Synthesis of Hyperbranched Poly(phenylacetylene)s Containing

Article pubs.acs.org/Macromolecules

Synthesis of Hyperbranched Poly(phenylacetylene)s Containing Pendant Alkyne Groups by One-Pot Pd-Catalyzed Copolymerization of Phenylacetylene with Diynes Zhongmin 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 of a class of hyperbranched poly(phenylacetylene)s (HBPPAs) containing various branching densities and different contents of pendant alkyne groups through one-pot chain-growth copolymerization of phenylacetylene (PA) with a diyne comonomer (1,3or 1,4-diethynylbenzene (DEB)). The polymerization was facilitated with the use of an in situ generated cationic diphosphine-ligated Pd(II) catalyst system. Serving effectively as difunctional cross-linker in the polymerization, the diyne comonomer first undergoes monoinsertion to render pendant alkyne groups, which can be further enchained to generate branching structures. A systematic investigation has been undertaken to study the effects of various polymerization parameters, including diyne/PA feed ratio, diyne type, temperature, and solvent, on the polymerization, polymer structure and topology. With either 1,3- or 1,4-DEB, a convenient tuning of polymer topology from linear to hyperbranched can be achieved by simply increasing diyne/PA feed ratio. Relative to 1,3-DEB, 1,4-DEB is more effective in rendering branching structures since the pendant alkyne groups suffer less steric effect from the polymer backbone and are thus more reactive. As to the solvent, a dichloromethane/methanol mixture (at vol. ratio of 13:3) was shown to better help the formation of branching structures than methanol alone as the resulting polymers can dissolve well in the former while precipitate in the latter. Because of their possession of the valuable pendant alkyne groups, these polymers have also been demonstrated for their use as building blocks in the synthesis of core−shell structured star polymers containing a HBPPA core and polystyrene arms through their Cu-catalyzed “click” reaction with an azide-ended polystyrene.

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INTRODUCTION As a novel family of polymers of unique chain architectures and some striking material properties, hyperbranched polymers have been at the forefront of polymer research for more than 2 decades by now. Resembling dendrimers, hyperbranched polymers possess the distinct three-dimensional spherical architecture featured with extensive branch-on-branch structures, which imparts them many unique properties including low melt/solution viscosity, good solubility, and abundance of reactive sites or functionalities.1 Different from dendrimers often requiring multistep sophisticated synthesis, hyperbranched polymers show the remarkable advantage in their convenient one-pot one-step synthesis, which promotes their large-scale production for commercial applications.1 Various synthetic strategies have been developed for the synthesis of a great variety of hyperbranched polymers from numerous monomer stocks. First described by Flory,2 condensation polymerization of ABx (x ≥ 2) monomers or A2 + By (y ≥ 3) monomer combinations, whereas A and B are complementary functionalities that could react with each other, is the classical synthetic strategy. Later on, other elegant selfcondensing chain-growth polymerization strategies, including vinyl polymerization,3 ring-opening polymerization,4 and proton transfer polymerization,5 have been developed with the use of latent ABx, AB*, or similar monomers.1 In particular, © 2012 American Chemical Society

self-condensing vinyl polymerization and proton transfer polymerization strategies have enabled the synthesis of hyperbranched polymers from vinyl monomers through different polymerization mechanisms, including various controlled/“living” radical polymerizations, anionic polymerization, etc.1 Furthermore, a chain walking polymerization strategy has been developed specifically for the synthesis of hyperbranched polyethylenes and functionalized polymers through ethylene coordination polymerization with the use of Brookhart Pd− diimine catalysts, which are featured with unique chain walking mechanism.6,7 In addition, copolymerization of monovinylic monomer with multivinylic monomer and homopolymerization of divinyl monomer have also been demonstrated to obtain hyperbranched structures conveniently.8 Involving only singlebond or double-bond reactions, the hyperbranched polymers synthesized through the above condensation or addition polymerization strategies are generally electronically inactive and are thus commonly used as commodity materials. With specific regard to electronically active alkyne polymers containing conjugated double or triple bonds, various polymerization strategies have also been developed for synthesis of their Received: April 12, 2012 Revised: May 28, 2012 Published: June 13, 2012 5020

dx.doi.org/10.1021/ma3007569 | Macromolecules 2012, 45, 5020−5031

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Table 1. HBPPAs Synthesized via Copolymerization of PA and Different Diynesa triple-detection GPC characterizationsd

NMR characterizationsc

samples

diyne

homoPPA HBPPA1 HBPPA2 HBPPA3 HBPPA4 HBPPA5 HBPPA6 homoDEBe HBPPA7 HBPPA8 HBPPA9 HBPPA10 HBPPA11f HBPPA12 HBPPA13 HBPPA14 HBPPA15 HBPPA16 HBPPA17 HBPPA18

1,3-DEB 1,3-DEB 1,3-DEB 1,3-DEB 1,3-DEB 1,3-DEB 1,3-DEB 1,3-DEB 1,4-DEB 1,4-DEB 1,4-DEB 1,4-DEB 1,4-DEB 1,4-DEB 1,4-DEB 1,4-DEB ODY ODY ODY

solv C C C C C C C C

b

+M +M +M +M +M +M +M +M M C+M C+M C+M C+M T+M C+M C+M C+M C+M C+M C+M

T (°C) 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 10 35 25 25 25

[diyne]0 /[PA]0 0.05 0.10 0.20 0.30 4.4 10.4 0.20 0.05 0.10 0.20 0.20 0.20 0.30 0.20 0.20 0.01 0.05 0.30

yield (%)

incorp [diyne]/ [PA]

34 31 32 28 26 20 25 44 5.8 32 30 30 58 29 25 14 32 16 13 1.3

0 0.04 0.09 0.18 0.30 0.87 1.69 0.03 0.04 0.08 0.19 0.20 0.20 0.28 0.18 0.17 0.003 0.03 0.09

diyne cont (mol %) 0 4 8 16 23 46 63 100 3 4 7 16 16 16 22 15 15 0.3 2 8

diyne dualinsert (%)

yne %

branch density (%)

Mw (kDa)

PDI

M−H α

[η]w (mL/g)

2.3 6.2 7.2 7.3 8.8 14

4.0 7.7 14 21 42 54

0.09 0.5 1.1 1.7 4.1 8.6

39 52 78 92 114 42 95

1.37 1.81 1.75 1.57 1.66 1.45 1.42

0.65 0.50 0.43 0.27 0.29 0.23 0.33

16.5 11.4 9.9 9.5 10.0 6.5 7.9

4.4 28 26 24 25 24 18 30 16 0.2 1.5 3.5

2.4 3.0 5.7 12 13 14 18 11 14 0.3 2.4 7.0

0.1 1.2 2.0 3.8 4.0 4.0 3.9 4.5 2.4 0.0 0.04 0.25

59 74 71 96 98 64 145 350 69 64 43 57

2.47 2.44 1.62 1.52 1.47 1.90 1.42 1.65 1.71 1.85 1.34 1.37

0.39 0.39 0.33 0.26 0.28 0.24 0.27 0.18 0.30 0.44 0.37 0.48

13.9 11.2 9.4 8.4 7.4 7.3 8.8 11.3 8.5 13.0 8.2 9.4

a

Other polymerization conditions: in all runs except HBPPA5, HBPPA6, HBPPA11, and homoDEB, [PA] = 0.61 M, [Pd(OAc)2] = [diphosphine ligand] = 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. b A solvent mixture of CH2Cl2:MeOH = 13:3 (C + M) was used in all runs except for HBPPA7, where pure MeOH (M) was used, and for HBPPA12, where solvent mixture of THF:MeOH = =13:3 (T + M) was used. cThe ratio of diyne molar content to PA content, diyne molar content in the copolymer, the percentage of diyne dual-insertion (both triple bonds enchained), the content of triple bond in the polymer (yne %), and branching density were calculated from 1H NMR spectrum of each polymer. These parameters are defined as follows: incrop. [diyne]/[PA] = the number of incorporated diyne units/the number of incorporated PA units; diyne cont. (%) = the number of incorporated diyne units/the total number of all incorporated monomer units ×100%; diyne dual-insert (%) = the number of dual-inserted diyne units/the number of incorporated diyne 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 calculation procedure and equations are detailed in Supporting Information. dTriple-detection GPC characterizations were carried out with THF as the eluent at 33 °C. Weight-average molecular weight (Mw) and polydispersity index (PDI) data were obtained from the light scattering detector. The intrinsic viscosity data were obtained from the viscosity detector. The Mark−Houwink α values (M−H α) were obtained with the use of the absolute molecular weight data determined with the light scattering detector. eMacrogelation occurred. Sample was not characterized. fA higher catalyst dosage (50% higher) was used in this run for HBPPA11 compared to that for HBPPA10. See above.

hemilabile phosphine ligands, contain branching structures.16 Therein, the branching structures were suggested to result from the incorporation of in situ generated macromonomer or chain transfer to polymer. Though interesting, the level of branching in those polymers was, however, found to be very low (e.g., average branching frequency about 0.86 per chain) and thus do not 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 reported using different transition metal catalysts.17 However, the resulting polymers often contain or appear as cross-linked gels. Though the polymers were soluble in some cases,17b−d their topological characterizations and control, however, were not 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 of phenylacetylene (PA) with diynes (1,3- or 1,4-DEB) with the

hyperbranched polymers though in general the advancements have lagged behind those of vinyl polymers.9 Hyperbranched alkyne polymers are particularly appealing given their valuable electronic, optical, and photonic properties, besides the generic advantageous properties of hyperbranched polymers. The polymerization strategies developed thus far often involve nonlinear polycondensation of specially synthesized di- or multifunctional alkyne monomers and necessary comonomers having complementary functionalities, and utilize the characteristic coupling/addition reactions of the alkyne triple bonds (such as Diels−Alder cycloaddition,10 alkyne cyclotrimerization,11 click cycloaddition,12 Sonogashira coupling,13 hydrosilylation, 14 Glaser−Hay coupling,15 etc.) for topology construction.9 The use of chain-growth polymerization for the synthesis of hyperbranched polymers from alkyne monomers, however, has not yet been demonstrated. Jiménez et al. have recently reported that some poly(phenylacetylene) samples, synthesized with several Rh(I) catalysts bearing 5021


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in THF and washed with a large amount of methanol for 3 times. The product 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, 15 mmol), and DMF (40 mL) were mixed in a round-bottom flask. The mixture was kept stirred at room temperature overnight to have all Br groups converted into azide groups. After the reaction, the azide-ended PS was precipitated with a methanol/water mixture (1:1 v/v) and washed with methanol three times before vacuum drying. Synthesis of Star Polymer Having a HBPPA Core and PS Arms by Cu-Catalyzed Azide−Alkyne Cycloaddition. CuBr (2 mg, 14 μmol) and 2,2′-bipyridine (4.7 mg, 28 μmol) were mixed in a dried test tube. The tube was sealed with a rubber septum, vacuumed, and charged with N2. Simultaneously, HBPPA10 (0.099 g, Mw = 96 kDa, 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 % in reference to the double bonds in HBPPA10), and THF (1 mL) were mixed in a sealed vial. After three freeze−pump−thaw cycles, the polymer solution was injected into the test tube. The reaction system was protected under dry nitrogen throughout the reaction process. Samples were taken at predetermined times during the reaction to monitor the PS conversion and molecular weight development of the star copolymer with GPC. For each reaction solution sampled from the reactor, the polymer was obtained by precipitating the polymer mixture in acidified methanol, followed with further wash with a large amount of methanol for three times, and subsequent drying under vacuum at room temperature. Characterizations and Measurements. Proton nuclear magnetic resonance (1H NMR) spectra of the polymers were all obtained on a Varian Gemini 2000 spectrometer (200 MHz) at ambient temperature with CDCl3 as solvent. 13C nuclear magnetic resonance (13C NMR) spectra of the polymers were obtained on a Bruker AV500 spectrometer (125 MHz) at ambient temperature with CDCl3 as the solvent. Fourier-transformed infrared (FTIR) spectra of the polymers were obtained on a Thermo Scientific Nicolet 6700 Analytical FTIR spectrometer. The samples were prepared as pellets using spectroscopic-grade KBr. Polymer characterizations with triple-detection gel permeation chromatography (GPC) were carried out on a Polymer Laboratories PL-GPC220 system equipped with a triple-detection array comprised of a differential refractive index (DRI) detector (from Polymer Laboratories), 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 Polymer Laboratories). One guard column (PL# 1110−1120) and three 30 cm columns (PLgel 10 μm MIXED-B 300 × 7.5 mm) were used for polymer fractionation. HPLC-grade THF was used as the mobile phase at a flow rate of 1.0 mL/min. The whole GPC system, including columns and detectors, was maintained at 33 °C. Polymer solutions with a concentration between 1.5 and 30 mg/mL were injected into the columns at an injection volume of 200 μL. Astra software from Wyatt Technology was used to collect and analyze the data from all three detectors. Two polystyrene narrow standards (from Pressure Chemicals) with a weight-average molecular weight (Mw) of 30 and 200 kg/mol, respectively, were used for the normalization of LS signals from three detecting angles, and for the determination of interdetector delay volume and band broadening, respectively. The DRI increment (dn/dc) value of 0.286 mL/g19 was used for all PPA samples, and a value of 0.185 mL/g was used for polystyrenes. For the star polymer product synthesized by “click” reaction between HBPPA10 and azideended PS, an average dn/dc value was calculated from the mass content of HBPPA core (xHBPPA) according to the following equation:

use of an in situ generated cationic diphosphine-ligated Pd(II) catalyst system, palladium acetate (Pd(OAc)2)/α,α′-bis(di-tertbutylphosphino)-o-xylene/methanesulfonic acid. In this polymerization strategy, the diyne comonomers act as difunctional cross-linker, generating the desired branch-on-branch topologies. A class of HBPPAs having tunable degrees of branching and molecular weights has thus been conveniently produced from commercially available alkyne monomers while at mild reaction conditions. Moreover, these hyperbranched polymers contain the valuable pendant alkyne moieties, which endows them the benefit for postpolymerization functionalization and the construction of polymers of more complex architectures as building blocks. To the best of our knowledge, this represents the first piece of work demonstrating the successful synthesis of HBPPAs via the chain-growth polymerization strategy.

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EXPERIMENTAL SECTION

Materials. HPLC-grade CH2Cl2 (99.5%, Fisher Scientific) and tetrahydrofuran (THF, 99.5%, Fisher Scientific) were deoxygenated and dried by using a solvent purification system (Innovative Technology) 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 the inhibitor 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-diethynylbenzene (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″-pentamethyldiethylenetriamine (PMDETA, 99%, Aldrich), ethyl 2-bromoisobutyrate (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-dried flask was charged with PA (3.0 g, 29.4 mmol), 1,3-DEB (185 mg, 1.47 mmol), and CH2Cl2 (39 mL) under nitrogen protection. The solution was mixed with a magnetic stirrer. The flask was covered with aluminum foil. One drop of methanesulfonic acid (ca. 0.05 mL) was added into the solution under stirring. In the meantime, Pd(OAc)2 (16.8 mg, 75 μmol), diphosphine ligand (29.6 mg, 75 μmol), and MeOH (9 mL) were mixed in a 20 mL vial under N2 protection. The mixture was sonicated at room temperature for 10 min until all the solids were dissolved. Then the catalyst mixture was injected into the flask under N2 protection to start the polymerization. The polymerization lasted for 1 h at room temperature under N2 protection. At the end, the polymerization mixture was poured into 2%-acidified methanol (ca. 300 mL) to terminate the polymerization. The orange polymer precipitate underwent a redissolution−precipitation procedure for 3 times to remove possible catalyst and monomer residues. Finally, the polymer was dried under vacuum at room temperature for 24 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 atomtransfer radical polymerization (ATRP).18 A flame-dried Schlenk flask was 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 N2 protection. The mixture was subject to three freeze−pump−thaw cycles, and the flask was finally charged with nitrogen. Degassed EBiB (175.6 mg 0.9 mmol) was then injected into the reactor under N2 protection. The reaction system was placed in a thermostated oil bath set at 90 °C to start the polymerization. The reaction was stopped at a predetermined monomer conversion by pouring the solution into 2%acidified methanol (100 mL). The polymer precipitate was redissolved

( dn/dc)avg = ( dn/dc)HBPPA × x HBPPA + ( dn/dc)PS × (1 − x HBPPA ) in which (dn/dc)HBPPA = 0.286 mL/g, (dn/dc)PS = 0.185 mL/g, and xHBPPA was calculated from the mass of the HBPPA10 used and the mass of the reacted PS. 5022


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RESULTS AND DISCUSSION Copolymerization of Phenylacetylene (PA) with Diynes. 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, was employed herein for the copolymerization of PA with diynes. This and similar diphosphine-ligated Pd(II) catalyst systems have been extensively used for the alternating copolymerization of alkenes with CO to afford polyketones,20 and have also been demonstrated for highly active polymerization of alkyne monomers.21,22 In the catalyst system, cationic (P∧P)Pd2+ species are generated in situ through protonation of the acetate ligands with methanesulfonic acid.20 Compared to early transition metal catalysts, the catalyst systems show remarkably low oxophilicity and enable polymerization in protic media (e.g., methanol and water). Similar catalyst systems have been successfully used by Mecking et al. to carry out emulsion polymerization of both acetylene and arylacetylene monomers in aqueous media for the preparation of polymer nanoparticle dispersions.23 Three diyne comonomers, 1,3-DEB, 1,4-DEB, and 1,7octadiyne (ODY), were employed herein to study the effect of diyne type on the synthesis of hyperbranched polymers. While the monofunctional PA contributes to the formation of polymer backbone, the diyne comonomer, which acts as a cross-linker, is key to the construction of hyperbranched topology in the polymerization. The incorporation of diyne comonomer renders first pendant alkyne groups and subsequent interchain enchainment of some of the pendant alkyne groups leads to the formation of branching structures (see Scheme 1) and enhanced polymer molecular weight.

Meanwhile, intrachain enchainment of the pendant alkyne groups can also occur, rendering intrachain cyclic structures but with no effect on polymer molecular weight. Herein, the intrachain cyclic structures are also counted in as branching structures as these two types of chain microstructures cannot be differentiated with the current nuclear magnetic resonance (NMR) technique used for polymer structural characterization. The extent of branching in the resulting polymers should depend on the feed concentration ratio of diyne to PA ([diyne]0/[PA]0). With the other polymerization parameters fixed, raising the ratio is expected to increase the extent of branching and render more compact chain topology. However, excessive diyne concentration (above critical gelation concentration) will lead to undesired macrogelation, which should be avoided. Table 1 lists all the polymers synthesized under various conditions. The polymerization runs were designed with the aim of investigating the effects of various polymerization parameters, including [diyne]0/[PA]0 ratio, diyne type, temperature, and solvent, on the polymerization, polymer structure, and topology. In the table, HBPPA1−HBPPA4, along with homoPPA as the linear PA homopolymer control sample, were synthesized 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 ratios but with the use of different diynes (1,4-DEB and ODY, respectively). In addition, HBPPA5 and HBPPA6 were synthesized at much higher [diyne]0/[PA]0 ratios (4.4 and 10.4, respectively) at a fixed 1,3-DEB concentration (0.64 M) to investigate the effect of further enhancing [diyne]0/[PA]0 ratios. Meanwhile, homopolymerization of 1,3-DEB (homoDEB in Table 1) was also carried out at the same temperature (25 °C) as a control run. While macrogelation occurred in homoDEB 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 of Pd catalyst dosage/concentration only causes the increase of polymer yield, whereas the molecular weight and chain topology of HBPPA11 are similar to those of HBPPA10. Moreover, to investigate the effects of temperature, HBPPA14 and HBPPA15 were synthesized with 1,4-DEB at the same conditions as HBPPA10 except the different polymerization temperature (10 and 35 °C, respectively, as opposed to 25 °C for HBPPA10). The polymerization time for all runs except the run of homoDEB is 1 h. A further extension of the reaction time to 3 h for a separate run carried out at the same condition as HBPPA10 was found to only affect the polymer yield with negligible changes in both molecular weight and chain topology of the polymer product. The solvent was found to greatly affect the polymerization, polymer structure, and topology. The use of methanol is essential to achieve successful polymerization with the Pd(II) catalyst system employed herein. While it occurred in methanol, homopolymerization of PA did not take place, with no polymer produced, when dichloromethane or THF alone was used as the solvent. This confirms the role of methanol in assisting chain initiation in the polymerization: the in situ generated (P∧P)Pd2+ species reacts with methanol to

Scheme 1. Synthesis of Hyperbranched Poly(phenylacetylene)s (HBPPAs) Bearing Multiple Alkyne Groups by Copolymerization of Phenylacetylene (PA) with 1,3- or 1,4-Diethynylbenzene (DEB), and Subsequent Synthesis of Core−Shell Structured Star Polymers with a HBPPA Core and Linear Polystyrene (PS) Arms by a CuCatalyzed Alkyne−Azide “Click” Reaction

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

form a metal hydride to promote polymerization.20,24 However, polymer precipitated out once produced in methanol due to its insolubility. In the case of copolymerization runs, we reasoned such polymer precipitation will reduce the extent of branching since the pendant alkyne groups in the precipitated polymer can not be further enchained. A homogeneous system with the produced polymer well dissolved in the solution should maximize the incorporation of the pendant alkyne groups to form hyperbranched topology. Considering this, we chose, after optimization, a dichloromethane/methanol mixture at a volumetric ratio of 13:3 as the solvent in all runs except those for HBPPA7 and HBPPA12, to render successful polymerization and meanwhile to maintain good polymer solubilization. All the polymers produced, except the gelled homoDEB, were found completely soluble in the solvent mixture. To demonstrate the solvent effect, HBPPA7 and HBPPA12 were synthesized for comparison. HBPPA7 was synthesized at the same condition as HBPPA3 except with pure MeOH as solvent, whereas HBPPA12 was synthesized at the same 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 characterized with NMR spectroscopy, Fourier-transformed infrared

(FTIR) spectroscopy, and triple-detection gel permeation chromatography (GPC) incorporating online differential refractive index (DRI), light scattering (LS), and viscosity detectors. In particular, 1H NMR technique was used to elucidate and quantify polymer microstructure. Figure 1 shows the 1H NMR spectra of three representative copolymers (HBPPA4, HBPPA13, and HBPPA17) obtained with the three respective diyne comonomers, along with that of linear homoPPA for comparison. With each diyne, two insertion modes should exist, with dual-insertion of both triple bonds rendering a branching structure and monoinsertion of one triple bond rendering a pendant alkyne group. The 1H NMR spectrum of homoPPA shows three signals at 5.87 ppm (attributed the protons on the double bonds on the polymer backbone), and 6.64 and 6.98 ppm (both attributed to the protons on the aromatic rings), respectively.22,25 From the chemical shift values of these signals, the polymer has a regular head-to-tail structure of a cis-transoidal configuration.22,25 In the spectra of both HBPPA4 and HBPPA13, a new signal, besides those dominant ones seen in the homoPPA, is observed at 2.90 and 3.00 ppm, respectively. This signal should be attributed to the protons on the pendant alkyne groups.17b−d The dualinserted 1,3-DEB or 1,4-DEB units, however, have their signals overlapping with those of PA units with no new characteristic 5024


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

presence of the pendant alkyne groups in HBPPA10 is clearly confirmed with the νCC band at 2107 cm−1 and the νCC−H band at 3295 cm−1, which are absent in the spectrum of homoPPA.17b−d Except the gelled homoDEB that could not be characterized, the other copolymers exhibit similar NMR spectra as the corresponding ones shown in Figure 1 but at different relative signal intensities. These NMR spectra thus confirm the successful incorporation of the diyne comonomers in the copolymerizations. Quantifications of the contents of three structural units (PA, dual-inserted diyne, and pendant alkyne) were undertaken by using the signal integrations of the three types of protons (alkynyl, vinylic, and aromatic) in the 1H NMR spectrum for each copolymer. Because of the presence of complex branching structures, the 1H NMR signals of the copolymers are relatively broader compared to those of homoPPA. However, the peaks of the three types of protons are fairly well separated with negligible overlapping, thus enabling structural quantifications. The results, including molar ratio of incorporated diyne to PA ([diyne]/[PA]), diyne molar content, percentage of diyne dual-insertion, pendant alkyne molar content (among all monomer units), and branching density (i.e., molar content of dual-inserted diyne among all monomer units), are summarized in Table 1. The detailed calculation procedure and equations are illustrated in the Supporting Information. Polymer characterizations with triple-detection GPC were carried out with THF as the eluent to determine average molecular weights, molecular weight distribution, and topology information (intrinsic viscosity, Mark−Houwink curve, and α value). The characterization results, including weight-average molecular weight (Mw), polydispersity index (PDI), weightaverage intrinsic viscosity ([η]w), and α value, are also summarized in Table 1. Compared to linear homoPPA, the copolymers have enhanced solubility in THF despite that they generally have much higher molecular weights to be discussed below. This should be attributed to the presence of branching structures in the copolymers. All the polymers showed monomodal GPC elution curves. Representatively, Figure 4

peaks. In the spectrum of HBPPA17 (copolymer of ODY), the protons for the pendant alkyne groups should be at about 1.92 ppm according to the spectra of some standards (e.g., 1octyne). Figure 2 shows the 13C NMR spectra of homoPPA and HBPPA13. In the spectrum of homoPPA, the vinylic carbons are at 142.8 (quaternary, 2 in Figure 2) and 131.8 ppm (tertiary, 1), and the quaternary carbon (3) on the aromatic ring is at 139.3 ppm, with the rest aromatic carbons in a region centered at 127.7 ppm. These 13C NMR signals also confirm that the polymer has a cis-transoidal configuration. By comparing the two spectra, the presence of the pendant alkyne groups in HBPPA13 is evidenced from the two additional characteristic signals (6′ and 7 in the figure) at 120 and 83.5 ppm, respectively.17c,d,26 The signals on the dual-inserted comonomer units overlap with those of PA units and cannot be differentiated. Figure 3 shows the FTIR spectra of homoPPA and HBPPA10. Three characteristic bands (739, 883, 1595 cm−1) are observed in both polymers. In particular, the one at 1595 cm−1 corresponds to the stretching vibrations of the conjugated double bonds. Those at 739 and 883 cm−1, respectively, are characteristic of cis-structured PPAs.25 The

Figure 3. FT-IR spectra of homoPPA and HBPPA10. 5025


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0.30 at 25 °C has ODY content of only 8 mol % and the polymer yield is only 1.3%. A significant reduction in polymer yield (from 16 to 1.3%) is observed in the ODY copolymers with the increase of [diyne]0/[PA]0 ratio (from 0.01 to 0.30), while it decreases only marginally in the copolymers with both DEB comonomers. This indicates that the incorporation of low-reactivity ODY significantly reduced the catalyst’s activity. Given this, the use of either DEB comonomer is preferred for the 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 1,4DEB copolymers show significant differences in the percentage of dual-insertion, with higher dual-insertion percentages found always in the 1,4-DEB copolymers. For example, the dualinsertion percentage values are 7.3 and 18% in HBPPA4 (1,3DEB copolymer) and HBPPA13 (1,4-DEB copolymer), respectively. This indicates that the pendant alkyne groups in 1,4-DEB copolymers (on fourth carbon of the aromatic ring) have higher reactivity than those in 1,3-DEB copolymers (on third carbon of the ring) for further enchainment to render branching structures. The difference should result from the steric reason since the alkyne group on the fourth position of the aromatic ring should suffer less steric hindering effect from the polymer backbone compared to that on the third position. Consequently, the 1,4-DEB copolymers have higher branching density and lower alkyne content data (see Table 1) relative to the parallel 1,3-DEB copolymers. In each set of polymers synthesized with the different diynes, diyne content, alkyne content, and branching density all increase with the increase of [diyne]0/[PA]0 ratio. This is also accompanied by the increase in Mw, and the decreases in both [η]w and α values. For example, in the set of 1,3-DEB copolymers, the diyne content and branching density increase from 4% and 0.09% for HBPPA1 to 23% and 1.7%, respectively. Meanwhile, the Mw value increases from 39 kDa for homoPPA to 52 kDa for HBPPA1, and to 114 kDa for HBPPA4. Despite the increasing Mw values, the [η]w value is reduced from 16.5 mL/g for homoPPA to 10.0 mL/g for HBPPA4, and the α value decreases from 0.65 for homoPPA to 0.29 for HBPPA4. Similar trends of change are also found in the set of polymers synthesized with 1,4-DEB. The PDI values of the polymers are generally in the range of 1.37−2.47 and tend to decrease slightly 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 1,3and 1,4-DEB copolymers synthesized at the same [diyne]0/ [PA]0 ratios (HBPPA1−HBPPA4, and HBPPA8−HBPPA10 and HBPPA13), along with that for homoPPA. Increasing the [diyne]0/[PA]0 ratio from 0 to 0.20 leads to consistent downshift of the intrinsic viscosity curve in each polymer set, along with the decrease of the curve slope (i.e., the α value). A further increase of the ratio from 0.20 to 0.30, however, shows only negligible changes in both polymer sets. The intrinsic viscosity curve and α value depend sensitively on polymer topology. A downshift of the intrinsic viscosity curve, accompanied by a decrease in the α value, is typically observed when polymer topology changes from linear to increasingly hyperbranched.6b,7,27 The α value of 0.65 found for the linear homoPPA indicates that the polymer synthesized with the Pd(II) catalyst herein has a flexible random coil conformation in THF.28 This differs from the rod-like conformation with a high α value of 2.4 (in benzene at 35 °C) found for some crystallizable PPAs synthesized with a ferric catalyst system.29

Figure 4. GPC elution curves of homoPPA and HBPPA1−HBPPA4 recorded from the differential refractive index (DRI) detector in tripledetection GPC measurement.

shows the GPC elution curves of homoPPA and HBPPA1− HBPPA4, which were recorded from the DRI detector. We found that both PPA homopolymer and copolymers synthesized herein tended to adsorb onto the packing of the GPC columns. This can be observed from the weak but persistent tails at the high-elution-time end of their DRI elution curves (see Figure 4), which should be attributed to the eluting desorbed polymers. This column adsorption phenomenon of PPA has been previously reported by Jiménez et al.16 The accuracy in determining polymer average molecular weight and polydispersity index (PDI) data may thus be slightly affected due to the possible overlap of the desorbed polymers with the regular unadsorbed eluting polymers. Because of this, we generally truncated the weak low-molecular-weight tail when performing the GPC peak selection and result quantification. This treatment was also adopted by Jiménez et al. in their analyses of GPC data.16 See Supporting Information for an example of GPC baseline setting and peak selection. Along with the structural data from 1H NMR spectroscopy, the results obtained from the GPC characterizations enable us to elucidate polymer topology and discuss below the effects of different 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 similar triple bond reactivity in the copolymerization while the reactivity of ODY appears to be much lower. Comparing the parallel polymers synthesized at the same conditions, the copolymers of 1,3- and 1,4-DEB have nearly identical diyne content and polymer yield (i.e., monomer conversion) data (see Table 1). For instance, comparing HBPPA4 and HBPPA13 synthesized at [diyne]0/[PA]0 ratio of 0.30 at 25 °C, the diyne content is 23 and 22 mol %, respectively, and the polymer yield is 26 and 25%, respectively. Compared to PA, the triple-bond reactivity in 1,3- and 1,4-DEB is, however, lower as the content ratio of diyne to PA in the copolymers (i.e., [diyne]/[PA] ratio) is about equal to or less than the corresponding [diyne]0/[PA]0 ratio in the monomer feed (see Table 1). If the triple bonds in both PA and DEB had the same reactivity, the [diyne]/[PA] ratio should theoretically be twice of the [diyne]0/[PA]0 ratio since both DEB monomers are difunctionalized. Different from the DEB copolymers, the ODY copolymers show much lower diyne content data and dramatically reduced polymer yield due to the low triple bond reactivity in ODY compared to that in DEB comonomers. For example, HBPPA18 synthesized at the [diyne]0/[PA]0 ratio of 5026


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

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

Compared to the linear polymer homoPPA, those copolymers having low α values, including HBPPA3, HBPPA4, HBPPA9, HBPPA10, and HBPPA13, should be typically hyperbranched as their α values (0.26−0.33) are in the typical range found for hyperbranched polymers, such as hyperbranched polyethylenes synthesized in our previous works through ethylene chain walking polymerization.27 These results thus confirm that increasing [diyne]0/[PA]0 ratio leads to polymers of enhanced molecular weight and increasingly compact hyperbranched topologies. 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 1,4DEB copolymer is always below that of the corresponding 1,3DEB copolymer, indicating more compact chain topology in the 1,4-DEB copolymer. This is in good consistency with the higher branching density found with the 1,4-DEB copolymer. It also confirms that 1,4-DEB is more effective than 1,3-DEB to render hyperbranched topology in the copolymer. While macrogelation occurred in the homopolymerization of 1,3-DEB for homoDEB, addition of a small amount of PA (only 22 and 10 mol % relative to 1,3-DEB in HBPPA5 and HBPPA6, respectively) effectively inhibited macrogelation and led to soluble polymers featured with high diyne content, alkyne content, and branching density (see Table 1). Though no visible gels could be found, these two copolymers generally had lower solubility in THF than other copolymers. This indicates the possible presence of microgelation in these two copolymers having high diyne contents. The intrinsic viscosity curves for both HBPPA5 and HBPPA6 are also displayed in Figure 5. Both curves nearly overlap and are further downshifted compared to HBPPA4, indicating these two polymers have even enhanced hyperbranched chain topologies given their higher branching densities. (2). Effect of Solvent. In agreement with our original reasoning, the solvent is confirmed herein to have an important

Figure 6. Mark−Houwink plot (intrinsic viscosity vs molecular weight) of HBPPAs synthesized in different solvents (HBPPA3 and HBPPA7) and under different polymerization temperatures (HBPPA10, HBPPA14, and HBPPA15). The curve for homoPPA is included for comparison.

structures are present in HBPPA7 on the basis of the downshifted curve and the reduced α value (0.39) relative to homoPPA, HBPPA3 clearly has a much more compact chain topology with a higher level of branching as its curve is well below that of HBPPA7 and the α value (0.27) is also much lower. On the other hand, polymerizations performed in dichloromethane/methanol mixture (HBPPA10) and THF/ methanol mixture (HBPPA12) resulted in polymers having very similar polymer yield, 1,4-DEB content and incorporation modes (see Table 1), and chain topology (similar intrinsic viscosity and α values, see Table 1). But the average molecular weights of HBPPA12 appear to be slightly lower than those of HBPPA10. Its molecular weight distribution is also slightly broader than HBPPA10, indicating that dichloromethane/ methanol mixture is the better solvent for the synthesis. (3). Effect of Polymerization Temperature. The effect of temperature is examined by comparing HBPPA10 (synthesized at 25 °C), HBPPA14 (10 °C), and HBPPA15 (35 °C) synthesized with 1,4-DEB at the same conditions except the temperature. The polymer yield is reduced significantly from 32 to 14% with the temperature decreasing from 35 to 10 °C. The diyne contents of the three polymers are similar (around 15%), showing the temperature change within the range studied herein does not have a significant effect on the reactivity of the triple bonds in 1,4-DEB relative to that in PA. However, slight increases in dual-insertion percentage (from 16 to 30%) and branching density (from 2.4 to 4.5%) are observed with the 5027


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decrease of the polymerization temperature from 35 to 10 °C. The intrinsic viscosity curves of the three polymers displayed in Figure 6 for comparison also confirm this trend. HBPPA10 and HBPPA14 have similar curves in their overlapping molecular weight range, and their curves are slightly below the curve for HBPPA15. The α value is also reduced from 0.30 to 0.18 with the decrease of reaction temperature. These results suggest that a lower temperature helps the formation of more compact hyperbranched structures. The Mw value of the polymers also shows a pronounced increase with the temperature decrease, with HBPPA14 possessing a much higher Mw value (350 kDa) compared to HBPPA10 and HBPPA15. This increase in molecular weight should be attributed to the reduced chain transfer reactions at a lower temperature. In particular, the reduced chain transfer following the incorporation of the pendant alkyne groups leads to the enhanced branching density, 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 or used as building blocks for constructing polymers of more complex architectures through “click” reaction, which is another advantage of this class of hyperbranched polymers. As a demonstration, we report herein some preliminary data on the synthesis of core−shell structured star polymers having a hyperbranched HBPPA core and linear polystyrene (PS) arms through Cu-catalyzed azide−alkyne cycloaddition (CuAAC) reaction or “click” reaction.30 For the purpose of “click” reaction, we synthesized an azide-ended narrow-distributed linear polystyrene (Mn = 6.6 kDa and PDI = 1.02, determined with the light scattering detector in triple-detection GPC measurement) through atom-transfer radical polymerization of styrene with ethyl 2-bromoisobutyrate as initiator at 90 °C, followed with end group transformation with NaN3.18 On the basis of 1H NMR quantification, it was confirmed that every chain contained an azide end group. HBPPA10 synthesized with 1,4-DEB as comonomer was selected as the alkynecontaining hyperbranched core. The “click” reaction was carried out in THF at room temperature with CuBr/2,2′-bipyridine (at a molar ratio of 1:2) as the catalyst. Considering that some alkyne groups buried within the hyperbranched core might not get reacted due to steric effect in this “graft-to” star construction, we selected a high [alkyne]0/[azide]0 feed ratio of 8:1 (at [azide]0 = 0.014 M) for the reaction. A radical inhibitor, BHT, was also added (at ca. 5% of the backbone double bonds present in HBPPA10) in the reaction system. In the absence of BHT, radical-induced cross-linking was often found to occur among the hyperbranched polymer chains containing 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 taken at 0 h, the azide-ended PS has a peak-maximum elution time of about 24.8 min and HBPPA10 has a broad elution curve with the peak-maximum elution time at about 22.6 min. With the progress of the reaction from 0 to 2 h, the relative intensity of the PS peak in the bimodal elution curve is quickly reduced while the high-molecular-weight peak corresponding to the star polymer gains intensity along with slight move toward left (i.e., reduced elution time). This demonstrates the successful occurrence of the “click” reaction. Further extension of the

Figure 7. GPC elution traces (recorded from DRI detector) of polymer samples taken at different time in “click” reaction between HBPPA10 and the azide-ended polystyrene (see Table 2 for reaction conditions).

reaction to 48 h leads to slight but consistent left-move of the high-molecular-weight peak, along with slight gain in its intensity relative to the PS peak. The conversion data for the azide-ended PS were determined by performing peak fitting on the GPC elution curves with two peaks, one for the lowmolecular-weight PS and the other for the high-molecularweight star polymer. In the calculation, it is assumed that no polymer adsorption on the packing of the GPC columns occurred. Table 2 summarizes the PS conversion data and GPC Table 2. Star Copolymers Synthesized via CuAAC Reactiona sample

time (h)

Mw (kDa)

PDI

M−H α

[η]w (mL/g)

PS convn (%)

narmb

1 2 3 4 5 6

1 2 4 8 23 48

321 338 398 445 488 645

1.8 1.8 2.0 2.0 2.4 2.3

0.22 0.24 0.19 0.13 0.11 0.13

16.5 17.6 16.4 16.2 15.7 16.1

8 68 71 75 76 81

34 36 45 52 59 82

a

Other reaction conditions: use HBPPA10 in Table 1 as core, use azide-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 = room temperature. 5% BHT (mol % in reference to the double bonds in HBPPA10) was added as radical inhibitor. bThe average PS arm number in the produced star copolymer (narm) is calculated from the following equation:

narm = (M w,star − M w,HBPPA )/M w,PS , whereas M w,HBPPA = 96 kDa

and M w,PS = 6.7 kDa

results of the star polymers. At 2 h, the conversion of the PS reached about 68%. Further extension of the reaction to 48 h led to a slightly enhanced conversion of 81%. The Mw value of the star polymer is enhanced significantly from 96 kDa for HBPPA10 to 645 kDa at 48 h of reaction, and the average PS arm number increases from 34 at 1 h to 82 at 48 h. Despite the increase in molecular weight and arm number, the [η]w value of the star polymer changes marginally (16.5 and 16.1 mL/g at 1 and 48 h, respectively), along with a slight decrease in the α value (from 0.26 for HBPPA10 to 0.13 for the star polymer at 48 h). These results are reflective of their star conformation and indicate that the star polymer approaches to behave more like 5028


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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. CDCl3 was used as the solvent.

(from 3 to 54%) have been synthesized. The presence of compact hyperbranched topology, though at different levels, in these polymers has been verified on the basis of the downshifted intrinsic viscosity curves and reduced α values (0.23−0.43 as opposed to 0.65 for homoPPA) relative to the linear control homoPPA. The effects of various polymerization parameters, including [diyne]0/[PA]0 ratio, diyne type, temperature, and solvent, on the polymerization, on polymer structure and topology have been examined. While both 1,3- and 1,4DEB are effective in rendering branching structures, ODY has been found to be much less effective as its incorporation reduces dramatically polymer yield while rendering only low branching density values. Compared to 1,3-DEB, 1,4-DEB is better for rendering branching structures as the resulting pendant akyne groups are more reactive for further enchainment due to less steric effect from the backbone. With both 1,3and 1,4-DEB, increasing [diyne]0/[PA]0 ratio leads to the increases in polymer molecular weight, diyne content, alkyne content, and branching density, and renders more compact hyperbranched structures. Decreasing polymerization temperature from 35 to 10 °C enhances branching density, alkyne content, and polymer molecular weight, but reduces polymer yield. The use of dichloromethane/methanol (volume ratio 13/ 3) mixture solvent has been found to be beneficial in forming the hyperbranched polymers. Meanwhile, the use of these HBPPAs for the construction of core−shell structured star polymers having a HBPPA core and PS arms has also been

rigid spheres (α = 0) with the increase of average arm number.28,31 It should be noted that radical-induced star−star coupling reaction could still possibly take place in the reaction system although the addition of BHT suppressed gelation during the reaction. Thus, the average arm numbers of the star polymers estimated herein are much higher than the theoretical values 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 of the original HBPPA core (HBPPA10) and PS arm. A new peak at 5.02 ppm (signal 4′ in Figure 8c) appears in the spectrum of the star polymer product, which should be attributed to the methine proton next to the triazole ring connecting each PS arm.18 Accordingly, the signal for the methine proton next to azide group originally in azide-ended PS is too weak to be detected. This is another proof of the success of “click” reaction. However, because the alkyne groups were used in large excess relative to the azide groups, the signal at 3.0 ppm for the pendant alkyne groups can still be observed.

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CONCLUSIONS A chain-growth polymerization strategy for the convenient synthesis of HBPPAs containing pendant alkyne groups has been successfully demonstrated in this work by copolymerization of PA with either 1,3- or 1,4-DEB as difunctional crosslinker using a cationic diphosphine-ligated Pd(II) catalyst system. A range of HBPPAs having tunable branching density (in the range from 0.09 to 8.6%) and pendant alkyne content 5029


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

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(9) See a recent review paper: Liu, J.; Lam, J. W. Y.; Tang, B. Z. Chem. Rev. 2009, 109, 5799. (10) (a) Morgenroth, F.; Müllen, K. Tetrahedron 1997, 53, 15349. (c) Berresheim, A. J.; Müllen, K. Chem. Rev. 1999, 99, 1747. (11) Some examples: (a) Häussler, M.; Tang, B. Z. Adv. Polym. Sci. 2007, 209, 1. (b) Häussler, M.; Qin, A.; Tang, B. Z. Polymer 2007, 48, 6181. (c) Xu, K.; Peng, H.; Sun, Q.; Dong, Y.; Salhi, F.; Luo, J.; Chen, J.; Huang, Y.; Zhang, D.; Xu, Z.; Tang, B. Z. Macromolecules 2002, 35, 5821. (d) Peng, H.; Lam, J. W. Y.; Tang, B. Z. Macromol. Rapid Commun. 2005, 26, 673. (e) Dong, H.; Zheng, R.; Lam, J. W. Y.; Häussler, M.; Tang, B. Z. Macromolecules 2005, 38, 6382. (12) Some examples: (a) Scheel, A.; Komber, H.; Voit, B. Macromol. Rapid Commun. 2004, 25, 1175. (b) Smet, M.; Metten, K.; Dehaen, W. Collect. Czech. Chem. Commun. 2004, 69, 1097. (c) Xie, J.; Hu, L.; Shi, W.; Deng, X.; Cao, Z.; Shen, Q. Polym. Int. 2008, 57, 965. (d) Qin, A.; Jim, C. K. W.; Lu, W.; Lam, J. W. Y.; Häussler, M.; Dong, Y.; Sung, H. H. Y.; Williams, I. D.; Wong, G. K. L.; Tang, B. Z. Macromolecules 2007, 40, 2308. (e) Qin, A.; Lam, J. W. Y.; Jim, C. K. W.; Zhang, L.; Yan, J.; Häussler, M.; Liu, J.; Dong, Y.; Liang, D.; Chen, E.; Jia, G.; Tang, B. Z. Macromolecules 2008, 41, 3808. (13) (a) Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1997, 119, 3391. (b) Bharathi, P.; Moore, J. S. Macromolecules 2000, 33, 3212. (c) Kim, C.; Chang, Y.; Kim, J. S. Macromolecules 1996, 29, 6353. (d) Li, Z.; Qin, A. J.; Lam, J. W. Y.; Dong, Y. P.; Dong, Y. Q.; Ye, C.; Williams, I. D.; Tang, B. Z. Macromolecules 2006, 39, 1436. (14) (a) Xiao, Y.; Wong, R. A.; Son, D. Y. Macromolecules 2000, 33, 7232. (b) Kwak, G.; Masuda, T. Macromol. Rapid Commun. 2002, 23, 68. (15) Häussler, M.; Zheng, R.; Lam, J. W. Y.; Tong, H.; Dong, H.; Tang, B. Z. J. Phys. Chem. B 2004, 108, 10645. (16) Angoy, M.; Bartolomé, M. I.; Vispe, E.; Lebeda, P.; Jiménez, M. V.; Pérez-Torrente, J. J.; Collins, S.; Podzimek, S. Macromolecules 2010, 43, 6278. (17) (a) Hanková, V.; Slováková, E.; Zedník, J.; Vohlídal, J.; Sivkova, R.; Balcar, H.; Zukal, A.; Brus, J.; Sedlácě k, J. Macromol. Rapid Commun. 2012, 33, 158. (b) Zhan, X.; Yang, M. Macromol. Rapid Commun. 2000, 21, 1263. (c) Zhan, X.; Yang, M. Eur. Polym. J. 2001, 37, 1649. (d) Zhan, X.; Yang, M.; Lei, Z. J. Mol. Catal. A: Chem. 2002, 184, 139. (18) Gao, H.; Min, K.; Matyjaszewski, K. Macromol. Chem. Phys. 2007, 208, 1370. (19) Sedlacek, J.; Vohlidal, J.; Grubisic-Gallot, Z. Makromol. Chem., Rapid. Commun 1993, 14, 51. (20) See a review: Drent, E.; Budzelaar, P. H. M. Chem. Rev. 1996, 96, 663. (21) Drent, E.; Pello, D. H. L. Eur. Pat. 576091 (Shell), 1993. (22) Li, K.; Wei, G.; Darkwa, J.; Pollack, S. K. Macromolecules 2002, 35, 4573. (23) (a) Huber, J.; Mecking, S. Angew. Chem., Int. Ed. 2006, 45, 6314. (b) Huber, J.; Mecking, S. Macromolecules 2010, 43, 8718. (24) (a) Clegg, W.; Eastham, G. R.; Elsegood, M. R. J.; Heaton, B. T.; Iggo, J. A.; Tooze, R. P.; Whyman, R.; Zacchini, S. Organometallics 2002, 21, 1832. (b) Lopez-Serrano, J.; Duckett, S. B.; Aiken, S.; Almeida Lenero, K. Q.; Drent, E.; Dunnel, J. P.; Konya, D.; Whitwood, A. C. J. Am. Chem. Soc. 2007, 129, 6513. (25) Furlani, A.; Napoletano, C.; Russo, M. V.; Feast, W. J. Polym. Bull. 1986, 16, 311. (26) Lavastre, O.; Cabioch, S.; Dixneuf, P. H.; Sedlacek, J.; Vohlidal, J. Macromolecules 1999, 32, 4477. (27) (a) Ye, Z.; Zhu, S. Macromolecules 2003, 36, 2194. (b) Xiang, P.; Ye, Z.; Morgan, S.; Xia, X.; Liu, W. Macromolecules 2009, 42, 4946. (c) Morgan, S.; Ye, Z.; Subramanian, R.; Wang, W.-J.; Ulibarri, G. Polymer 2010, 51, 597. (d) Xu, Y.; Xiang, P.; Ye, Z.; Wang, W.-J. Macromolecules 2010, 43, 8026. (28) Hiemenz, P. C.; Lodge, T. P.; Polymer Chemistry, 2nd ed.; CRC Press: Boca Raton, FL, 2007. (29) Biyani, B.; Campagna, A. J.; Daruwalla, D.; Srivastava, C. M.; Ehrlich, P. J. Macromol. Sci.: Part AChem. 1975, 9, 327.

ASSOCIATED CONTENT

S Supporting Information *

Detailed calculation procedure and equations for determining the ratio of incorporated comonomers ([diyne]/[PA]), diyne content, percentage of diyne dual-insertion, pendant alkyne content (yne %), and branch density in copolymers (listed in Table 1), and GPC curves for typical HBPPA core and PS arm including the baseline setting and peak selection. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 1(705)675-4862. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the Natural Science and Engineering Research Council (NSERC) of Canada, Canada Research Chair (CRC), and Ontario Ministry of Research and Innovation is greatly appreciated. The authors also thank the Canadian Foundation for Innovation (CFI) for funding research equipment and infrastructure. Bienvenu Muboyayi and Brendan Mirka performed some preliminary polymerization experiments.

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REFERENCES

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