Controlling polymer structures by atom transfer radical polymerization and other controlled/living radical polymerizations

Fundamentals of controlled/living radical polymerization (CRP) and Atom Transfer Radical Polymerization (ATRP), relevant to the synthesis of controlled polymer structures are described. Macromolecular brushes with star like structure are used as an example to illustrate synthetic power of ATRP.     

Eutectic mixtures as a green alternative for efficient catalyst recycling in atom transfer radical polymerizations

A new solvent mixture, based on ethanol/reline (EM: eutectic mixture), was investigated for the supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP) of methyl acrylate (MA) near room temperature, for the first time, affording complete catalyst recovery and reuse. The kinetic results revealed that the polymerizations were controlled, with polymers having narrow molecular weight distributions (? < 1.2). The “living” character of the resultant PMA was confirmed by the synthesis of a well‐defined PMA‐b‐PBA block copolymer. Remarkably, it was demonstrated that the Cu(0)/CuBr₂/Me₆TREN (Me₆TREN: tris[2‐(dimethylamino)ethyl]amine) could be recovered from the final reaction mixture and reused for new successful SARA ATRP of MA, suggesting that the reported system could be very attractive from both the economic and environmental perspectives. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 371–381     

Preparation of H‐shaped ABCAB terpolymers by atom transfer radical coupling

H‐shaped ABCAB terpolymers composed of polystyrene (PS) (A), poly(ethylene oxide) (PEO) (B), and poly(tert‐butyl acrylate) (PtBA) (C) were prepared by atom transfer radical coupling reaction using ABC star terpolymers as precursors, CuBr and N,N,N′,N″,N″‐pentamethyldiethylenetriamine (PMDETA) as catalysts, and nanosize copper as the reducing agent. The synthesis of 3‐miktoarm star terpolymer PS‐PEO‐(PtBA‐Br) involved following steps: (1) the preparation of PS with an active and an ethoxyethyl‐ptotected hydroxyl group at the same end; (2) the preparation of diblock copolymer PS‐b‐PEO with ethoxyethyl‐protected group at the junction point through the ring‐opening polymerization (ROP) of EO; (3) after de‐protection of ethoxyethyl group and further modification of hydroxyl group, tBA was polymerized by atom transfer radical polymerization using PS‐b‐PEO with 2‐bromoisobutyryl functional group as macroinitiator. The H‐shaped terpolymer could be successfully formed by atom transfer radical coupling reaction in the presence of small quantity of styrene, CuBr/PMDETA, and Cu at 90 °C. The copolymers were characterized by SEC, ¹H NMR, and FTIR in detail. The optimized coupling temperature is 90 °C. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 59–68, 2009     

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis of atom transfer radical polymerization macroinitiators derived from poly(3-ethyl-3-hydroxymethyloxetane)

The matrix-assisted laser desorption/ionization time-of-flight analysis of macroinitiators for atom transfer radical polymerization prepared by the functionalization of a multihydroxyl, highly branched polyether, poly(3-ethyl-3-hydroxymethyloxetane), allows the exact determination of the macroinitiator functionality, providing information on the number of bromine atoms in individual macromolecules, in contrast to NMR analysis, which gives average values. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 608–614, 2004     

Synthesis and characterization of star polymers initiated by hexafunctional discotic initiator through atom transfer radical polymerization

A novel hexafunctional discotic initiator, 2,3,6,7,11,12‐hexakis(2‐bromobutyryloxy)triphenylene (HBTP), was synthesized by the esterification of 2,3,6,7,11,12‐hexahydroxytriphenylene with 2‐bromobutyryl chloride. Atom transfer radical polymerizations of styrene, methyl acrylate, and n‐butyl acrylate were carried out in 50 vol % tetrahydrofuran with HBTP/copper(I) bromide/2,2′‐bipyridyl as an initiation system. The polymers produced had well‐controlled molecular weights and narrow molecular weight distributions (<1.2). On the basis of ¹H NMR spectra of the star polymer and its hydrolyzed products, we can conclude that the initiator quantitatively initiated the polymerization of vinyl monomers and that a star polymer with a discotic core was obtained. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2233–2243, 2001     

Facile synthesis of AB2‐type miktoarm star polymers through the combination of atom transfer radical polymerization and ring‐opening polymerization

A novel miktofunctional initiator ( 1 ), 2‐hydroxyethyl 3‐[(2‐bromopropanoyl)oxy]‐2‐{[(2‐bromopropanoyl)oxy]methyl}‐2‐methyl‐propanoate, possessing one initiating site for ring‐opening polymerization (ROP) and two initiating sites for atom transfer radical polymerization (ATRP), was synthesized in a three‐step reaction sequence. This initiator was first used in the ROP of ?‐caprolactone, and this led to a corresponding polymer with secondary bromide end groups. The obtained poly(?‐caprolactone) (PCL) was then used as a macroinitiator for the ATRP of tert‐butyl acrylate or methyl methacrylate, and this resulted in AB₂‐type PCL–[poly(tert‐butyl acrylate)]₂ or PCL–[poly(methyl methacrylate)]₂ miktoarm star polymers with controlled molecular weights and low polydispersities (weight‐average molecular weight/number‐average molecular weight < 1.23) via the ROP–ATRP sequence. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2313–2320, 2004     

A new tetradentate ligand for atom transfer radical polymerization

The properties of a ligand, including molecular structure and substituents, strongly affect the catalyst activity and control of the polymerization in atom transfer radical polymerization (ATRP). A new tetradentate ligand, N,N′‐bis(pyridin‐2‐ylmethyl‐3‐hexoxo‐3‐oxopropyl)ethane‐1,2‐diamine (BPED) was synthesized and examined as the ligand of copper halide for ATRP of styrene (St), methyl acrylate (MA), and methyl methacrylate (MMA), and compared with other analogous linear tetrdendate ligands. The BPED ligand was found to significantly promote the activation reaction: the CuBr/BPED complex reacted with the initiators so fast that a large amount of Cu(II)Br₂/BPED was produced and thus the polymerizations were slow for all the monomers. The reaction of CuCl/BPED with the initiator was also fast, but by reducing the catalyst concentration or adding CuCl₂, the activation reaction could be slowed to establish the equilibrium of ATRP for a well‐controlled living polymerization of MA. CuCl/BPED was found very active for the polymerization of MA. For example, 10 mol% of the catalyst relatively to the initiator was sufficient to mediate a living polymerization of MA. The CuCl/BPED, however, could not catalyze a living polymerization of MMA because the resulting CuCl₂/BPED could not deactivate the growing radicals. The effects of the ligand structures on the catalysis of ATRP are also discussed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3553–3562, 2004     

Reverse atom transfer radical polymerization of butyl methacrylate in a miniemulsion stabilized with a cationic surfactant

The miniemulsion reverse atom transfer radical polymerization of butyl methacrylate was carried out with cetyltrimethylammonium bromide (CTAB) as the sole surfactant. The polymerizations were initiated with 2,2′‐azobis[2‐(2‐imidazolin‐2‐yl)propane] dihydrochloride and mediated with copper(II) bromide/tris[2‐di(2‐ethylhexyl acrylate)aminoethyl]amine. The living character was demonstrated by the linear increase in the number‐average molecular weight with conversion and the decreasing polydispersity index with conversion. The polymerizations were conducted at 90 °C with 1 wt % CTAB with respect to the monomer and produced a coagulum‐free latex with a mean particle diameter of 155 nm. The resulting latexes exhibited good shelf‐life stability. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1628–1634, 2006     

Living radical emulsion polymerization using the nanoprecipitation technique: An extension to atom transfer radical polymerization

Living radical polymerizations of styrene were performed under emulsion atom transfer radical polymerization conditions with latexes prepared by a nanoprecipitation technique recently developed for the stable free‐radical polymerization process. Latexes were prepared by the precipitation of a solution of low‐molecular‐weight polystyrene in acetone into a solution of a surfactant in water. The resulting particles were swollen with styrene and then heated. The effects of various surfactants and hydrophobic ligands, the reaction temperature, and the ligand/copper(I) bromide ratio were studied. The best results were obtained with the nonionic surfactant Brij 98 in combination with the hydrophobic ligand N,N‐bis(2‐pyridylmethyl)octadecylamine and a ligand/copper(I) bromide ratio of 1.5 at a reaction temperature of 85–90 °C. Under these conditions, latexes with good colloidal stability with average particle diameters of 200 nm were obtained. The molecular weight distributions of the polystyrenes were narrow, although the experimental molecular weights were slightly larger than the theoretical ones because not all the macroinitiator appeared to reinitiate. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4027–4038, 2006     

Synthesis of miktoarm star and miktoarm star block copolymers via a combination of atom transfer radical polymerization and stable free‐radical polymerization

A trifunctional initiator, 2‐phenyl‐2‐[(2,2,6,6‐tetramethyl)‐1‐piperidinyloxy] ethyl 2,2‐bis[methyl(2‐bromopropionato)] propionate, was synthesized and used for the synthesis of miktoarm star AB₂ and miktoarm star block AB₂C₂ copolymers via a combination of stable free‐radical polymerization (SFRP) and atom transfer radical polymerization (ATRP) in a two‐step or three‐step reaction sequence, respectively. In the first step, a polystyrene (PSt) macroinitiator with dual ω‐bromo functionality was obtained by SFRP of styrene (St) in bulk at 125 °C. Next, this PSt precursor was used as a macroinitiator for ATRP of tert‐butyl acrylate (tBA) in the presence of Cu(I)Br and pentamethyldiethylenetriamine at 80 °C, affording miktoarm star (PSt)(PtBA)₂ [where PtBA is poly(tert‐butyl acrylate)]. In the third step, the obtained St(tBA)₂ macroinitiator with two terminal bromine groups was further polymerized with methyl methacrylate by ATRP, and this resulted in (PSt)(PtBA)₂(PMMA)₂‐type miktoarm star block copolymer [where PMMA is poly(methyl methacrylate)] with a controlled molecular weight and a moderate polydispersity (weight‐average molecular weight/number‐average molecular weight < 1.38). All polymers were characterized by gel permeation chromatography and ¹H NMR. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2542–2548, 2003     

Block copolymer preparation by atom transfer radical polymerization under emulsion conditions using a nanoprecipitation technique

Living‐radical polymerization of acrylates were performed under emulsion atom transfer radical polymerization (ATRP) conditions using latexes prepared by a nanoprecipitation technique previously employed and optimized for the polymerization of styrene. A macroinitiator of poly(n‐butyl acrylate) prepared under bulk ATRP was dissolved in acetone and precipitated in an aqueous solution of Brij 98 to preform latex particles, which were then swollen with monomer and heated. Various monomers (i.e. n‐butyl acrylate, styrene, and tert‐butyl acrylate) were used to swell the particles to prepare homo‐ and block copolymers from the poly(n‐butyl acrylate) macroinitiator. Under these conditions latexes with a relatively good colloidal stability were obtained. Furthermore, amphiphilic block copolymers were prepared by hydrolysis of the tert‐butyl groups and the resulting block copolymers were characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The bulk morphologies of the polystyrene‐b‐poly(n‐butyl acrylate) and poly(n‐butyl acrylate)‐b‐poly(acrylic acid) copolymers were investigated by atomic force microscopy (AFM) and small angle X‐ray scattering (SAXS). © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 625–635, 2008     

Polymer–silicate nanocomposites produced by in situ atom transfer radical polymerization

Polymer–silicate nanocomposites were synthesized with atom transfer radical polymerization (ATRP). An ATRP initiator, consisting of a quaternary ammonium salt moiety and a 2‐bromo‐2‐methyl propionate moiety, was intercalated into the interlayer spacings of the layered silicate. Subsequent ATRP of styrene, methyl methacrylate, or n‐butyl acrylate with Cu(I)X/N,N‐bis(2‐pyridiylmethyl) octadecylamine, Cu(I)X/N,N,N′,N′,N″‐pentamethyldiethylenetriamine, or Cu(I)X/1,1,4,7,10,10‐hexamethyltriethylenetetramine (X = Br or Cl) catalysts with the initiator‐modified silicate afforded homopolymers with predictable molecular weights and low polydispersities, both characteristics of living radical polymerization. The polystyrene nanocomposites contained both intercalated and exfoliated silicate structures, whereas the poly(methyl methacrylate) nanocomposites were significantly exfoliated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 916–924, 2004     

Coupled atom transfer radical coupling and atom transfer radical polymerization approach for controlled grafting from poly(vinylidene fluoride) backbone

A new “grafting from” strategy for grafting of different monomers (methacrylates, acrylates, and acrylamide) on poly(vinylidene fluoride) (PVDF) backbone is designed using atom transfer radical coupling (ATRC) and atom transfer radical polymerization (ATRP). 4‐Hydroxy TEMPO moieties are anchored on PVDF backbone by ATRC followed by attachment of ATRP initiating sites chosen according to the reactivity of different monomers. High graft conversion is achieved and grafting of poly(methyl methacrylate) (PMMA) exhibits high degree of polymerization (DPn = 770) with a very low graft density (0.18 per hundred VDF units) which has been increased to 0.44 by regenerating the active catalyst with the addition of Cu(0). A significant impact on thermal and stress–strain property of graft copolymers on the graft density and graft length is noted. Higher tensile strain and toughness are observed for PVDF‐g‐PMMA produced from model initiator but graft copolymer from pure PVDF exhibits higher tensile strength and Young's modulus. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 995–1008     

Synthesis and characterization of sulfonated block copolymers by atom transfer radical polymerization

Well‐defined sulfonated polystyrene and block copolymers with n‐butyl acrylate (nBA) were synthesized by CuBr catalyzed living radical polymerization. Neopentyl p‐styrene sulfonate (NSS) was polymerized with ethyl‐2‐bromopropionate initiator and CuBr catalyst with N,N,N′,N′‐pentamethylethyleneamine to give poly(NSS) (PNSS) with a narrow molecular weight distribution (MWD < 1.12). PNSS was then acidified by thermolysis resulting in a polystyrene backbone with 100% sulfonic acid groups. Random copolymers of NSS and styrene with various composition ratios were also synthesized by copolymerization of NSS and styrene with different feed ratios (MWD < 1.11). Well defined block copolymers with nBA were synthesized by sequential polymerization of NSS from a poly(n‐butyl acrylate) (PnBA) precursor using CuBr catalyzed living radical polymerization (MWD < 1.29). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5991–5998, 2008     

Kinetic analysis of styrene atom transfer radical polymerization: Extraction of radical‐radical termination rate coefficients

Kinetic studies of the atom transfer radical polymerization (ATRP) of styrene are reported, with the particular aim of determining radical‐radical termination rate coefficients (<k_t>). The reactions are analyzed using the persistent radical effect (PRE) model. Using this model, average radical‐radical termination rate coefficients are evaluated. Under appropriate ATRP catalyst concentrations, <k_t> values of approximately 2 × 10⁸ L mol^?1 s^?1 at 110 °C in 50 vol % anisole were determined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5548–5558, 2004     

High molecular weight polyacrylamides by atom transfer radical polymerization: Enabling advancements in water‐based applications

Through the use of copper (I) chloride (CuCl) and tris(2‐dimethylaminoethyl)amine (Me₆‐TREN) as a metal/ligand pair, conditions for the robust, fast, and controlled radical polymerization of high molecular weight N‐hydroxyethylacrylamide (HEAm),N‐isopropylacrylamide (NIPAm), N,N′‐dimethylacrylamide (DMAm), and acrylamide (Am) at ambient temperature are reported. Linear evolution of molecular weight and narrow molecular weight distribution was observed for all monomers with degrees of polymerization ranging from 50 to 5000. Random copolymers of several acrylamide‐based monomers are also reported with excellent control over molecular weight and polydispersity. Characterization of high molecular weight poly (NIPAm) demonstrated large changes in the lower critical solution temperature observed on heating and cooling, and this hysteresis was exploited for the controlled release of doxorubicin from poly(NIPAm) spheres. This study represents the first example of preparation of high molecular weight acrylamide polymers by a metal‐mediated controlled radical polymerization technique. Access to these materials, as well as to NIPAm polymers in particular, opens new doors for interesting applications in a variety of fields including tissue engineering, drug delivery, and controlled solution viscosity. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Synthesis of diblock copolymers by combining stable free radical polymerization and atom transfer radical polymerization

A stable nitroxyl radical functionalized with an initiating group for atom transfer radical polymerization (ATRP), 4‐(2‐bromo‐2‐methylpropionyloxy)‐2,2,6,6‐tetramethyl‐1‐piperidinyloxy (Br‐TEMPO), was synthesized by the reaction of 4‐hydroxyl‐2,2,6,6‐tetramethyl‐1‐piperidinyloxy with 2‐bromo‐2‐methylpropionyl bromide. Stable free radical polymerization of styrene was then carried out using a conventional thermal initiator, dibenzoyl peroxide, along with Br‐TEMPO. The obtained polystyrene had an active bromine atom for ATRP at the ω‐end of the chain and was used as the macroinitiator for ATRP of methyl acrylate and ethyl acrylate to prepare block copolymers. The molecular weights of the resulting block copolymers at different monomer conversions shifted to higher molecular weights and increased with monomer conversion. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2468–2475, 2006     

Preparation of poly(ethylene oxide) star polymers and poly(ethylene oxide)–polystyrene heteroarm star polymers by atom transfer radical polymerization

Poly(ethylene oxide) (PEO) star polymer with a microgel core was prepared by atom transfer radical poylmerization (ATRP) of divinyl benzene (DVB) with mono‐2‐bromoisobutyryl PEO ester as a macroinitiator. Several factors, such as the feed ratio of DVB to the initiator, type of catalysts, and purity of DVB, play important roles during star formation. The crosslinked poly(divinyl benzene) (PDVB) core was further obtained by the hydrolysis of PEO star to remove PEO arms. Size exclusion chromatography (SEC) traces revealed the bare core has a broad molecular weight distribution. PEO–polystyrene (PS) heteroarm star polymer was synthesized through grafting PS from the core of PEO star by another ATRP of styrene (St) because of the presence of initiating groups in the core inherited from PEO star. Characterizations by SEC, ¹H NMR, and DSC revealed the successful preparation of the target star copolymers. Scanning electron microscopy images suggested that PEO–PS heteroarm star can form spherical micelles in water/tetrahydrofuran mixture solvents, which further demonstrated the amphiphilic nature of the star polymer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2263–2271, 2004     

New antibacterial cationic surfactants prepared by atom transfer radical polymerization

Efficient antibacterial surfactants have been prepared by the quaternization of the amino groups of poly(ethylene‐co‐butylene)‐b‐poly[2‐(dimethylamino)ethylmethacrylate] (PEB‐b‐PDMAEMA) diblock copolymers by octyl bromide. The diblock copolymers have been synthesized by ATRP of 2‐(dimethylamino)ethylmethacrylate (DMAEMA) initiated by an activated bromide‐end‐capped poly(ethylene‐co‐butylene). In the presence of CuBr, 1,4,7,10,10‐hexamethyl‐triethylenetetramine (HMTETA), and toluene at 50 °C, the initiation is slow in comparison with propagation. This situation has been improved by the substitution of CuCl for CuBr, all the other conditions being the same. Finally, the addition of an excess of CuCl₂ (deactivator) to the CuCl/HMTETA catalyst is very beneficial in making the agreement between the theoretical and experimental number‐average molecular weights excellent. The antibacterial activity of PEB‐b‐PDMAEMA quaternized by octyl bromide has been assessed against bacteria and is comparable to the activity of a commonly used disinfectant, that is, benzalkonium chloride. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1214‐1224, 2006     

Synthesis of star polymer by atom transfer radical polymerization with resorcinol‐core multifunctional initiator: The construction of nanocapsule via hydrolysis and olefin metathesis reaction of the obtained star polymer

Olefin group‐carrying styrene, 1‐but‐3‐enyl‐4‐vinylbenznene (BVB), was polymerized via atom transfer radical polymerization (ATRP) initiated from C‐methylcalix [4]resorcinarene‐based multifunctional initiator (CRA‐bib) at low conversion to produce star polymer [poly(BVB)] with narrow molecular weight distribution (M_w/M_n < 1.35). The copolymerization of styrene (St) with poly(BVB) (M_n = 11,000, M_w/M_n = 1.23) as a macroinitiator afforded star block copolymer [poly(BVB‐b‐St)] with M_n = 35,000 and M_w/M_n = 1.44. The BVB layer of poly(BVB‐b‐St), located between the St shell and the CRA core, was crosslinked by olefin metathesis reaction of olefin groups o the BVB moieties. The removal of the CRA core of the crosslinked poly(BVB‐b‐St) by hydrolysis using KOH as a base gave polymeric hollow sphere [poly(cored crossBVB‐b‐St)] with good solubility in organic solvents. The morphological structure of the poly(cored crossBVB‐b‐St) showed spherical aggregates in THF by scanning electron microscopy (SEM). Furthermore, the nanocapsule structure of poly(cored crossBVB‐b‐St) with hollow spheres was found to be observed by transmission electron microscopy (TEM). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4879–4888, 2008