Miktoarm star copolymers of poly(ϵ‐caprolactone) from a novel heterofunctional initiator

A novel heterofunctional initiator, synthesized from pentaerythritol in a three step reaction sequence with two ring opening polymerization (ROP) and two atom transfer radical polymerization (ATRP) initiating sites, was used to prepare A₂B₂ miktoarm star copolymers of poly(ε‐caprolactone), PεCL, with polystyrene, PS, poly(methyl methacrylate), PMMA, poly(dimethylaminoethyl methacrylate), PDMAEMA, and poly(2‐hydroxyethyl methacrylate), PHEMA. A₂B miktoarm stars, A being PεCL or poly(δ‐valerolactone), PδVL and B PS were also prepared from ω,ω‐dihydroxy‐PS, synthesized from ω‐Br‐PS and serinol, by ROP of εCL or δVL. All polymers were characterized by size exclusion chromatography, ¹H NMR spectroscopy, and membrane osmometry. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5164–5181, 2007     

Thermo‐responsive fluorescent micelles from amphiphilic A3B miktoarm star copolymers prepared via a combination of SET‐LRP and RAFT polymerization

A novel amphiphilic A₃B miktoarm star copolymer poly(N‐isopropylacrylamide)₃‐poly(N‐vinylcarbazole) ((PNIPAAM)₃(PVK)) was successfully synthesized by a combination of single‐electron transfer living radical polymerization (SET‐LRP) and reversible addition‐fragmentation chain transfer (RAFT) polymerization. First, the well‐defined three‐armed poly(N‐isopropylacrylamide) (PNIPAAM)₃ was prepared via SET‐LRP of N‐isopropylacrylamide in acetone at 25 °C using a tetrafunctional bromoxanthate iniferter (Xanthate‐Br₃) as the initiator and Cu(0)/PMDETA as a catalyst system. Secondly, the target amphiphilic A₃B miktoarm star copolymer ((PNIPAAM)₃(PVK)) was prepared via RAFT polymerization of N‐vinylcarbazole (NVC) employing (PNIPAAM)₃ as the macro‐RAFT agent. The architecture of the amphiphilic A₃B miktoarm star copolymers were characterized by GPC, ¹H‐NMR spectra. Furthermore, the fluorescence intensity of micelle increased with the temperature and had a good temperature reversibility, which was investigated by dynamic light scattering (DLS), fluorescent and UV‐vis spectra. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4268–4278, 2010     

Synthesis,self‐assembly and drug‐loading capacity of well‐defined drug‐conjugated amphiphilic A2B2 type miktoarm star copolymers based on poly(ε‐caprolactone) and poly(ethylene glycol)

Well‐defined drug‐conjugated amphiphilic A₂B₂ miktoarm star copolymers [(PCL)₂‐(PEG)₂‐D] were prepared by the combination of controlled ring‐opening polymerization (CROP) and “click” reaction strategy. First, bromide functionalized poly(ε‐caprolactone) (PCL‐Br) with double hydroxyl end groups was synthesized by the CROP of ε‐caprolactone using 2,2‐bis(bromomethyl)propane‐1,3‐diol as a difunctional initiator in the presence of Sn(Oct)₂ at 110 °C. Next, the bromide groups of PCL‐Br were quantitatively converted to azide form by NaN₃ to give PCL‐N₃. Subsequently, the end hydroxyl groups of PCL‐N₃ were capped with ibuprofen as a model drug at room temperature. Finally, copper(I)‐catalyzed cycloaddition reaction between ibuprofen‐conjugated PCL‐N₃ and slightly excess alkyne‐terminated poly(ethylene glycol) (A‐PEG) led to ibuprofen‐conjugated A₂B₂ miktoarm star copolymer [(PCL)₂‐(PEG)₂‐D]. The excess A‐PEG was removed by dialysis. ¹H NMR, FTIR and SEC analyzes confirmed the expected miktoarm star architecture. These amphiphilic miktoarm star copolymers could self‐assemble into multimorphological aggregates in aqueous solution, which were characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM). In addition, the drug‐loading capacity of these drug‐conjugated miktoarm star copolymers as well as their nondrug‐conjugated analogs were also investigated in detail. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2009     

Synthesis and self‐assembly of amphiphilic macrocyclic block copolymer topologies

We demonstrated the synthesis of miktoarm star block copolymers of AB, AB₂, and A₂B, in which block A consisted of linear poly(tert‐butyl acrylate) (P^tBA) and block B consisted of cyclic polystyrene. These structures were produced using the atom transfer radical polymerization to make telechelic polymers that, after modification, were further coupled together by copper‐catalyzed “click” reactions with high coupling efficiency. Deprotection of P^tBA to poly(acrylic acid) (PAA) afforded amphiphilic miktoarm structures that when micellized in water gave vesicle morphologies when the block length of PAA was 21 units. Increasing the PAA block length to 46 units produced spherical core‐shell micelles. AB₂ miktoarm stars packed more densely into the core compared to its linear counterpart (i.e., a four times greater aggregation number with approximately the same hydrodynamic diameter), resulting in the PAA arms being more compressed in the corona and extending into the water phase beyond its normal Gaussian chain conformation. These results show that the cyclic structure attached to an amphiphilic block has a significant influence on increasing the aggregation number through a greater packing density. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011.     

Synthesis of low polydispersity polybutadiene and polyethylene stars by convergent living anionic polymerization

Star‐shaped polybutadiene stars were synthesized by a convergent coupling of polybutadienyllithium with 4‐(chlorodimethylsilyl)styrene (CDMSS). CDMSS was added slowly and continuously to the living anionic chains until a stoichiometric equivalent was reached. Gel permeation chromatography‐multi‐angle laser light scattering (GPC‐MALLS) was used to determine the molecular weights and molecular weight distribution of the polybutadiene polymers. The number of arms incorporated into the star depended on the molecular weight of the initial chains and the rate of addition of the CDMSS. Low molecular weight polybutadiene arms (M_n = 640 g/mol) resulted in polybutadiene star polymers with an average of 12.6 arms, while higher molecular weight polybutadiene arms (M_n = 16,000 g/mol) resulted in polybutadiene star polymers with an average of 5.3 arms. The polybutadiene star polymers exhibited high 1,4‐polybutadiene microstructure (88.3–93.1%), and narrow molecular weight distributions (M_w/M_n = 1.11–1.20). Polybutadiene stars were subsequently hydrogenated by two methods, heterogeneous catalysis (catalytic hydrogenation using Pd/CaCO₃) or reaction with p‐toluenesulfonhydrazide (TSH), to transform the polybutadiene stars into polyethylene stars. The hydrogenation of the polybutadiene stars was found to be close to quantitative by ¹H NMR and FTIR spectroscopy. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 828–836, 2006     

Syntheses and characterization of 16‐arm star and star diblock copolymer and AB8 9‐miktoarm star copolymer via NMRP and ROP from dendritic multifunctional macroinitiators

A dendritic macroinitiator having 16 TEMPO‐based alkoxyamines, Star‐16 , was prepared by the reaction of a dendritic macroinitiator having eight TEMPO‐based alkoxyamines, [G‐3]‐OH , with 4,4′‐bis(chlorocarbonyl)biphenyl. The nitroxide‐mediated radical polymerization (NMRP) of styrene (St) from Star‐16 gave 16‐arm star polymers with PDI of 1.19–1.47, and NMPR of 4‐vinylpyridine from the 16‐arm star polymer gave 16‐arm star diblock copolymers with PDI of 1.30–1.43. The ring‐opening polymerization of ε‐caprolactone from [G‐3]‐OH and the subsequent NMRP of St gave AB₈ 9‐miktoarm star copolymers with PDI of 1.30–1.38. The benzyl ether linkages of the 16‐arm star polymers and the AB₈ 9‐miktoarm star copolymers were cleaved by treating with Me₃SiI, and the resultant poly(St) arms were investigated by size exclusion chromatography (SEC). The SEC results showed PDIs of 1.23–1.28 and 1.18–1.22 for the star polymers and miktoarm stars copolymers, respectively, showing that they have well‐controlled poly(St) arms. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1159–1169, 2007.     

Calixarene‐centered amphiphilic A2B2 miktoarm star copolymers based on poly(ε‐caprolactone) and poly(ethylene glycol): Synthesis and self‐assembly behaviors in water

Novel calixarene‐centered amphiphilic A₂B₂ miktoarm star copolymers composed of two PCL arms and two PEG arms with calix[4]arene as core moiety were synthesized by the combination of CROP and “click” chemistry. First, a heterotetrafunctional calix[4]arene derivative with two hydroxyl groups and two alkyne groups was designed as a macroinitiator to prepare calixarene‐centered PCL homopolymers (C4‐PCL) by CROP in the presence of Sn(Oct)₂ as catalyst at 110 °C. Next, azide‐terminated PEG (A‐PEG) was synthesized by tandem treating methoxy poly(ethylene glycol)s (mPEG) with 4‐chlorobutyryl chloride and NaN₃. Finally, copper(I)‐catalyzed cycloaddition reaction between C4‐PCL and A‐PEG led to A₂B₂ miktoarm star copolymer [C4S(PCL)₂‐(PEG)₂]. ¹H NMR, FT‐IR, and SEC analyses confirmed the well‐defined miktoarm star architecture. These amphiphilic miktoarm star copolymers could self‐assemble into multimorphological aggregates in water. The calix[4]arene moieties with a cavity <1 nm on the hydrophilic/hydrophobic interface of these aggregates may provide potential opportunities to entrap guest molecules for special applications in supermolecular science. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Synthesis,characterization, and crosslinking of novel stars comprising eight poly(isobutylene‐azeotropic‐styrene) copolymer arms with allyl or hydroxyl termini. II. Stars of eight isobutylene/styrene azeotropic copolymer arms emanating from a calix[8]arene core

Our objective was the precision synthesis of novel stars consisting of a well‐defined calix[8]arene core out of which radiate eight poly(isobutylene‐aze‐styrene) [P(IB‐aze‐St)] arms fitted with crosslinkable end groups. We reached our objective by preparing the octafunctional calixarene derivative C[8]OCH₃, inducing the living azeotropic copolymerization of IB/St charges with the C[8]OCH₃/BCl₃·TiCl₄ initiating system, and end‐quenching living IB/St copolymerizations with allyltrimethylsilane. With this strategy, we obtained stars C[8]? [P(IB‐aze‐St)? CH₂CH?CH₂]₈ of various molecular weights. The number of ? CH₂CH?CH₂ termini of the arms was 8.0 ± 0.2 by quantitative ¹H NMR analysis. The eight allyl termini were quantitatively converted to ? CH₂CH₂CH₂OH termini by hydroboration/oxidation. To confirm that the latter second‐generation stars possessed eight primary alcohol end groups, we quantitatively converted the ? CH₂OH termini to ? OSi(CH₃)₃ termini, the concentration of which was quantitated by ¹H NMR spectroscopy. According to this analysis, the stars contained 8.0 ± 0.3 hydroxyl termini. The glass‐transition temperatures of the P(IB‐aze‐St) arms increased from 59 to 65 °C as the weight‐average molecular weights of the arms increased from about 2500 to about 4300 g/mol, respectively. The α and K constants of the Mark–Houwink–Sakurada relationship and the intrinsic viscosity of a representative allyl‐telechelic star were determined and compared with a linear azeotropic IB/St copolymer of similar molecular weight. The crosslinking of C[8]? [P(IB‐aze‐St)CH₂CH₂CH₂OH]₈ stars with 4,4′‐methylene bis(phenyl) diisocyanate and 2,4‐tolylene diisocyanate in various solvents afforded tightly crosslinked films of potential interest for scratch‐resistant coatings, mar‐resistant coatings, or both. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1525–1532, 2001     

A New Approach for the Synthesis of Miktoarm Star Polymers Through a Combination of Thiol–Epoxy “Click” Chemistry and ATRP/Ring‐Opening Polymerization Techniques

A new approach was developed for synthesis of certain A₃B₃‐type of double hydrophilic or amphiphilic miktoarm star polymers using a combination of “grafting onto” and “grafting from” methods. To achieve the synthesis of desired miktoarm star polymers, acetyl protected poly(ethylene glycol) (PEG) thiols (M_n = 550 and 2000 g mol^?1) were utilized to generate A₃‐type of homoarm star polymers through an in situ protective group removal and a subsequent thiol–epoxy “click” reaction with a tris‐epoxide core viz. 1,1,1‐tris(4‐hydroxyphenyl)ethane triglycidyl ether. The secondary hydroxyl groups generated adjacent to the core upon the thiol–epoxy reaction were esterified with α‐bromoisobutyryl bromide to install atom transfer radical polymerization (ATRP) initiating sites. ATRP of N‐isopropylacrylamide (NIPAM) using the three‐arm star PEG polymer fitted with ATRP initiating sites adjacent to the core afforded A₃B₃‐type of double hydrophilic (PEG)₃[poly(N‐isopropylacrylamide)] (PNIPAM)₃ miktoarm star polymers. Furthermore, the generated hydroxyl groups were directly used as initiator for ring‐opening polymerization of ε‐caprolactone to prepare A₃B₃‐type of amphiphilic (PEG)₃[poly(ε‐caprolactone)]₃ miktoarm star polymers. The double hydrophilic (PEG)₃(PNIPAM)₃ miktoarm star polymers showed lower critical solution temperature around 34 °C. The preliminary transmission electron microscopy analysis indicated formation of self‐assembly of (PEG)₃(PNIPAM)₃ miktoarm star polymer in aqueous solution. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 146–156     

Microphase separation in block CO- and terpolymers of novel macromolecular architectures

The synthesis of model miktoarm stars of the A₂B, A₃B, (AB)₂(BA)₂ and ABC (A:polyisoprene, B: polystyrene C: poly (methyl methacrylate)) was achieved by using anionic polymerization techniques and controlled chlorosilane chemistry. The microphase separation was studied by TEM, SAXS and image simulation. In the case of A₂B and A₃B miktoarm star copolymers, the morphology results are in agreement with the newly and after the experimental results developed theory of Milner. One of the symmetric in composition inverse block copolymers ((AB)₂(BA)₂) shows the OBDD morphology due to the special spatial arrangements of the arms. The first morphological results of ABC miktoarm star terpolymer show interesting future.     

Synthesis and characterization of resorcinarene‐centered amphiphilic A8B4 miktoarm star copolymers based on poly(ε‐caprolactone) and poly(ethylene glycol) by combination of CROP and “click” chemistry

Well‐defined amphiphilic A₈B₄ miktoarm star copolymers with eight poly(ethylene glycol) chains and four poly(ε‐caprolactone) arms (R‐8PEG‐4PCL) were prepared using “click” reaction strategy and controlled ring‐opening polymerization (CROP). First, multi‐functional precursor (R‐8N₃‐4OH) with eight azides and four hydroxyls was synthesized based on the derivatization of resorcinarene. Then eight‐PEG‐arm star polymer (R‐8PEG‐4OH) was prepared through “click” reaction of R‐8N₃‐4OH with pre‐synthesized alkyne‐terminated monomethyl PEG (mPEG‐A) in the presence of CuBr/N,N,N′,N″,N″′‐ pentamethyldiethylenetriamine (PMDETA) in DMF. Finally, R‐8PEG‐4OH was used as tetrafunctional macroinitiator to prepare resorcinarene‐centered A₈B₄ miktoarm star copolymers via CROP of ε‐caprolactone utilizing Sn(Oct)₂ as catalyst at 100 °C. These miktoarm star copolymers could self‐assemble into spherical micelles in aqueous solution with resorcinarene moieties on the hydrophobic/hydrophilic interface, and the particle sizes could be controlled by the ratio of PCL to PEG. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2824–2833.     

Hydrodynamic properties of A8B8 type miktoarm (Vergina) stars

The dilute solution properties of three (PS)₈(PI)₈ miktoarm (Vergina) stars were investigated by viscometry and dynamic light scattering in toluene and tetrahydrofuran (THF) (common good solvents), cyclohexane at 34.5°C (theta solvent for PS and good for PI) and dioxane at 34°C (theta solvent for PI and good for PS). Experimental intrinsic viscosity [η] and hydrodynamic radii, R_h, values in all solvents were larger for the miktoarm stars in comparison to the calculated ones using a simple model which describes the size of the copolymers as a weighted average of the sizes of the homopolymer stars with the same total molecular weight and number of arms as the copolymer. This expansion is discussed on the basis of the increased number of heterocontacts, the topological constrains imposed by the common junction point in this highly branched miktoarm architecture and the asymmetry in molecular weights of the different kinds of arms. The conformation adopted in dilute solutions can explain, to some extent, the morphological results obtained on the same materials. The ratios of viscometric to hydrodynamic radii are consistent with previous investigations on linear and star polymers and in accord with the hard sphere model. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1329–1335, 1999     

Synthesis of well‐defined miktoarm star polymers of poly(dimethylsiloxane) by the combination of chlorosilane and benzyl chloride linking chemistry

A series of novel four‐arm A₂B₂ and A₂BC and five‐arm A₂B₂C miktoarm star polymers, where A is poly(dimethylsiloxane) (PDMS), B is polystyrene (PS), and C is polyisoprene (PI), were successfully synthesized by the combination of chlorosilane and benzyl chloride linking chemistry. This new and general methodology is based on the linking reaction of in‐chain benzyl chloride functionalized poly(dimethylsiloxane) (icBnCl–PDMS) with the in‐chain diphenylalkyl (icD) living centers of PS‐DLi‐PS, PS‐DLi‐PI, or (PS)₂‐DLi‐PI. icBnCl–PDMS was synthesized by the selective reaction of lithium PDMS enolate (PDMSOLi) with the chlorosilane groups of dichloro[2‐(chloromethylphenyl)ethyl]methylsilane, leaving the benzyl chloride group intact. The icD living polymers, characterized by the low basicity of DLi to avoid side reactions with PDMS, were prepared by the reaction of the corresponding living chains with the appropriate chloro/bromo derivatives of diphenylethylene, followed by a reaction with BuLi or the living polymer. The combined molecular characterization results of size exclusion chromatography, ¹H NMR, and right‐angle laser light scattering revealed a high degree of structural and compositional homogeneity in all miktoarm stars prepared. The power of this general approach was demonstrated by the synthesis of a morphologically interesting complex miktoarm star polymer composed of two triblock terpolymer (PS‐b‐PI‐b‐PDMS) and two diblock copolymer (PS‐b‐PI) arms. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6587–6599, 2006     

Transition‐metal compatibilization of poly(4‐bromostyrene) and polybutadiene via palladium(0) catalyzed reactions

Immiscible blends of 1,2‐polybutadiene and poly(4‐bromostyrene) can be compatibilized by rather low concentrations of Pd⁽⁰⁾[P(C₆H₅)₃]₄ at ambient temperature and 60 °C under argon. Two distinct glass‐transition temperatures merge into a single glass‐transition temperature at high enough concentrations of Pd⁽⁰⁾ (i.e., 2 or 3 mol %). Compatibilization does not occur if Pd⁽⁰⁾ is absent, triphenylphosphine is added without Pd⁽⁰⁾, or polystyrene is not functionalized. The methodology described herein is also useful for inducing melting‐point depression of 2,7‐dibromofluorene in ternary complexes with 1,2‐polybutadiene and Pd⁽⁰⁾. A 72/28 complex of poly(4‐bromostyrene) and 1,2‐polybutadiene with 5.5 mol % Pd⁽⁰⁾ exhibits a reinforced rubbery response with a modulus of 1.2 × 10⁷ N/m², a fracture strain of 235%, and a single glass‐transition temperature. Mechanical properties of these compatibilized ternary systems compare well with those of styrene–butadiene block copolymers, particularly above 100% strain. A five‐step mechanism that includes oxidative addition, olefin coordination, migratory insertion, β‐hydride elimination, and reductive elimination in the coordination sphere of the transition metal is proposed to illustrate how either poly(4‐bromostyrene) or 2,7‐dibromofluorene is linked covalently to alkene side groups in the diene polymer via the Heck reaction. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 677–688, 2001     

Well‐defined non‐linear polyethylene‐based macromolecular architectures

Polyethylene (PE)‐based 3‐ and 4‐miktoarm star [PE(PCL)₂, PE(PCL)₃] and H‐type [(PCL)₂PE(PCL)₂] block copolymers [polycaprolactone (PCL)] were synthesized by a combination of polyhomologation, chlorosilane chemistry, and ring opening polymerization (ROP). The following steps were used for the synthesis of the miktoarm stars: (a) reaction of a hydroxy‐terminated polyethylene (PE‐OH), prepared by polyhomologation of dimethylsulfoxonium methylide with a monofunctional boron initiator followed by oxidation/hydrolysis, with chloromethyl(methyl)dimethoxysilane or chloromethyltrimethoxysilane; (b) hydrolysis of the produced ω‐di(tri)methoxysilyl‐polyethylenes to afford ω‐dihydroxy‐polyethylene (difunctional initiator) and ω‐trihydroxy‐polyethylene (trifunctional initiator); and (c) ROP of ɛ‐caprolactone with the difunctional (3‐miktoarm star) or trifunctional macroinitiator (4‐miktoarm star), in the presence of 1‐tert‐butyl‐2,2,4,4,4‐pentakis(dimethylamino)‐2λ⁵,4λ⁵‐catenadi(phosphazene) (t‐BuP₂). The H‐type block copolymers were synthesized using the same strategy, but with a difunctional polyhomologation initiator. All intermediates and final products were characterized by HT‐GPC, ¹H NMR and FTIR analyses. Thermal properties of the PE precursors and all final products were investigated by DSC and TGA. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2129–2136     

Cyclodextrin‐overhanging hyperbranched core‐double‐shell miktoarm architectures: Synthesis and gradient stimuli‐responsive properties

We report the synthesis and gradient stimuli‐responsive properties of cyclodextrin‐overhanging hyperbranched core‐double‐shell miktoarm architectures. A ionic hyperbranched poly(β‐cyclodextrin) (β‐CD) core was firstly synthesized via a convenient “A₂+B₃” approach. Double‐layered shell architectures, composed of poly(N‐isopropyl acrylamide) (PNIPAm) and poly(N,N‐dimethylaminoethyl methacrylate) (PDMAEMA) miktoarms as the outermost shell linked to poly(N,N‐diethylaminoethyl methacrylate) (PDEAEMA) homoarms which form the inner shell, were obtained by a sequential atom transfer radical polymerization (ATRP) and parallel click chemistry from the modified hyperbranched poly(β‐CD) macroinitiator. The combined characterization by ¹H NMR, ¹³C NMR, ¹H‐²⁹Si heteronuclear multiple‐bond correlation (HMBC), FTIR and size exclusion chromatography/multiangle laser light scattering (SEC/MALLS) confirms the remarkable hyperbranched poly(β‐CD) core and double‐shell miktoarm architectures. The gradient triple‐stimuli‐responsive properties of hyperbranched core‐double‐shell miktoarm architectures and the corresponding mechanisms were investigated by UV–vis spectrophotometer and dynamic light scattering (DLS). Results show that this polymer possesses three‐stage phase transition behaviors. The first‐stage phase transition comes from the deprotonation of PDEAEMA segments at pH 9–10 aqueous solution under room temperature. The confined coil‐globule conformation transition of PNIPAm and PDMAEMA arms gives rise to the second‐stage hysteretic cophase transition between 38 and 44 °C at pH 10. The third‐stage phase transition occurs above 44 °C at pH = 10 attributed to the confined secondary conformation transition of partial PDMAEMA segments. This cyclodextrin‐overhanging hyperbranched core‐double‐shell miktoarm architectures are expected to solve the problems of inadequate functionalities from core layer and lacking multiresponsiveness for shell layers existing in the dendritic core‐multishell architectures. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Micelles possessing mixed cores and thermoresponsive shells fabricated from well‐defined amphiphilic ABC miktoarm star terpolymers

Amphiphilic ABC miktoarm star terpolymers consisting of polystyrene, poly(ε‐caprolactone), and poly(N‐isopropylacrylamide) arms, PS(‐b‐PNIPAM)‐b‐PCL, were synthesized via a combination of atom transfer radical polymerization, ring‐opening polymerization (ROP), and click chemistry. Difunctional PS bearing an alkynyl and a primary hydroxyl moiety at the chain end, PS‐alknyl‐OH, was prepared by reacting azido‐terminated PS with an excess of 3,5‐bis(propargyloxy)benzyl alcohol (BPBA) under click conditions. The subsequent ROP of ε‐caprolactone using PS‐alknyl‐OH macroinitiator afforded PS(‐alkynyl)‐b‐PCL copolymer bearing an alkynyl moiety at the diblock junction point. Target PS(‐b‐PNIPAM)‐b‐PCL amphiphilic ABC miktoarm star terpolymers were then prepared via click reaction between PS(‐alkynyl)‐b‐PCL and an excess of azido‐terminated PNIPAM (PNIPAM‐N₃). The removal of excess PNIPAM‐N₃ was accomplished by “clicking” onto alkynyl‐functionalized Wang resin. All the intermediate and final products were characterized by gel permeation chromatography, ¹H NMR, and FTIR. In aqueous solution, the obtained amphiphilic ABC miktoarm star terpolymer self‐assembles into micelles possessing mixed PS/PCL cores and thermoresponsive shells, which were further characterized by dynamic laser light scattering and transmission electron microscopy. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1636–1650, 2009     

Controlled synthesis of pH‐responsive amphiphilic A2B2 miktoarm star block copolymer by combination of SET‐LRP and RAFT polymerization

The pH‐responsive amphiphilic A₂B₂ miktoarm star block copolymer, poly(acrylic acid)₂‐poly(vinyl acetate)₂ [(PAA)₂(PVAc)₂], with controlled molecular weight and well‐defined structure was successfully synthesized via combination of single‐electron transfer‐mediated living radical polymerization (SET‐LRP) and reversible addition‐fragmentation chain transfer (RAFT) polymerization methods. First, the precursor two‐armed poly(t‐butyl acrylate) (PtBA)₂ functionalized with two xanthate groups was prepared by SET‐LRP of t‐butyl acrylate in acetone at 25 °C using the novel tetrafunctional bromoxanthate (Xanthate₂‐Br₂) as an Iniferter (initiator‐transfer agent‐terminator) agent. The polymerization behavior showed typical LRP natures by the first‐order polymerization kinetics and the linear dependence of molecular weight of the polymer on the monomer conversion. Second, the A₂B₂ miktoarm star block copolymer (PtBA)₂(PVAc)₂ was prepared by RAFT polymerization of VAc using (PtBA‐N₃)₂(Xanthate)₂ obtained as the macro‐RAFT agent. Finally, the pH‐sensitive A₂B₂ amphiphilic miktoarm star block copolymer poly(acrylic acid)₂‐poly(vinyl acetate)₂ ((PAA)₂(PVAc)₂) was obtained by selectively cleavage of t‐butyl esters of (PtBA)₂(PVAc)₂. All the miktoarm star block copolymers were characterized by GPC, ¹H‐NMR, and FT‐IR spectra. The self‐assembly behaviors of the amphiphilic A₂B₂ miktoarm block copolymers (PAA)₂(PVAc)₂ were also investigated by transmission electron microscopy. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2009     

Thermodynamic interaction parameter of star–star polybutadiene blends

The value of the thermodynamic interaction parameter, χ_eff, for star–star polybutadiene blends was determined with small‐angle neutron scattering. Blends in which the stars have the same number of arms and blends in which the stars have different numbers of arms are investigated. For star–star isotopic blends with components having the same number of arms, the presence of the junction point of the star leads to a value of χ_eff that is larger than that for an analogous linear–linear isotopic blend. However, changes in the value of χ_eff resulting from small dissimilarities in the number of arms of the two components in the isotopic star–star blends were too small to resolve. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 247–257, 2003     

Synthesis and morphological characterization of miktoarm star copolymers (PCL)2(PS)2 of poly(ε‐caprolactone) and polystyrene

An heterofunctional initiator combining two reactive sites for ring opening polymerization and two for atom transfer radical polymerization was used to prepare three A₂B₂ miktoarm star copolymers of poly(ε‐caprolactone) (PCL) and polystyrene (PS). The morphology and thermal properties were studied by transmission electron microscopy, polarized light optical microscopy, and differential scanning calorimetry. The (PCL)₂(PS)₂ 72/28 (72 wt % PCL) sample was crystallized from a disordered melt. In this case, crystallization drove the structure formation and a lamellar morphology was obtained at the microdomain level, while spherulites were observed at a superstructural level. The other two samples, 39/61 and 27/73, with lower PCL content and higher total molecular weight, were not able to form spherulites. Surprisingly, these miktoarm star copolymers exhibited hexagonally packed cylinders and spheres morphologies, respectively, instead of lamellar and cylindrical morphology. Such unexpected and novel behavior was explained in terms of the higher resistance of the arms to be stretched in a miktoarm star copolymer when compared with the corresponding linear diblocks. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5387–5397, 2007