inverse molecular weight and polymer concentration dependences. Moreover, the presence of a carboxyl group instead of an alkyl one at the PEMA chain end can elevate its CP by ～3–4 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 60–69, 2008     

Macroinitiator halide effects in galactoglucomannan‐mediated single electron transfer‐living radical polymerization

Chloro (Cl)‐ and bromo (Br)‐functionalized macroinitiators were successfully prepared from the softwood hemicellulose O‐acetylated galactoglucomannan (AcGGM) and then explored and evaluated with respect to their ability and efficiency of initiating single electron transfer‐living radical polymerization (SET‐LRP). Both halogenated species effectively initiate SET‐LRP of an acrylate and a methacrylate monomer, respectively, yielding brushlike AcGGM graft copolymers, where the molecular weights are accurately controlled via the monomer:macroinitiator ratio and polymerization time over a broad range: from oligomeric to ultrahigh. The nature of the halogen does not influence the kinetics of polymerization strongly, however, for acrylate graft polymerization, AcGGM‐Cl gives a somewhat higher rate constant of propagation, while methacrylate grafting proceeds slightly faster when the initiating species is AcGGM‐Br. For both monomers, the macroinitiator efficiency is superior in the case of AcGGM‐Br. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis of bio‐based poly(N‐phenylitaconimide) by atom transfer radical polymerization

Poly(N‐phenylitaconimide) (polyPhII) was prepared using initiators for continuous activator regeneration atom transfer radical polymerization of PhII using FeBr₃ complexes as catalysts. Conversion reached 69% in 24 h, yielding polyPhII with a number average molecular weight M_n = 11,900 and a molecular weight distribution M_w/M_n = 1.52. Copolymerizations of PhII with styrene at various molar ratios were performed providing a range of polyPhII‐copolySt polymers. When the copolymerization was carried out with higher [St]₀ > [PhII]₀ ratio, a one‐pot synthesis of poly(St‐alt‐PhII)‐b‐polySt was achieved. The thermal properties of the obtained copolymers were studied by differential scanning calorimetry. PolyPhII prepared by ATRP showed high glass transition temperature (T_g) of 216 °C and the poly(St‐alt‐PhII)‐b‐polySt exhibited two T_gs, at 162 and 104 °C, corresponding to a poly(St‐alt‐PhII) and polySt segments, respectively. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 822–827     

Ligand‐free Cu(0)‐mediated controlled radical polymerization of methyl methacrylate at ambient temperature

For the first time, ligand‐free Cu(0)‐mediated polymerization of methyl methacrylate (MMA) was realized by the selection of ethyl‐2‐bromo‐2‐phenylacetate as initiator at ambient temperature. The polymerization can reach up to 90% conversion within 5 h with dimethyl sulfoxide (DMSO) as solvent, while keeping manners of the controlled radical polymerization. Extensive investigation of this system revealed that for a well‐controlled Cu(0)‐mediated polymerization of MMA, the initiator should be selected with the structure as alkyl 2‐bromo‐2‐phenylacetate, and the solvent should be DMSO or N,N‐dimethylformamide. The selectivity for solvents indicated a typical single‐electron transfer‐living radical polymerization process. Scanning for other monomers indicated that under equal conditions, the polymerizations of other alkyl (meth)acrylates were uncontrollable. Based on these results, plausible reasons were discussed. The ligand‐free Cu(0)‐mediated polymerization showed its superiority with economical components and needless removal of Cu species from the resultant products. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Iron halide mediated atom transfer radical polymerization of methyl methacrylate with N‐alkyl‐2‐pyridylmethanimine as the ligand

The controlled atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) catalyzed by iron halide/N‐(n‐hexyl)‐2‐pyridylmethanimine (NHPMI) is described. The ethyl 2‐bromoisobutyrate (EBIB)‐initiated ATRP with [MMA]₀/[EBIB]₀/[iron halide]₀/[NHPMI]₀ = 150/1/1/2 was better controlled in 2‐butanone than in p‐xylene at 90 °C. Initially added iron(III) halide improved the controllability of the reactions in terms of molecular weight control. The p‐toluenesulfonyl chloride (TsC1)‐initiated ATRP were uncontrolled with [MMA]₀/[TsC1]₀/[iron halide]₀/[NHPMI]₀ = 150/1/1/2 in 2‐butanone at 90 °C. In contrast to the EBIB‐initiated system, the initially added iron(III) halide greatly decreased the controllability of the TsC1‐initiated ATRP. The ration of iron halide to NHPMI significantly influenced the controllability of both EBIB and TsC1‐initiated ATRP systems. The ATRP with [MMA]₀/[initiator]₀/[iron halide]₀/[NHPMI]₀ = 150/1//1/2 provided polymers with PDIs ≥ 1.57, whereas those with [iron halide]₀/[NHPMI]₀ = 1 resulted in polymers with PDIs as low as 1.35. Moreover, polymers with PDIs of approximately 1.25 were obtained after their precipitation from acidified methanol. The high functionality of the halide end group in the obtained polymer was confirmed by both ¹H NMR and a chain‐extenstion reaction. Cyclic voltammetry was utilized to explain the differing catalytic behaviors of the in situ‐formed complexes by iron halide and NHPMI with different molar ratios. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4882–4894, 2004     

Atom transfer radical polymerization through N‐chlorosulfonamides

Controlled polymerizations of vinyl monomers such as methyl methacrylate and styrene are achieved using N‐chloro,N‐propyl‐p‐toluenesulfonamide (NCPT) together with a cuprous bromide/hexahexyl triethylenetetramine (CuBr/H‐TETA) complex. Although N‐halosulfonamides are known to decompose radically to give free chlorine, NCPT alone (without a cuprous complex) does not initiate any polymerization even in prolonged reaction times. Instead these add to the double bonds to give 2‐chloroethylsulfonamides. In the present polymerization system a good chlorine donator (NCPT) is combined with an organic soluble complex (CuBr/H‐TETA) to perform atom transfer radical polymerizations (ATRPs) in homogenous conditions. The linear proportionality of the molecular weights to the conversions and straight lines observed in ln(M₀/M) (where M₀ and M are the monomer contents at the beginning and at any time, respectively) versus time plots indicate typical controlled polymerization characteristics. The use of freshly prepared NCPT is advisable due to its slow and spontaneous decomposition when standing at room temperatures. Because of their easy preparation, N‐chlorosulfonamides can be used and are preferred instead of special halogen compounds commonly used in copper mediated ATRP. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2691–2695, 2001     

Synthesis and self‐assembly of thermosensitive double‐hydrophilic poly(N‐vinylcaprolactam)‐b‐poly(N‐vinyl‐2‐pyrrolidone) diblock copolymers

We report on novel diblock copolymers of poly(N‐vinylcaprolactam) (PVCL) and poly(N‐vinyl‐2‐pyrrolidone) (PVPON) (PVCL‐b‐PVPON) with well‐defined block lengths synthesized by the MADIX/reversible addition‐fragmentation chain transfer (RAFT) process. We show that the lower critical solution temperatures (LCST) of the block copolymers are controllable over the length of PVCL and PVPON segments. All of the diblock copolymers dissolve molecularly in aqueous solutions when the temperature is below the LCST and form spherical micellar or vesicular morphologies when temperature is raised above the LCST. The size of the self‐assembled structures is controlled by the molar ratio of PVCL and PVPON segments. The synthesized homopolymers and diblock copolymers are demonstrated to be nontoxic at 0.1–1 mg mL^?1 concentrations when incubated with HeLa and HEK293 cancer cells for various incubation times and have potential as nanovehicles for drug delivery. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2725–2737