Stimuli-responsive star polymers

Stimuli-responsive star polymers gain more and more interest over the last decades due to their unique properties compared to their linear counterparts. The branched structure for instance has influence on the responsive behavior of these polymers. This review offers an overview of stimuli-responsive star polymers generated by different polymerization techniques, e.g. anionic and controlled radical polymerization (CRP). Beside conventional branched homopolymers different other types like block copolymers, miktoarm star copolymers, core crosslinked star polymers (CCS) and comb polymers are also presented. Furthermore their responsive behavior in solution or immobilized on a substrate, and their applications are outlined. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2980–2994     

Investigation of the influence of the architectures of poly(vinyl pyrrolidone) polymers made via the reversible addition–fragmentation chain transfer/macromolecular design via the interchange of xanthates mechanism on the stabilization of suspension polymerizations

Linear, star, and block copolymers based on poly(vinyl pyrrolidone) (PVP) were synthesized with the macromolecular design via the interchange of xanthates (MADIX) process for use as potential stabilizers in suspension polymerization. The design of the leaving group of the dithioxanthate‐based transfer agent was shown to be key to the successful preparation of well‐defined PVP architectures. A linear correlation of the monomer conversion and molecular weight was found in the synthesis of star polymers, whereas the molecular weight distribution remained narrow (polydispersity index < 1.3). Significant side reactions, which typically broaden the molecular weight distribution when R‐designed MADIX agents are used, were absent. The living behavior of the PVP polymerization was furthermore confirmed via chain extension with vinyl acetate, which resulted in the formation of PVP–PVAc block copolymers [where PVAc is poly(vinyl acetate)]. The prepared polymers were used as stabilizers in suspension polymerization to prepare crosslinked poly(vinyl neodecanoate)/ethylene glycol dimethacrylate microspheres. The ratio of the interfacial tension of the aqueous and monomer phases and the overall viscosity were found to have an effect on the diameter of the particles, with PVP star polymers as stabilizers resulting in smaller particles. A smaller interfacial tension, measured when star polymers and block copolymers were used, resulted in the appearance of smaller particles, probably because of more breakup events of the monomer droplets and the enhanced stabilization of the particle surface area. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4372–4383, 2006     

Cyclic polymers: Synthetic strategies and physical properties

Syntheses of cyclic polymers including cyclic homopolymers, cyclic block copolymers, sun‐shaped polymers, and tadpole polymers are discussed on the basis of a differentiation between synthetic methods and synthetic strategies (e.g., polycondensation, ring–ring equilibration, or ring‐expansion polymerization). Furthermore, all synthetic methods are classified as kinetically or thermodynamically controlled reactions. Characteristic properties of cyclic polymers such as smaller hydrodynamic volume, lower melt viscosities, and higher thermostabilities are compared to the properties of their linear counterparts. Furthermore, the nanophase separation of cyclic diblock copolymers is discussed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 251–284, 2010     

Synthesis and stability of linear and star polymers containing [C60] fullerene

Linear and symmetric star block copolymers of styrene and isoprene containing [C₆₀] fullerene were synthesized by anionic polymerization and appropriate linking postpolymerization chemistry. In all block copolymers, the C₆₀ was connected to the terminal polyisoprene (PI) block. The composition of the copolymers was kept constant (～30% wt PI), whereas the molecular weight of the diblock chains was varied. The polymers were characterized with a number of techniques, including size exclusion chromatography, membrane osmometry, and ¹H NMR spectroscopy. The combined characterization results showed that the synthetic procedures followed led to well‐defined materials. However, degradation of the fractionated star‐shaped copolymers was observed after storage for 2 months at 4 °C, whereas the nonfractionated material was stable. To further elucidate the reasons for this degradation, we prepared and studied a four‐arm star copolymer with the polystyrene part connected to C₆₀ and a six‐arm star homopolymer of styrene. These polymers as well as linear copolymers end‐capped, through ? N<, with C₆₀ were stable. Possible reasons are discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2494–2507, 2001     

Synthesis,characterization, and thermal properties of dendrimer‐star,block‐comb copolymers by ring‐opening polymerization and atom transfer radical polymerization

Novel and well‐defined dendrimer‐star, block‐comb polymers were successfully achieved by the combination of living ring‐opening polymerization and atom transfer radical polymerization on the basis of a dendrimer polyester. Star‐shaped dendrimer poly(?‐caprolactone)s were synthesized by the bulk polymerization of ?‐caprolactone with a dendrimer initiator and tin 2‐ethylhexanoate as a catalyst. The molecular weights of the dendrimer poly(?‐caprolactone)s increased linearly with an increase in the monomer. The dendrimer poly(?‐caprolactone)s were converted into macroinitiators via esterification with 2‐bromopropionyl bromide. The star‐block copolymer dendrimer poly(?‐caprolactone)‐block‐poly(2‐hydroxyethyl methacrylate) was obtained by the atom transfer radical polymerization of 2‐hydroxyethyl methacrylate. The molecular weights of these copolymers were adjusted by the variation of the monomer conversion. Then, dendrimer‐star, block‐comb copolymers were prepared with poly(L ‐lactide) blocks grafted from poly(2‐hydroxyethyl methacrylate) blocks by the ring‐opening polymerization of L ‐lactide. The unique and well‐defined structure of these copolymers presented thermal properties that were different from those of linear poly(?‐caprolactone). © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6575–6586, 2006     

Photocrosslinkable liquid‐crystalline block copolymers with coumarin units synthesized with atom transfer radical polymerization

A new class of liquid‐crystalline (LC) homopolymers of poly{11‐[4‐(3‐ethoxycarbonyl‐coumarin‐7‐oxy)‐carbonylphenyloxy]‐undecyl methacrylate} containing a coumarin moiety as a photocrosslinkable unit with various polymerization degrees and their LC‐coil diblock and LC‐coil‐LC triblock copolymers with polystyrene as the coil segment was synthesized with the atom transfer radical polymerization method. All the homopolymers and block copolymers synthesized here exhibited narrow polydispersities, indicating well‐controlled living polymerization. Differential scanning calorimetry, polarized optical microscopy, and wide‐angle X‐ray diffraction confirmed that all the homopolymers and block copolymers exhibit a monolayer smectic A phase. Coumarin moieties in the polymers can be photodimerized under λ > 300 nm light irradiation to yield crosslinked network structures, which improve the thermal stability of a polymer nanostructure because of microphase separation. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2197–2206, 2003     

Three‐Arm Star Homo‐ and Block Co‐Polymers Via Atom Transfer Radical Polymerization

Star homopolymers of some vinyl monomers such as methyl methacrylate, n‐butyl methacrylate and styrene (MMA, nBMA, St.) were prepared by using a N,N,N′N′‐tetramethylethylenediamine ligand/CuBr catalytic system via atom transfer radical polymerization (ATRP). A three armed benzene based core was successfully used as initiator. Low polydispersities and regular molecular weight values were obtained in most cases, especially at low conversions. MMA and BMA showed comparable behavior where controlled and true ATRP was observed even at high conversions. However, styrene monomer recorded irregular high polydispersities at high conversions in spite of the relatively low molecular weight values. Some block copolymers were obtained using MMA homopolymer as macroinitiator with the same strategy of ATRP. ¹H‐NMR confirmed the structures of the resulting polymers. Transmission electron microscopy (TEM) proved the nano‐structure of the star polymers. The thermal behavior of the MMA star homo and copolymers was studied. The effect of the star shape on thermal behavior was very clear with respect to the linear ones.     

Synthesis of tertiary amine‐based pH‐responsive polymers by RAFT Polymerization

Well‐defined tertiary amine‐based pH‐responsive homopolymers and block copolymers were synthesized via reversible addition‐fragmentation chain transfer (RAFT) polymerization using 4‐cyanopentanoic acid dithiobenzoate (CPAD) as the RAFT agent for homopolymers and a poly(ethylene glycol) (PEG) macro‐RAFT agent for the block copolymers. ¹H NMR and gel permeation chromatography results confirmed the successful synthesis of these homopolymers and block copolymers. Kinetics studies indicated that the formation of both the homopolymers and the block copolymers were well defined. The pKa titration experiments suggested that the homopolymers and the related block copolymers have a similar pKa. The dynamic light scattering investigation showed that all of the block copolymers underwent a sharp transition from unimers to micelles around their pKa and the hydrodynamic diameter (D_h) was not only dependent on the molecular weight but also on the composition of the block copolymers. The polymer solution of PEG‐b‐PPPDEMA formed the largest micelle compare to the PEG‐b‐PDPAEMA and PEG‐b‐PDBAEMA with a similar molecular weight. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1010–1022     

R‐RAFT approach for the polymerization of N‐isopropylacrylamide with a star poly(ε‐caprolactone) core

Reversible addition‐fragmentation chain transfer (RAFT) polymerization is a more robust and versatile approach than other living free radical polymerization methods, providing a reactive thiocarbonylthio end group. A series of well‐defined star diblock [poly(ε‐caprolactone)‐b‐poly(N‐isopropylacrylamide)]₄ (SPCLNIP) copolymers were synthesized by R‐RAFT polymerization of N‐isopropylacrylamide (NIPAAm) using [PCL‐DDAT]₄ (SPCL‐DDAT) as a star macro‐RAFT agent (DDAT: S‐1‐dodecyl‐S′‐(α, α′‐dimethyl‐α″‐acetic acid) trithiocarbonate). The R‐RAFT polymerization showed a controlled/“living” character, proceeding with pseudo‐first‐order kinetics. All these star polymers with different molecular weights exhibited narrow molecular weight distributions of less than 1.2. The effect of polymerization temperature and molecular weight of the star macro‐RAFT agent on the polymerization kinetics of NIPAAm monomers was also addressed. Hardly any radical–radical coupling by‐products were detected, while linear side products were kept to a minimum by careful control over polymerization conditions. The trithiocarbonate groups were transferred to polymer chain ends by R‐RAFT polymerization, providing potential possibility of further modification by thiocarbonylthio chemistry. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Biocompatible amphiphilic conetwork based on crosslinked star copolymers: A potential drug carrier

A series of amphiphilic conetworks (APCNs) is synthesized through crosslinking of well‐defined tri‐arm star diblock copolymers via atom transfer radical polymerization. A new three‐arm initiator is synthesized to initiate the polymerization of 2‐hydroxyethyl methacrylate (HEMA) via “core‐first” method. The resulting star HEMA homopolymers with well‐defined molecular weight and narrow polydispersity are used as macroinitiator to incorporate allyl methacrylate to get the star diblock copolymers. Then, the precursors with allyl pendant groups are fully crosslinked with polyhydrosiloxanes through hydrosilylation. The so‐prepared APCNs exhibit unique properties of microphase separation of hydrophilic (HI) and hydrophobic (HO) phases with small channel size, a variable swelling capacity, excellent biocompatibility, and outstanding mechanical strength (2 ± 0.5 MPa). The properties of APCNs depend on the ratio of HI to HO, which can be regulated via precise synthesis of the star diblock copolymers. The APCNs show well‐controlled drug release to choline theophyllinate, suggesting a promising intelligent drug carrier for controlled release. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2537–2545     

Dendrimer‐star polymer and block copolymer prepared by reversible addition‐fragmentation chain transfer (RAFT) polymerization with dendritic chain transfer agent

A new reversible addition‐fragmentation chain transfer (RAFT) agent, dendritic polyester with 16 dithiobenzoate terminal groups, was prepared and used in the RAFT polymerization of styrene (St) to produce star polystyrene (PSt) with a dendrimer core. It was found that this polymerization was of living characters, the molecular weight of the dendrimer‐star polymers could be controlled and the polydispersities were narrow. The dendrimer‐star block copolymers of St and methyl acrylate (MA) were also prepared by the successive RAFT polymerization using the dendrimer‐star PSt as macro chain transfer agent. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6379–6393, 2005     

Morphology and microphase separation of star copolymers

Star copolymers have attracted significant interest due to their different characteristics compared with diblock copolymers, including higher critical micelle concentration, lower viscosity, unique spatial shape, or morphologies. Development of synthetic skills such as anionic polymerization and controlled radical polymerization have made it possible to make diverse architectures of polymers. Depending on the molecular architecture of the copolymer, numerous morphologies are possible, for instance, Archimedean tiling patterns and cylindrical microdomains at symmetric volume fraction for miktoarm star copolymers as well as asymmetric lamellar microdomains for star‐shaped copolymers, which have not been reported for linear block copolymers. In this review, we focus on morphologies and microphase separations of miktoarm (A_mB_n and ABC miktoarm) star copolymers and star‐shaped [(A‐b‐B)_n] copolymers with nonlinear architecture. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1–21     

Synthesis of high‐order multiblock core cross‐linked star polymers

Core cross‐linked star (CCS) polymers with radiating arms composed of high‐order multiblock copolymers have been synthesized in a one‐pot system via iterative copper‐mediated radical polymerization. The employed “arm‐first” technique ensures the multiblock sequence of the macroinitiator is carried through to the star structure with no arm defects. The versatility of this approach is demonstrated by the synthesis of three distinct star polymers with differing arm compositions, two with an alternating ABABAB block sequence and one with six different block units (i.e. ABCDEF). Owing to the star architecture, CCS polymers in which the arm composition consists of alternating hydrophilic–hydrophobic (ABABAB) segments undergo supramolecular self‐assembly in selective solvents, whereas linear polymers with the same block sequence did not yield self‐assembled structures, as evidenced by DLS analysis. The combination of microstructural and topological control in CCS polymers offers exciting possibilities for the development of tailor‐made nanoparticles with spatially defined regions of functionality. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 135–143     

Living radical polymerization of diisopropyl fumarate to obtain block copolymers containing rigid poly(substituted methylene) and flexible polyacrylate segments

Living radical polymerizations of diisopropyl fumarate (DiPF) are carried out to synthesize poly(diisopropyl fumarate) (PDiPF) as a rigid poly(substituted methylene) and its block copolymers combined with a flexible polyacrylate segment. Reversible addition‐fragmentation chain transfer (RAFT) polymerization is suitable to obtain a high‐molecular‐weight PDiPF with well‐controlled molecular weight, molecular weight distribution, and chain‐end structures, while organotellurium‐mediated living radical polymerization (TERP) and reversible chain transfer catalyzed polymerization (RTCP) give PDiPF with controlled chain structures under limited polymerization conditions. In contrast, controlled polymerization for the production of high‐molecular‐weight and well‐defined PDiPF is not achieved by atom transfer radical polymerization (ATRP) and nitroxide‐mediated radical polymerization (NMP). The block copolymers consisting of rigid poly(substituted methylene) and flexible polyacrylate segments are synthesized by the RAFT polymerization. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2136–2147     

Functional polymers by living anionic polymerization

The application of living anionic polymerization techniques for the functionalization of polymers and block copolymers is reviewed. The attachment of functional groups to polymeric chains of predetermined lengths and narrow molecular weight distributions is described. Carboxyls, hydroxyls, amines, halogens, double bonds, and many other functional groups can be placed at one or two ends in the center or evenly spaced along polymeric chains. Subsequent transformations of the functional groups further contribute to the versatility of such treatments. General methods based on the use, as terminators, of substituted haloalkanes, as well as the addition of living polymers or their initiators to diphenylethylenes, substituted with appropriate functional groups or molecules, are discussed. Another approach, based on the living polymerization of monomers with protected functional groups, is also discussed. It has been used for the preparation of polymers and copolymers with evenly spaced functional groups. The combination of living anionic polymerization techniques with controlled radical and cationic polymerizations is also described. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2116–2133, 2002     

Hyperbranched polymers as scaffolds for multifunctional reversible addition–fragmentation chain‐transfer agents: A route to polystyrene‐core‐polyesters and polystyrene‐block‐poly(butyl acrylate)‐core‐polyesters

Polydisperse hyperbranched polyesters were modified for use as novel multifunctional reversible addition–fragmentation chain‐transfer (RAFT) agents. The polyester‐core‐based RAFT agents were subsequently employed to synthesize star polymers of n‐butyl acrylate and styrene with low polydispersity (polydispersity index < 1.3) in a living free‐radical process. Although the polyester‐core‐based RAFT agent mediated polymerization of n‐butyl acrylate displayed a linear evolution of the number‐average molecular weight (M_n) up to high monomer conversions (>70%) and molecular weights [M_n > 140,000 g mol^?1, linear poly(methyl methacrylate) equivalents)], the corresponding styrene‐based system reached a maximum molecular weight at low conversions (≈30%, M_n = 45,500 g mol^?1, linear polystyrene equivalents). The resulting star polymers were subsequently used as platforms for the preparation of star block copolymers of styrene and n‐butyl acrylate with a polyester core with low polydispersities (polydispersity index < 1.25). The generated polystyrene‐based star polymers were successfully cast into highly regular honeycomb‐structured microarrays. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3847–3861, 2003     

Facile one‐pot synthesis of linear and radial block copolymers of styrene and isoprene through a novel coupling agent by living anionic polymerization

A new methodology is successfully used for the concurrent synthesis of three different copolymers; diblock, triblock, and three‐armed star‐block copolymers of styrene and isoprene via the living anionic polymerization with control over the molecular weight and weight fractions of each block. The room temperature polymerization process has resulted in the well defined linear and radial block copolymers, when the living di‐block of poly(styrene‐b‐isoprene) was coupled using cheap and readily available malonyl chloride as a novel coupling agent giving nearly 100% yield. The resulting block copolymers have narrow polydispersity index (PDI = 1.01–1.09) with a good agreement between the calculated and the observed molecular weights. The results are further supported by fractionation of the block copolymers by reversed‐phase temperature gradient interaction chromatography (RP‐TGIC) technique followed by size exclusion chromatography (SEC). © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2636–2641, 2010     

Controlled polymerization of carbazole‐based vinyl and methacrylate monomers at ambient temperature: A comparative study through ATRP,SET, and SET‐RAFT polymerizations

The polymerization of N‐vinylcarbazole (NVK) and carbazole methacrylate (CMA) was carried out using controlled radical polymerization methods such as atom transfer radical polymerization (ATRP), single electron transfer (SET)‐LRP, and single electron transfer initiation followed by reversible addition fragmentation chain transfer (SET‐RAFT). Well‐controlled polymerization with narrow molecular weight distribution (M_w/M_n) < 1.25 was achieved in the case of NVK by high‐temperature ATRP while ambient temperature SET‐RAFT polymerization was relatively slow and controlled. In the case of CMA, SET‐RAFT is found to be more suitable for the ambient temperature polymerization. The polymerization rate followed first order kinetics with respect to monomer conversion and the molecular weight of the polymer increased linearly with conversion. The controlled nature of the polymerization is further demonstrated by the synthesis of diblock copolymers from PNVK and PCMA macroinitiators using a new flavanone‐based methacrylate (FMA) as the second monomer. All the polymers exhibited fluorescence. The excimer bands in the homopolymers of PNVK and PCMA were very broad, which may be attributed to the carbazole–carbazole overlap interaction. The scanning electron microscopy analysis of the block copolymer reveals interesting morphological features. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis of well‐defined comb‐like graft (co)polymers by nucleophilic substitution reaction between living polymers and polyhalohydrocarbon

A series of graft (co)polymers were synthesized by nucleophilic substitution reaction between iodinated 1,2‐polybutadiene (PB‐I, backbone) and living polymer lithium (side chains). The coupling reaction between PB‐I and living polymers can finish within minutes at room temperature, and high conversion (up to 92%) could be obtained by effectively avoiding side reaction of dimerization when living polymers were capped with 1,1‐diphenylethylene. By virtue of living anionic polymerization, backbone length, side chain length, and side chain composition, as well as graft density, were well controlled. Tunable molecular weight of graft (co)polymers with narrow molecular weight distribution can be obtained by changing either the lengths of side chain and backbone, or the graft density. Graft copolymers could also be synthesized with side chains of multicomponent polymers, such as block polymer (polystyrene‐b‐polybutadiene) and even mixed polymers (polystyrene and polybutadiene) as hetero chains. Thus, based on living anionic polymerization, this work provides a facile way for modular synthesis of graft (co)polymers via nucleophilic substitution reaction between living polymers and polyhalohydrocarbon (PB‐I). © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Novel synthesis of biodegradable star poly(ethylene glycol)‐block‐poly(lactide) copolymers

Biodegradable star‐shaped poly(ethylene glycol)‐block‐poly(lactide) copolymers were synthesized by ring‐opening polymerization of lactide, using star poly(ethylene glycol) as an initiator and potassium hexamethyldisilazide as a catalyst. Polymerizations were carried out in toluene at room temperature. Two series of three‐ and four‐armed PEG‐PLA copolymers were synthesized and characterized by gel permeation chromatography (GPC) as well as ¹H and ¹³C NMR spectroscopy. The polymerization under the used conditions is very fast, yielding copolymers of controlled molecular weight and tailored molecular architecture. The chemical structure of the copolymers investigated by ¹H and ¹³C NMR indicates the formation of block copolymers. The monomodal profile of molecular weight distribution by GPC provided further evidence of controlled and defined star‐shaped copolymers as well as the absence of cyclic oligomeric species. The effects of copolymer composition and lactide stereochemistry on the physical properties were investigated by GPC and differential scanning calorimetry. For the same PLA chain length, the materials obtained in the case of linear copolymers are more viscous, whereas in the case of star copolymer, solid materials are obtained with reduction in their T_g and T_m temperatures. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3966–3974, 2007