Synthesis of Hyperbranched Poly(phenylacetylene)s ContainingPendant Alkyne Groups by One-Pot Pd-Catalyzed Copolymerizationof Phenylacetylene with DiynesZhongmin Dong and Zhibin Ye*

Bharti School of Engineering, Laurentian University, Sudbury, Ontario P3E 2C6, Canada

*S Supporting Information

ABSTRACT: We report in this article the convenient synthesis ofa class of hyperbranched poly(phenylacetylene)s (HBPPAs)containing various branching densities and different contents ofpendant alkyne groups through one-pot chain-growth copoly-merization of phenylacetylene (PA) with a diyne comonomer (1,3-or 1,4-diethynylbenzene (DEB)). The polymerization wasfacilitated with the use of an in situ generated cationicdiphosphine-ligated Pd(II) catalyst system. Serving effectively asdifunctional cross-linker in the polymerization, the diyne comonomer first undergoes monoinsertion to render pendant alkynegroups, which can be further enchained to generate branching structures. A systematic investigation has been undertaken to studythe effects of various polymerization parameters, including diyne/PA feed ratio, diyne type, temperature, and solvent, on thepolymerization, polymer structure and topology. With either 1,3- or 1,4-DEB, a convenient tuning of polymer topology fromlinear to hyperbranched can be achieved by simply increasing diyne/PA feed ratio. Relative to 1,3-DEB, 1,4-DEB is more effectivein rendering branching structures since the pendant alkyne groups suffer less steric effect from the polymer backbone and arethus more reactive. As to the solvent, a dichloromethane/methanol mixture (at vol. ratio of 13:3) was shown to better help theformation of branching structures than methanol alone as the resulting polymers can dissolve well in the former while precipitatein the latter. Because of their possession of the valuable pendant alkyne groups, these polymers have also been demonstrated fortheir use as building blocks in the synthesis of core−shell structured star polymers containing a HBPPA core and polystyrenearms through their Cu-catalyzed “click” reaction with an azide-ended polystyrene.

■ INTRODUCTIONAs a novel family of polymers of unique chain architectures andsome striking material properties, hyperbranched polymershave been at the forefront of polymer research for more than 2decades by now. Resembling dendrimers, hyperbranchedpolymers possess the distinct three-dimensional sphericalarchitecture featured with extensive branch-on-branch struc-tures, which imparts them many unique properties includinglow melt/solution viscosity, good solubility, and abundance ofreactive sites or functionalities.1 Different from dendrimersoften requiring multistep sophisticated synthesis, hyper-branched polymers show the remarkable advantage in theirconvenient one-pot one-step synthesis, which promotes theirlarge-scale production for commercial applications.1

Various synthetic strategies have been developed for thesynthesis of a great variety of hyperbranched polymers fromnumerous monomer stocks. First described by Flory,2

condensation polymerization of ABx (x ≥ 2) monomers orA2 + By (y ≥ 3) monomer combinations, whereas A and B arecomplementary functionalities that could react with each other,is the classical synthetic strategy. Later on, other elegant self-condensing chain-growth polymerization strategies, includingvinyl polymerization,3 ring-opening polymerization,4 andproton transfer polymerization,5 have been developed withthe use of latent ABx, AB*, or similar monomers.1 In particular,

self-condensing vinyl polymerization and proton transferpolymerization strategies have enabled the synthesis ofhyperbranched polymers from vinyl monomers throughdifferent polymerization mechanisms, including various con-trolled/“living” radical polymerizations, anionic polymerization,etc.1 Furthermore, a chain walking polymerization strategy hasbeen developed specifically for the synthesis of hyperbranchedpolyethylenes and functionalized polymers through ethylenecoordination polymerization with the use of Brookhart Pd−diimine catalysts, which are featured with unique chain walkingmechanism.6,7 In addition, copolymerization of monovinylicmonomer with multivinylic monomer and homopolymerizationof divinyl monomer have also been demonstrated to obtainhyperbranched structures conveniently.8 Involving only single-bond or double-bond reactions, the hyperbranched polymerssynthesized through the above condensation or additionpolymerization strategies are generally electronically inactiveand are thus commonly used as commodity materials.With specific regard to electronically active alkyne polymers

containing conjugated double or triple bonds, various polymer-ization strategies have also been developed for synthesis of their

Received: April 12, 2012Revised: May 28, 2012Published: June 13, 2012

Article

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hyperbranched polymers though in general the advancementshave lagged behind those of vinyl polymers.9 Hyperbranchedalkyne polymers are particularly appealing given their valuableelectronic, optical, and photonic properties, besides the genericadvantageous properties of hyperbranched polymers. Thepolymerization strategies developed thus far often involvenonlinear polycondensation of specially synthesized di- ormultifunctional alkyne monomers and necessary comonomershaving complementary functionalities, and utilize the character-istic coupling/addition reactions of the alkyne triple bonds(such as Diels−Alder cycloaddition,10 alkyne cyclotrimeriza-tion,11 click cycloaddition,12 Sonogashira coupling,13 hydro-silylation,14 Glaser−Hay coupling,15 etc.) for topologyconstruction.9 The use of chain-growth polymerization forthe synthesis of hyperbranched polymers from alkynemonomers, however, has not yet been demonstrated. Jimenezet al. have recently reported that some poly(phenylacetylene)samples, synthesized with several Rh(I) catalysts bearing

hemilabile phosphine ligands, contain branching structures.16

Therein, the branching structures were suggested to result fromthe incorporation of in situ generated macromonomer or chaintransfer to polymer. Though interesting, the level of branchingin those polymers was, however, found to be very low (e.g.,average branching frequency about 0.86 per chain) and thus donot qualify as commonly defined hyperbranched polymers.16

Some studies on the chain-growth polymerization of diynes(such as 1,4-diethynylbenzene (1,4-DEB)) have been reportedusing different transition metal catalysts.17 However, theresulting polymers often contain or appear as cross-linkedgels. Though the polymers were soluble in some cases,17b−d

their topological characterizations and control, however, werenot reported or demonstrated.In this article, we report a chain-growth polymerization

strategy for the synthesis of hyperbranched poly-(phenylacetylene)s (HBPPAs) via the copolymerization ofphenylacetylene (PA) with diynes (1,3- or 1,4-DEB) with the

Table 1. HBPPAs Synthesized via Copolymerization of PA and Different Diynesa

NMR characterizationsctriple-detection GPCcharacterizationsd

samples diyne solvbT

(°C)[diyne]0/[PA]0

yield(%)

incorp [diyne]/[PA]

diynecont

(mol %)

diynedual-insert(%) yne %

branchdensity(%)

Mw(kDa) PDI M−H α

[η]w(mL/g)

homoPPA C + M 25 34 0 0 39 1.37 0.65 16.5HBPPA1 1,3-DEB C + M 25 0.05 31 0.04 4 2.3 4.0 0.09 52 1.81 0.50 11.4HBPPA2 1,3-DEB C + M 25 0.10 32 0.09 8 6.2 7.7 0.5 78 1.75 0.43 9.9HBPPA3 1,3-DEB C + M 25 0.20 28 0.18 16 7.2 14 1.1 92 1.57 0.27 9.5HBPPA4 1,3-DEB C + M 25 0.30 26 0.30 23 7.3 21 1.7 114 1.66 0.29 10.0HBPPA5 1,3-DEB C + M 25 4.4 20 0.87 46 8.8 42 4.1 42 1.45 0.23 6.5HBPPA6 1,3-DEB C + M 25 10.4 25 1.69 63 14 54 8.6 95 1.42 0.33 7.9homoDEBe 1,3-DEB C + M 25 44 100HBPPA7 1,3-DEB M 25 0.20 5.8 0.03 3 4.4 2.4 0.1 59 2.47 0.39 13.9HBPPA8 1,4-DEB C + M 25 0.05 32 0.04 4 28 3.0 1.2 74 2.44 0.39 11.2HBPPA9 1,4-DEB C + M 25 0.10 30 0.08 7 26 5.7 2.0 71 1.62 0.33 9.4HBPPA10 1,4-DEB C + M 25 0.20 30 0.19 16 24 12 3.8 96 1.52 0.26 8.4HBPPA11f 1,4-DEB C + M 25 0.20 58 0.20 16 25 13 4.0 98 1.47 0.28 7.4HBPPA12 1,4-DEB T + M 25 0.20 29 0.20 16 24 14 4.0 64 1.90 0.24 7.3HBPPA13 1,4-DEB C + M 25 0.30 25 0.28 22 18 18 3.9 145 1.42 0.27 8.8HBPPA14 1,4-DEB C + M 10 0.20 14 0.18 15 30 11 4.5 350 1.65 0.18 11.3HBPPA15 1,4-DEB C + M 35 0.20 32 0.17 15 16 14 2.4 69 1.71 0.30 8.5HBPPA16 ODY C + M 25 0.01 16 0.003 0.3 0.2 0.3 0.0 64 1.85 0.44 13.0HBPPA17 ODY C + M 25 0.05 13 0.03 2 1.5 2.4 0.04 43 1.34 0.37 8.2HBPPA18 ODY C + M 25 0.30 1.3 0.09 8 3.5 7.0 0.25 57 1.37 0.48 9.4

aOther polymerization conditions: in all runs except HBPPA5, HBPPA6, HBPPA11, and homoDEB, [PA] = 0.61 M, [Pd(OAc)2] = [diphosphineligand] = 1.56 mM, polymerization time =1 h, total volume =48 mL; in runs for HBPPA5 and HBPPA6, [1,3-DEB] = 0.64 M, [Pd(OAc)2] =[diphosphine ligand] = 6 mM, polymerization time =1 h, total volume =3.8 mL; in the run for homoDEB, [1,3-DEB] = 0.96 M, [Pd(OAc)2] =[diphosphine ligand] = 2.48 mM, polymerization time =6 h, total volume =2.5 mL; in the run for HBPPA11, [PA] = 0.61 M, [Pd(OAc)2] =[diphosphine ligand] = 2.34 mM, polymerization time =1 h, total volume =48 mL. For all runs, one drop of MSA was added to activate the catalyst.bA solvent mixture of CH2Cl2:MeOH = 13:3 (C + M) was used in all runs except for HBPPA7, where pure MeOH (M) was used, and forHBPPA12, where solvent mixture of THF:MeOH = =13:3 (T + M) was used. cThe ratio of diyne molar content to PA content, diyne molar contentin the copolymer, the percentage of diyne dual-insertion (both triple bonds enchained), the content of triple bond in the polymer (yne %), andbranching density were calculated from 1H NMR spectrum of each polymer. These parameters are defined as follows: incrop. [diyne]/[PA] = thenumber of incorporated diyne units/the number of incorporated PA units; diyne cont. (%) = the number of incorporated diyne units/the totalnumber of all incorporated monomer units ×100%; diyne dual-insert (%) = the number of dual-inserted diyne units/the number of incorporateddiyne units in the polymer ×100%; yne % = the number of pendant triple bonds/the total number of all incorporated monomer units ×100%;branch density (%) = the number of dual-inserted diyne units/the total number of all incorporated monomer units ×100%. The calculationprocedure and equations are detailed in Supporting Information. dTriple-detection GPC characterizations were carried out with THF as the eluent at33 °C. Weight-average molecular weight (Mw) and polydispersity index (PDI) data were obtained from the light scattering detector. The intrinsicviscosity data were obtained from the viscosity detector. The Mark−Houwink α values (M−H α) were obtained with the use of the absolutemolecular weight data determined with the light scattering detector. eMacrogelation occurred. Sample was not characterized. fA higher catalystdosage (50% higher) was used in this run for HBPPA11 compared to that for HBPPA10. See above.

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use of an in situ generated cationic diphosphine-ligated Pd(II)catalyst system, palladium acetate (Pd(OAc)2)/α,α′-bis(di-tert-butylphosphino)-o-xylene/methanesulfonic acid. In this poly-merization strategy, the diyne comonomers act as difunctionalcross-linker, generating the desired branch-on-branch top-ologies. A class of HBPPAs having tunable degrees of branchingand molecular weights has thus been conveniently producedfrom commercially available alkyne monomers while at mildreaction conditions. Moreover, these hyperbranched polymerscontain the valuable pendant alkyne moieties, which endowsthem the benefit for postpolymerization functionalization andthe construction of polymers of more complex architectures asbuilding blocks. To the best of our knowledge, this representsthe first piece of work demonstrating the successful synthesis ofHBPPAs via the chain-growth polymerization strategy.

■ EXPERIMENTAL SECTIONMaterials. HPLC-grade CH2Cl2 (99.5%, Fisher Scientific) and

tetrahydrofuran (THF, 99.5%, Fisher Scientific) were deoxygenatedand dried by using a solvent purification system (InnovativeTechnology) before use. Methanol (ACS reagent, Fisher Scientific)was dried by 3 Å/5 Å molecular sieves before use. CuBr (98%,Aldrich) was purified before use by stirring in glacial acetic acid,followed with washing with methanol and drying under vacuum.Styrene (99%, Aldrich) was distilled under vacuum to remove theinhibitor before use. Other chemicals, including palladium(II) acetate(Pd(OAc)2, 98%, Strem Chemicals), α,α′-bis(di-tert-butylphosphino)-o-xylene (97%, Strem Chemicals), phenylacetylene (PA, 98%,Aldrich), 1,3-diethynylbenzene (1,3-DEB, 97%, Aldrich), 1,4-dieth-ynylbenzene (1,4-DEB, 96%, Aldrich), 1,7-octadiyne (98%, Aldrich),methanesulfonic acid (99.5%, Aldrich), 2,2′-bipyridine (bipy, 99%,Aldrich), sodium azide (99.5%, Aldrich), N,N,N′,N″,N″-pentamethyl-diethylenetriamine (PMDETA, 99%, Aldrich), ethyl 2-bromoisobuty-rate (EBiB, 99%, Aldrich), and 2,6-di-tert-butyl-4-methylphenol (BHT,> 99%, Aldrich) were all used as received.General Polymerization Procedure for Synthesizing

HBPPAs. The following is the typical polymerization procedure,which was used for the synthesis of HBPPA1 in Table 1. A flame-driedflask was charged with PA (3.0 g, 29.4 mmol), 1,3-DEB (185 mg, 1.47mmol), and CH2Cl2 (39 mL) under nitrogen protection. The solutionwas mixed with a magnetic stirrer. The flask was covered withaluminum foil. One drop of methanesulfonic acid (ca. 0.05 mL) wasadded into the solution under stirring. In the meantime, Pd(OAc)2(16.8 mg, 75 μmol), diphosphine ligand (29.6 mg, 75 μmol), andMeOH (9 mL) were mixed in a 20 mL vial under N2 protection. Themixture was sonicated at room temperature for 10 min until all thesolids were dissolved. Then the catalyst mixture was injected into theflask under N2 protection to start the polymerization. The polymer-ization lasted for 1 h at room temperature under N2 protection. At theend, the polymerization mixture was poured into 2%-acidifiedmethanol (ca. 300 mL) to terminate the polymerization. The orangepolymer precipitate underwent a redissolution−precipitation proce-dure for 3 times to remove possible catalyst and monomer residues.Finally, the polymer was dried under vacuum at room temperature for24 h and then weighed (0.989 g).Synthesis of Linear Narrow-Distributed Polystyrene (PS)

with a Br Chain End. The Br-ended PS was synthesized via atom-transfer radical polymerization (ATRP).18 A flame-dried Schlenk flaskwas charged with 10 mL of styrene (9.09 g, 0.09 mol), 129 mg of CuBr(0.9 mmol), and 156 mg of PMDETA (0.9 mmol) under N2protection. The mixture was subject to three freeze−pump−thawcycles, and the flask was finally charged with nitrogen. Degassed EBiB(175.6 mg 0.9 mmol) was then injected into the reactor under N2protection. The reaction system was placed in a thermostated oil bathset at 90 °C to start the polymerization. The reaction was stopped at apredetermined monomer conversion by pouring the solution into 2%-acidified methanol (100 mL). The polymer precipitate was redissolved

in THF and washed with a large amount of methanol for 3 times. Theproduct was finally dried overnight under vacuum at 60 °C.

Synthesis of Azide-Ended PS. The Br-ended polystyrene (6.0 g,Mn = 6.6 kDa, contain 0.9 mmol Br- chain ends), NaN3 (0.1 g, 15mmol), and DMF (40 mL) were mixed in a round-bottom flask. Themixture was kept stirred at room temperature overnight to have all Brgroups converted into azide groups. After the reaction, the azide-endedPS was precipitated with a methanol/water mixture (1:1 v/v) andwashed with methanol three times before vacuum drying.

Synthesis of Star Polymer Having a HBPPA Core and PSArms by Cu-Catalyzed Azide−Alkyne Cycloaddition. CuBr (2mg, 14 μmol) and 2,2′-bipyridine (4.7 mg, 28 μmol) were mixed in adried test tube. The tube was sealed with a rubber septum, vacuumed,and charged with N2. Simultaneously, HBPPA10 (0.099 g, Mw = 96kDa, PDI = 1.52, yne % = 12%, containing 112 μmol alkyne groups,see Table 1), azide-ended PS (92 mg, Mn = 6.6 kDa, PDI = 1.02,containing 14 μmol Br group), BHT (10 mg, 0.05 mmol, 5 mol % inreference to the double bonds in HBPPA10), and THF (1 mL) weremixed in a sealed vial. After three freeze−pump−thaw cycles, thepolymer solution was injected into the test tube. The reaction systemwas protected under dry nitrogen throughout the reaction process.Samples were taken at predetermined times during the reaction tomonitor the PS conversion and molecular weight development of thestar copolymer with GPC. For each reaction solution sampled fromthe reactor, the polymer was obtained by precipitating the polymermixture in acidified methanol, followed with further wash with a largeamount of methanol for three times, and subsequent drying undervacuum at room temperature.

Characterizations and Measurements. Proton nuclear magneticresonance (1H NMR) spectra of the polymers were all obtained on aVarian Gemini 2000 spectrometer (200 MHz) at ambient temperaturewith CDCl3 as solvent.

13C nuclear magnetic resonance (13C NMR)spectra of the polymers were obtained on a Bruker AV500spectrometer (125 MHz) at ambient temperature with CDCl3 as thesolvent. Fourier-transformed infrared (FTIR) spectra of the polymerswere obtained on a Thermo Scientific Nicolet 6700 Analytical FTIRspectrometer. The samples were prepared as pellets using spectro-scopic-grade KBr.

Polymer characterizations with triple-detection gel permeationchromatography (GPC) were carried out on a Polymer LaboratoriesPL-GPC220 system equipped with a triple-detection array comprisedof a differential refractive index (DRI) detector (from PolymerLaboratories), a three-angle (45, 90, and 135°) light scattering (LS)detector (from Wyatt Technology) at a laser wavelength of 687 nm,and a four-bridge capillary viscosity detector (from PolymerLaboratories). One guard column (PL# 1110−1120) and three 30cm columns (PLgel 10 μm MIXED-B 300 × 7.5 mm) were used forpolymer fractionation. HPLC-grade THF was used as the mobilephase at a flow rate of 1.0 mL/min. The whole GPC system, includingcolumns and detectors, was maintained at 33 °C. Polymer solutionswith a concentration between 1.5 and 30 mg/mL were injected intothe columns at an injection volume of 200 μL. Astra software fromWyatt Technology was used to collect and analyze the data from allthree detectors. Two polystyrene narrow standards (from PressureChemicals) with a weight-average molecular weight (Mw) of 30 and200 kg/mol, respectively, were used for the normalization of LS signalsfrom three detecting angles, and for the determination of interdetectordelay volume and band broadening, respectively. The DRI increment(dn/dc) value of 0.286 mL/g19 was used for all PPA samples, and avalue of 0.185 mL/g was used for polystyrenes. For the star polymerproduct synthesized by “click” reaction between HBPPA10 and azide-ended PS, an average dn/dc value was calculated from the masscontent of HBPPA core (xHBPPA) according to the following equation:

= × +

× −

n c n c x n c

x

( d /d ) ( d /d ) ( d /d )

(1 )

avg HBPPA HBPPA PS

HBPPA

in which (dn/dc)HBPPA = 0.286 mL/g, (dn/dc)PS = 0.185 mL/g, andxHBPPA was calculated from the mass of the HBPPA10 used and themass of the reacted PS.

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■ RESULTS AND DISCUSSION

Copolymerization of Phenylacetylene (PA) withDiynes. An in situ generated cationic diphosphine-ligatedPd(II) catalyst system, palladium acetate (Pd(OAc)2)/α,α′-bis(di-tert-butylphosphino)-o-xylene/methanesulfonic acid, wasemployed herein for the copolymerization of PA with diynes.This and similar diphosphine-ligated Pd(II) catalyst systemshave been extensively used for the alternating copolymerizationof alkenes with CO to afford polyketones,20 and have also beendemonstrated for highly active polymerization of alkynemonomers.21,22 In the catalyst system, cationic (P∧P)Pd2+

species are generated in situ through protonation of the acetateligands with methanesulfonic acid.20 Compared to earlytransition metal catalysts, the catalyst systems show remarkablylow oxophilicity and enable polymerization in protic media(e.g., methanol and water). Similar catalyst systems have beensuccessfully used by Mecking et al. to carry out emulsionpolymerization of both acetylene and arylacetylene monomersin aqueous media for the preparation of polymer nanoparticledispersions.23

Three diyne comonomers, 1,3-DEB, 1,4-DEB, and 1,7-octadiyne (ODY), were employed herein to study the effect ofdiyne type on the synthesis of hyperbranched polymers. Whilethe monofunctional PA contributes to the formation ofpolymer backbone, the diyne comonomer, which acts as across-linker, is key to the construction of hyperbranchedtopology in the polymerization. The incorporation of diynecomonomer renders first pendant alkyne groups andsubsequent interchain enchainment of some of the pendantalkyne groups leads to the formation of branching structures(see Scheme 1) and enhanced polymer molecular weight.

Meanwhile, intrachain enchainment of the pendant alkynegroups can also occur, rendering intrachain cyclic structures butwith no effect on polymer molecular weight. Herein, theintrachain cyclic structures are also counted in as branchingstructures as these two types of chain microstructures cannot bedifferentiated with the current nuclear magnetic resonance(NMR) technique used for polymer structural characterization.The extent of branching in the resulting polymers shoulddepend on the feed concentration ratio of diyne to PA([diyne]0/[PA]0). With the other polymerization parametersfixed, raising the ratio is expected to increase the extent ofbranching and render more compact chain topology. However,excessive diyne concentration (above critical gelation concen-tration) will lead to undesired macrogelation, which should beavoided.Table 1 lists all the polymers synthesized under various

conditions. The polymerization runs were designed with theaim of investigating the effects of various polymerizationparameters, including [diyne]0/[PA]0 ratio, diyne type, temper-ature, and solvent, on the polymerization, polymer structure,and topology. In the table, HBPPA1−HBPPA4, along withhomoPPA as the linear PA homopolymer control sample, weresynthesized with 1,3-DEB as the diyne at different [diyne]0/[PA]0 ratios (in the range of 0−0.30) while at a fixed [PA]0(0.61 M) at 25 °C. Similarly, two other sets of polymers(HBPPA8−HBPPA10 and HBPPA13; HBPPA16−HBPPA18)were synthesized at the same or similar [diyne]0/[PA]0 ratiosbut with the use of different diynes (1,4-DEB and ODY,respectively). In addition, HBPPA5 and HBPPA6 weresynthesized at much higher [diyne]0/[PA]0 ratios (4.4 and10.4, respectively) at a fixed 1,3-DEB concentration (0.64 M)to investigate the effect of further enhancing [diyne]0/[PA]0ratios. Meanwhile, homopolymerization of 1,3-DEB (homo-DEB in Table 1) was also carried out at the same temperature(25 °C) as a control run. While macrogelation occurred inhomoDEB run with completely insoluble polymer produced,no gelation was observed in all the other runs.HBPPA11 was synthesized at the same conditions as

HBPPA10 except the use of a higher Pd catalyst dosage(50% higher) to investigate the effect of catalyst concentration.However, the experiment results indicated that this increase ofPd catalyst dosage/concentration only causes the increase ofpolymer yield, whereas the molecular weight and chaintopology of HBPPA11 are similar to those of HBPPA10.Moreover, to investigate the effects of temperature, HBPPA14and HBPPA15 were synthesized with 1,4-DEB at the sameconditions as HBPPA10 except the different polymerizationtemperature (10 and 35 °C, respectively, as opposed to 25 °Cfor HBPPA10). The polymerization time for all runs except therun of homoDEB is 1 h. A further extension of the reactiontime to 3 h for a separate run carried out at the same conditionas HBPPA10 was found to only affect the polymer yield withnegligible changes in both molecular weight and chain topologyof the polymer product.The solvent was found to greatly affect the polymerization,

polymer structure, and topology. The use of methanol isessential to achieve successful polymerization with the Pd(II)catalyst system employed herein. While it occurred inmethanol, homopolymerization of PA did not take place,with no polymer produced, when dichloromethane or THFalone was used as the solvent. This confirms the role ofmethanol in assisting chain initiation in the polymerization: thein situ generated (P∧P)Pd2+ species reacts with methanol to

Scheme 1. Synthesis of HyperbranchedPoly(phenylacetylene)s (HBPPAs) Bearing Multiple AlkyneGroups by Copolymerization of Phenylacetylene (PA) with1,3- or 1,4-Diethynylbenzene (DEB), and SubsequentSynthesis of Core−Shell Structured Star Polymers with aHBPPA Core and Linear Polystyrene (PS) Arms by a Cu-Catalyzed Alkyne−Azide “Click” Reaction

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form a metal hydride to promote polymerization.20,24 However,polymer precipitated out once produced in methanol due to itsinsolubility. In the case of copolymerization runs, we reasonedsuch polymer precipitation will reduce the extent of branchingsince the pendant alkyne groups in the precipitated polymercan not be further enchained. A homogeneous system with theproduced polymer well dissolved in the solution shouldmaximize the incorporation of the pendant alkyne groups toform hyperbranched topology. Considering this, we chose, afteroptimization, a dichloromethane/methanol mixture at avolumetric ratio of 13:3 as the solvent in all runs exceptthose for HBPPA7 and HBPPA12, to render successfulpolymerization and meanwhile to maintain good polymersolubilization. All the polymers produced, except the gelledhomoDEB, were found completely soluble in the solventmixture. To demonstrate the solvent effect, HBPPA7 andHBPPA12 were synthesized for comparison. HBPPA7 wassynthesized at the same condition as HBPPA3 except with pureMeOH as solvent, whereas HBPPA12 was synthesized at thesame condition as HBPPA10 except with tetrahydrofuran/methanol mixture (13:3 v/v) as solvent.Following their synthesis, all the polymers, which were

colored between orange and red, were subsequently charac-terized with NMR spectroscopy, Fourier-transformed infrared

(FTIR) spectroscopy, and triple-detection gel permeationchromatography (GPC) incorporating online differentialrefractive index (DRI), light scattering (LS), and viscositydetectors. In particular, 1H NMR technique was used toelucidate and quantify polymer microstructure. Figure 1 showsthe 1H NMR spectra of three representative copolymers(HBPPA4, HBPPA13, and HBPPA17) obtained with the threerespective diyne comonomers, along with that of linearhomoPPA for comparison. With each diyne, two insertionmodes should exist, with dual-insertion of both triple bondsrendering a branching structure and monoinsertion of onetriple bond rendering a pendant alkyne group. The 1H NMRspectrum of homoPPA shows three signals at 5.87 ppm(attributed the protons on the double bonds on the polymerbackbone), and 6.64 and 6.98 ppm (both attributed to theprotons on the aromatic rings), respectively.22,25 From thechemical shift values of these signals, the polymer has a regularhead-to-tail structure of a cis-transoidal configuration.22,25 In thespectra of both HBPPA4 and HBPPA13, a new signal, besidesthose dominant ones seen in the homoPPA, is observed at 2.90and 3.00 ppm, respectively. This signal should be attributed tothe protons on the pendant alkyne groups.17b−d The dual-inserted 1,3-DEB or 1,4-DEB units, however, have their signalsoverlapping with those of PA units with no new characteristic

Figure 1. 1H NMR (200 MHz) spectra of (a) homoPPA, (b) HBPPA4 (with 1,3-DEB as comonomer), (c) HBPPA13 (with 1,4-DEB ascomonomer), and (d) HBPPA17 (with 1,7-octadiyne as comonomer) in CDCl3.

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peaks. In the spectrum of HBPPA17 (copolymer of ODY), theprotons for the pendant alkyne groups should be at about 1.92ppm according to the spectra of some standards (e.g., 1-octyne).Figure 2 shows the 13C NMR spectra of homoPPA and

HBPPA13. In the spectrum of homoPPA, the vinylic carbonsare at 142.8 (quaternary, 2 in Figure 2) and 131.8 ppm(tertiary, 1), and the quaternary carbon (3) on the aromaticring is at 139.3 ppm, with the rest aromatic carbons in a regioncentered at 127.7 ppm. These 13C NMR signals also confirmthat the polymer has a cis-transoidal configuration. Bycomparing the two spectra, the presence of the pendant alkynegroups in HBPPA13 is evidenced from the two additionalcharacteristic signals (6′ and 7 in the figure) at 120 and 83.5ppm, respectively.17c,d,26 The signals on the dual-insertedcomonomer units overlap with those of PA units and cannotbe differentiated. Figure 3 shows the FTIR spectra of homoPPAand HBPPA10. Three characteristic bands (739, 883, 1595cm−1) are observed in both polymers. In particular, the one at1595 cm−1 corresponds to the stretching vibrations of theconjugated double bonds. Those at 739 and 883 cm−1,respectively, are characteristic of cis-structured PPAs.25 The

presence of the pendant alkyne groups in HBPPA10 is clearlyconfirmed with the νCC band at 2107 cm−1 and the νCC−Hband at 3295 cm−1, which are absent in the spectrum ofhomoPPA.17b−d

Except the gelled homoDEB that could not be characterized,the other copolymers exhibit similar NMR spectra as thecorresponding ones shown in Figure 1 but at different relativesignal intensities. These NMR spectra thus confirm thesuccessful incorporation of the diyne comonomers in thecopolymerizations. Quantifications of the contents of threestructural units (PA, dual-inserted diyne, and pendant alkyne)were undertaken by using the signal integrations of the threetypes of protons (alkynyl, vinylic, and aromatic) in the 1HNMR spectrum for each copolymer. Because of the presence ofcomplex branching structures, the 1H NMR signals of thecopolymers are relatively broader compared to those ofhomoPPA. However, the peaks of the three types of protonsare fairly well separated with negligible overlapping, thusenabling structural quantifications. The results, including molarratio of incorporated diyne to PA ([diyne]/[PA]), diyne molarcontent, percentage of diyne dual-insertion, pendant alkynemolar content (among all monomer units), and branchingdensity (i.e., molar content of dual-inserted diyne among allmonomer units), are summarized in Table 1. The detailedcalculation procedure and equations are illustrated in theSupporting Information.Polymer characterizations with triple-detection GPC were

carried out with THF as the eluent to determine averagemolecular weights, molecular weight distribution, and topologyinformation (intrinsic viscosity, Mark−Houwink curve, and αvalue). The characterization results, including weight-averagemolecular weight (Mw), polydispersity index (PDI), weight-average intrinsic viscosity ([η]w), and α value, are alsosummarized in Table 1. Compared to linear homoPPA, thecopolymers have enhanced solubility in THF despite that theygenerally have much higher molecular weights to be discussedbelow. This should be attributed to the presence of branchingstructures in the copolymers. All the polymers showedmonomodal GPC elution curves. Representatively, Figure 4

Figure 2. 13C NMR (125 MHz) spectra of homoPPA and HBPPA13.

Figure 3. FT-IR spectra of homoPPA and HBPPA10.

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shows the GPC elution curves of homoPPA and HBPPA1−HBPPA4, which were recorded from the DRI detector. Wefound that both PPA homopolymer and copolymers synthe-sized herein tended to adsorb onto the packing of the GPCcolumns. This can be observed from the weak but persistenttails at the high-elution-time end of their DRI elution curves(see Figure 4), which should be attributed to the elutingdesorbed polymers. This column adsorption phenomenon ofPPA has been previously reported by Jimenez et al.16 Theaccuracy in determining polymer average molecular weight andpolydispersity index (PDI) data may thus be slightly affecteddue to the possible overlap of the desorbed polymers with theregular unadsorbed eluting polymers. Because of this, wegenerally truncated the weak low-molecular-weight tail whenperforming the GPC peak selection and result quantification.This treatment was also adopted by Jimenez et al. in theiranalyses of GPC data.16 See Supporting Information for anexample of GPC baseline setting and peak selection.Along with the structural data from 1H NMR spectroscopy,

the results obtained from the GPC characterizations enable usto elucidate polymer topology and discuss below the effects ofdifferent polymerization parameters.(1). Effects of Diyne Type and [Diyne]0/[PA]0 Ratio. Among

the three diynes used herein, 1,3- and 1,4-DEB show similartriple bond reactivity in the copolymerization while thereactivity of ODY appears to be much lower. Comparing theparallel polymers synthesized at the same conditions, thecopolymers of 1,3- and 1,4-DEB have nearly identical diynecontent and polymer yield (i.e., monomer conversion) data(see Table 1). For instance, comparing HBPPA4 andHBPPA13 synthesized at [diyne]0/[PA]0 ratio of 0.30 at 25°C, the diyne content is 23 and 22 mol %, respectively, and thepolymer yield is 26 and 25%, respectively. Compared to PA, thetriple-bond reactivity in 1,3- and 1,4-DEB is, however, lower asthe content ratio of diyne to PA in the copolymers (i.e.,[diyne]/[PA] ratio) is about equal to or less than thecorresponding [diyne]0/[PA]0 ratio in the monomer feed(see Table 1). If the triple bonds in both PA and DEB had thesame reactivity, the [diyne]/[PA] ratio should theoretically betwice of the [diyne]0/[PA]0 ratio since both DEB monomersare difunctionalized. Different from the DEB copolymers, theODY copolymers show much lower diyne content data anddramatically reduced polymer yield due to the low triple bondreactivity in ODY compared to that in DEB comonomers. Forexample, HBPPA18 synthesized at the [diyne]0/[PA]0 ratio of

0.30 at 25 °C has ODY content of only 8 mol % and thepolymer yield is only 1.3%. A significant reduction in polymeryield (from 16 to 1.3%) is observed in the ODY copolymerswith the increase of [diyne]0/[PA]0 ratio (from 0.01 to 0.30),while it decreases only marginally in the copolymers with bothDEB comonomers. This indicates that the incorporation oflow-reactivity ODY significantly reduced the catalyst’s activity.Given this, the use of either DEB comonomer is preferred forthe effective synthesis of hyperbranched polymers herein.Though having similar diyne content data in the parallel

polymers synthesized at the same condition, the 1,3- and 1,4-DEB copolymers show significant differences in the percentageof dual-insertion, with higher dual-insertion percentages foundalways in the 1,4-DEB copolymers. For example, the dual-insertion percentage values are 7.3 and 18% in HBPPA4 (1,3-DEB copolymer) and HBPPA13 (1,4-DEB copolymer),respectively. This indicates that the pendant alkyne groups in1,4-DEB copolymers (on fourth carbon of the aromatic ring)have higher reactivity than those in 1,3-DEB copolymers (onthird carbon of the ring) for further enchainment to renderbranching structures. The difference should result from thesteric reason since the alkyne group on the fourth position ofthe aromatic ring should suffer less steric hindering effect fromthe polymer backbone compared to that on the third position.Consequently, the 1,4-DEB copolymers have higher branchingdensity and lower alkyne content data (see Table 1) relative tothe parallel 1,3-DEB copolymers.In each set of polymers synthesized with the different diynes,

diyne content, alkyne content, and branching density allincrease with the increase of [diyne]0/[PA]0 ratio. This isalso accompanied by the increase in Mw, and the decreases inboth [η]w and α values. For example, in the set of 1,3-DEBcopolymers, the diyne content and branching density increasefrom 4% and 0.09% for HBPPA1 to 23% and 1.7%, respectively.Meanwhile, the Mw value increases from 39 kDa for homoPPAto 52 kDa for HBPPA1, and to 114 kDa for HBPPA4. Despitethe increasing Mw values, the [η]w value is reduced from 16.5mL/g for homoPPA to 10.0 mL/g for HBPPA4, and the αvalue decreases from 0.65 for homoPPA to 0.29 for HBPPA4.Similar trends of change are also found in the set of polymerssynthesized with 1,4-DEB. The PDI values of the polymers aregenerally in the range of 1.37−2.47 and tend to decreaseslightly with the increase of [diyne]0/[PA]0 ratio.Figure 5 shows and compares the Mark−Houwink curves

(i.e., [η] vs molecular weight) of the two parallel sets of 1,3-and 1,4-DEB copolymers synthesized at the same [diyne]0/[PA]0 ratios (HBPPA1−HBPPA4, and HBPPA8−HBPPA10and HBPPA13), along with that for homoPPA. Increasing the[diyne]0/[PA]0 ratio from 0 to 0.20 leads to consistentdownshift of the intrinsic viscosity curve in each polymer set,along with the decrease of the curve slope (i.e., the α value). Afurther increase of the ratio from 0.20 to 0.30, however, showsonly negligible changes in both polymer sets. The intrinsicviscosity curve and α value depend sensitively on polymertopology. A downshift of the intrinsic viscosity curve,accompanied by a decrease in the α value, is typically observedwhen polymer topology changes from linear to increasinglyhyperbranched.6b,7,27 The α value of 0.65 found for the linearhomoPPA indicates that the polymer synthesized with thePd(II) catalyst herein has a flexible random coil conformationin THF.28 This differs from the rod-like conformation with ahigh α value of 2.4 (in benzene at 35 °C) found for somecrystallizable PPAs synthesized with a ferric catalyst system.29

Figure 4. GPC elution curves of homoPPA and HBPPA1−HBPPA4recorded from the differential refractive index (DRI) detector in triple-detection GPC measurement.

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Compared to the linear polymer homoPPA, those copolymershaving low α values, including HBPPA3, HBPPA4, HBPPA9,HBPPA10, and HBPPA13, should be typically hyperbranchedas their α values (0.26−0.33) are in the typical range found forhyperbranched polymers, such as hyperbranched polyethylenessynthesized in our previous works through ethylene chainwalking polymerization.27 These results thus confirm thatincreasing [diyne]0/[PA]0 ratio leads to polymers of enhancedmolecular weight and increasingly compact hyperbranchedtopologies.Meanwhile, comparing each pair of parallel polymers

synthesized with both DEB comonomers at the same[diyne]0/[PA]0 ratio, the intrinsic viscosity curve of the 1,4-DEB copolymer is always below that of the corresponding 1,3-DEB copolymer, indicating more compact chain topology inthe 1,4-DEB copolymer. This is in good consistency with thehigher branching density found with the 1,4-DEB copolymer. Italso confirms that 1,4-DEB is more effective than 1,3-DEB torender hyperbranched topology in the copolymer.While macrogelation occurred in the homopolymerization of

1,3-DEB for homoDEB, addition of a small amount of PA (only22 and 10 mol % relative to 1,3-DEB in HBPPA5 and HBPPA6,respectively) effectively inhibited macrogelation and led tosoluble polymers featured with high diyne content, alkynecontent, and branching density (see Table 1). Though novisible gels could be found, these two copolymers generally hadlower solubility in THF than other copolymers. This indicatesthe possible presence of microgelation in these two copolymershaving high diyne contents. The intrinsic viscosity curves forboth HBPPA5 and HBPPA6 are also displayed in Figure 5.Both curves nearly overlap and are further downshiftedcompared to HBPPA4, indicating these two polymers haveeven enhanced hyperbranched chain topologies given theirhigher branching densities.(2). Effect of Solvent. In agreement with our original

reasoning, the solvent is confirmed herein to have an important

effect on the copolymerization, polymer structure, and topologyby comparing HBPPA3 (synthesized in dichloromethane/methanol mixture) and HBPPA7 (in methanol). Much higherpolymer yield, diyne content, diyne dual-insertion percentage,and branching density data (see Table 1) are observed withHBPPA3, which dissolved well in the polymerization solutionthroughout the reaction with no polymer precipitation. In adifferent manner, the immediate polymer precipitation inmethanol during the synthesis of HBPPA7 reduced significantlythe incorporation of the comonomer and the pendant alkynegroups in the copolymers. Meanwhile, HBPPA7 is also found tohave a significantly lower Mw value (59 kDa as opposed to 92kDa for HBPPA3). The intrinsic viscosity curves of HBPPA3and HBPPA7 are compared in Figure 6. Though branching

structures are present in HBPPA7 on the basis of thedownshifted curve and the reduced α value (0.39) relative tohomoPPA, HBPPA3 clearly has a much more compact chaintopology with a higher level of branching as its curve is wellbelow that of HBPPA7 and the α value (0.27) is also muchlower. On the other hand, polymerizations performed indichloromethane/methanol mixture (HBPPA10) and THF/methanol mixture (HBPPA12) resulted in polymers havingvery similar polymer yield, 1,4-DEB content and incorporationmodes (see Table 1), and chain topology (similar intrinsicviscosity and α values, see Table 1). But the average molecularweights of HBPPA12 appear to be slightly lower than those ofHBPPA10. Its molecular weight distribution is also slightlybroader than HBPPA10, indicating that dichloromethane/methanol mixture is the better solvent for the synthesis.

(3). Effect of Polymerization Temperature. The effect oftemperature is examined by comparing HBPPA10 (synthesizedat 25 °C), HBPPA14 (10 °C), and HBPPA15 (35 °C)synthesized with 1,4-DEB at the same conditions except thetemperature. The polymer yield is reduced significantly from 32to 14% with the temperature decreasing from 35 to 10 °C. Thediyne contents of the three polymers are similar (around 15%),showing the temperature change within the range studiedherein does not have a significant effect on the reactivity of thetriple bonds in 1,4-DEB relative to that in PA. However, slightincreases in dual-insertion percentage (from 16 to 30%) andbranching density (from 2.4 to 4.5%) are observed with the

Figure 5. Mark−Houwink plot (intrinsic viscosity vs molecularweight) of homoPPA and HBPPAs synthesized at different DEB feedratios at 25 °C. The red curves are for the polymers (HBPPA1−HBPPA6) synthesized with 1,3-DEB as comonomer whereas the blueones are for HBPPAs (HBPPA8−HBPPA10, and HBPPA13)synthesized with 1,4-DEB as comonomer.

Figure 6. Mark−Houwink plot (intrinsic viscosity vs molecularweight) of HBPPAs synthesized in different solvents (HBPPA3 andHBPPA7) and under different polymerization temperatures(HBPPA10, HBPPA14, and HBPPA15). The curve for homoPPA isincluded for comparison.

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decrease of the polymerization temperature from 35 to 10 °C.The intrinsic viscosity curves of the three polymers displayed inFigure 6 for comparison also confirm this trend. HBPPA10 andHBPPA14 have similar curves in their overlapping molecularweight range, and their curves are slightly below the curve forHBPPA15. The α value is also reduced from 0.30 to 0.18 withthe decrease of reaction temperature. These results suggest thata lower temperature helps the formation of more compacthyperbranched structures. The Mw value of the polymers alsoshows a pronounced increase with the temperature decrease,with HBPPA14 possessing a much higher Mw value (350 kDa)compared to HBPPA10 and HBPPA15. This increase inmolecular weight should be attributed to the reduced chaintransfer reactions at a lower temperature. In particular, thereduced chain transfer following the incorporation of thependant alkyne groups leads to the enhanced branchingdensity, in addition to the enhanced molecular weight.Synthesis of Core−Shell Star Polymers with a HBPPA

Core and Polystyrene (PS) Arms via Postpolymerization“Click” Reaction. Containing valuable pendant alkyne groups,the above-synthesized HBPPAs can be further functionalized orused as building blocks for constructing polymers of morecomplex architectures through “click” reaction, which is anotheradvantage of this class of hyperbranched polymers. As ademonstration, we report herein some preliminary data on thesynthesis of core−shell structured star polymers having ahyperbranched HBPPA core and linear polystyrene (PS) armsthrough Cu-catalyzed azide−alkyne cycloaddition (CuAAC)reaction or “click” reaction.30 For the purpose of “click”reaction, we synthesized an azide-ended narrow-distributedlinear polystyrene (Mn = 6.6 kDa and PDI = 1.02, determinedwith the light scattering detector in triple-detection GPCmeasurement) through atom-transfer radical polymerization ofstyrene with ethyl 2-bromoisobutyrate as initiator at 90 °C,followed with end group transformation with NaN3.

18 On thebasis of 1H NMR quantification, it was confirmed that everychain contained an azide end group. HBPPA10 synthesizedwith 1,4-DEB as comonomer was selected as the alkyne-containing hyperbranched core. The “click” reaction was carriedout in THF at room temperature with CuBr/2,2′-bipyridine (ata molar ratio of 1:2) as the catalyst. Considering that somealkyne groups buried within the hyperbranched core might notget reacted due to steric effect in this “graft-to” starconstruction, we selected a high [alkyne]0/[azide]0 feed ratioof 8:1 (at [azide]0 = 0.014 M) for the reaction. A radicalinhibitor, BHT, was also added (at ca. 5% of the backbonedouble bonds present in HBPPA10) in the reaction system. Inthe absence of BHT, radical-induced cross-linking was oftenfound to occur among the hyperbranched polymer chainscontaining numerous double bonds on the polymer backbone,rendering insoluble gels.Figure 7 shows the GPC elution curves of the polymer

samples taken at different time during the “click” reaction.From the bimodal elution curve of the polymer mixture takenat 0 h, the azide-ended PS has a peak-maximum elution time ofabout 24.8 min and HBPPA10 has a broad elution curve withthe peak-maximum elution time at about 22.6 min. With theprogress of the reaction from 0 to 2 h, the relative intensity ofthe PS peak in the bimodal elution curve is quickly reducedwhile the high-molecular-weight peak corresponding to the starpolymer gains intensity along with slight move toward left (i.e.,reduced elution time). This demonstrates the successfuloccurrence of the “click” reaction. Further extension of the

reaction to 48 h leads to slight but consistent left-move of thehigh-molecular-weight peak, along with slight gain in itsintensity relative to the PS peak. The conversion data for theazide-ended PS were determined by performing peak fitting onthe GPC elution curves with two peaks, one for the low-molecular-weight PS and the other for the high-molecular-weight star polymer. In the calculation, it is assumed that nopolymer adsorption on the packing of the GPC columnsoccurred. Table 2 summarizes the PS conversion data and GPC

results of the star polymers. At 2 h, the conversion of the PSreached about 68%. Further extension of the reaction to 48 hled to a slightly enhanced conversion of 81%. The Mw value ofthe star polymer is enhanced significantly from 96 kDa forHBPPA10 to 645 kDa at 48 h of reaction, and the average PSarm number increases from 34 at 1 h to 82 at 48 h. Despite theincrease in molecular weight and arm number, the [η]w value ofthe star polymer changes marginally (16.5 and 16.1 mL/g at 1and 48 h, respectively), along with a slight decrease in the αvalue (from 0.26 for HBPPA10 to 0.13 for the star polymer at48 h). These results are reflective of their star conformation andindicate that the star polymer approaches to behave more like

Figure 7. GPC elution traces (recorded from DRI detector) ofpolymer samples taken at different time in “click” reaction betweenHBPPA10 and the azide-ended polystyrene (see Table 2 for reactionconditions).

Table 2. Star Copolymers Synthesized via CuAAC Reactiona

sampletime(h)

Mw(kDa) PDI M−H α

[η]w(mL/g)

PS convn(%) narm

b

1 1 321 1.8 0.22 16.5 8 342 2 338 1.8 0.24 17.6 68 363 4 398 2.0 0.19 16.4 71 454 8 445 2.0 0.13 16.2 75 525 23 488 2.4 0.11 15.7 76 596 48 645 2.3 0.13 16.1 81 82

aOther reaction conditions: use HBPPA10 in Table 1 as core, useazide-functionalized PS with Mn = 6.6 kDa, PDI = 1.02, solvent =THF, catalyst = CuBr/2,2′-bipyridine, [PS] = 0.014 M, [alkyne]:[azide]:[CuBr]:[bipy] = 8:1:1:2, total volume =1 mL, T = roomtemperature. 5% BHT (mol % in reference to the double bonds inHBPPA10) was added as radical inhibitor. bThe average PS armnumber in the produced star copolymer (narm) is calculated from thefollowing equation:

= −

= =

n M M M

M M

( )/ ,

whereas 96 kDa and 6.7 kDa

arm w,star w,HBPPA w,PS

w,HBPPA w,PS

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rigid spheres (α = 0) with the increase of average armnumber.28,31 It should be noted that radical-induced star−starcoupling reaction could still possibly take place in the reactionsystem although the addition of BHT suppressed gelationduring the reaction. Thus, the average arm numbers of the starpolymers estimated herein are much higher than the theoreticalvalues obtained by assuming the absence of star−star coupling.The 1H NMR spectrum of the final polymer product

obtained at 48 h is illustrated in Figure 8, together with those ofthe original HBPPA core (HBPPA10) and PS arm. A new peakat 5.02 ppm (signal 4′ in Figure 8c) appears in the spectrum ofthe star polymer product, which should be attributed to themethine proton next to the triazole ring connecting each PSarm.18 Accordingly, the signal for the methine proton next toazide group originally in azide-ended PS is too weak to bedetected. This is another proof of the success of “click”reaction. However, because the alkyne groups were used inlarge excess relative to the azide groups, the signal at 3.0 ppmfor the pendant alkyne groups can still be observed.

■ CONCLUSIONSA chain-growth polymerization strategy for the convenientsynthesis of HBPPAs containing pendant alkyne groups hasbeen successfully demonstrated in this work by copolymeriza-tion of PA with either 1,3- or 1,4-DEB as difunctional cross-linker using a cationic diphosphine-ligated Pd(II) catalystsystem. A range of HBPPAs having tunable branching density(in the range from 0.09 to 8.6%) and pendant alkyne content

(from 3 to 54%) have been synthesized. The presence ofcompact hyperbranched topology, though at different levels, inthese polymers has been verified on the basis of thedownshifted intrinsic viscosity curves and reduced α values(0.23−0.43 as opposed to 0.65 for homoPPA) relative to thelinear control homoPPA. The effects of various polymerizationparameters, including [diyne]0/[PA]0 ratio, diyne type, temper-ature, and solvent, on the polymerization, on polymer structureand topology have been examined. While both 1,3- and 1,4-DEB are effective in rendering branching structures, ODY hasbeen found to be much less effective as its incorporationreduces dramatically polymer yield while rendering only lowbranching density values. Compared to 1,3-DEB, 1,4-DEB isbetter for rendering branching structures as the resultingpendant akyne groups are more reactive for further enchain-ment due to less steric effect from the backbone. With both 1,3-and 1,4-DEB, increasing [diyne]0/[PA]0 ratio leads to theincreases in polymer molecular weight, diyne content, alkynecontent, and branching density, and renders more compacthyperbranched structures. Decreasing polymerization temper-ature from 35 to 10 °C enhances branching density, alkynecontent, and polymer molecular weight, but reduces polymeryield. The use of dichloromethane/methanol (volume ratio 13/3) mixture solvent has been found to be beneficial in formingthe hyperbranched polymers. Meanwhile, the use of theseHBPPAs for the construction of core−shell structured starpolymers having a HBPPA core and PS arms has also been

Figure 8. 1H NMR (200 MHz) spectra of (a) azide-ended PS; (b) HBPPA10; (c) star polymer product obtained at 48 h in “click” reaction. CDCl3was used as the solvent.

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demonstrated by “clicking” grafting of azide-ended PS armsonto HBPPA10.

■ ASSOCIATED CONTENT*S Supporting InformationDetailed calculation procedure and equations for determiningthe ratio of incorporated comonomers ([diyne]/[PA]), diynecontent, percentage of diyne dual-insertion, pendant alkynecontent (yne %), and branch density in copolymers (listed inTable 1), and GPC curves for typical HBPPA core and PS armincluding the baseline setting and peak selection. This materialis available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: zye@laurentian.ca. Fax: 1(705)675-4862.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe financial support from the Natural Science and Engineer-ing Research Council (NSERC) of Canada, Canada ResearchChair (CRC), and Ontario Ministry of Research andInnovation is greatly appreciated. The authors also thank theCanadian Foundation for Innovation (CFI) for fundingresearch equipment and infrastructure. Bienvenu Muboyayiand Brendan Mirka performed some preliminary polymer-ization experiments.

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