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Investigate the Glass Transition Temperature of Hyperbranched Copolymers with Segmented Monomer Sequence Yi Shi,† Xiaosong Cao,† Shuangjiang Luo,‡ Xiaofeng Wang,† Robert W. Graff,† Daqiao Hu,† Ruilan Guo,‡ and Haifeng Gao*,† †

Department of Chemistry and Biochemistry and ‡Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Hyperbranched copolymers with segmented structures were synthesized using a chain-growth coppercatalyzed azide−alkyne cycloaddition (CuAAC) polymerization via sequential monomer addition in one pot. Three AB2-type monomers that contained one alkynyl group (A), two azido groups (B), and one dangling group, either benzyl or oligo(ethylene oxide) (EOx, x = 3 and 7.5), were used in these CuAAC reactions. Varying the addition sequences and feed ratios of the monomers produced a variety of hyperbranched copolymers with tunable compositions, molecular weights, segmented structures, and consequently glass transition temperature (Tg). It was found that the Tg of hyperbranched copolymers was little affected by the polymer molecular weights when Mn ≥ 5000. However, the values of Tg were significantly determined by the compositions of the terminal groups and the outermost segment of the hyperbranched copolymers. The last added AB2 monomer in the polymerization formed an outermost “shell” and shielded the contribution of inner segments to the glass transition of the copolymers, reflecting a chain sequence effect of hyperbranched polymers on the thermal properties.

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consistent.42,43 For instance, Moore and McHugh44 synthesized polyether hyperbranched polymers by copolymerization of AB2 and AB monomers. They found that the Tg decreased with the increased DB although the compared hyperbranched copolymers also had different numbers of terminal B groups. A similar strategy was applied by Jayakannan et al. for synthesis of polyester hyperbranched polymers, in which it was reported that the Tg increased with the DB when the molar ratio of AB2 in the feed was 0 < fAB2 < 0.35.45 To produce hyperbranched polymers with identical terminal groups and only different DBs, Lederer’s group46,47 copolymerized AB2 and AB(BP) monomers for synthesis of polyester hyperbranched polymers, in which B was the OH group and P was the t-butyldimethylsilyl (TBS) group. Lederer found that the TBS-terminated hyperbranched polymers showed a decreased Tg with the increase of DB. However, this dependence of Tg on DB became random in the OH-terminated hyperbranched polymers. In another study, Yan et al. used ring-opening cationic polymerization of oxetanes to synthesize hyperbranched polyethers with tunable DBs by varying reaction temperature. All polymers in this study carried the same number of terminal OH groups, and the results indicated that the Tg decreased with the increase of DB.48,49 In

INTRODUCTION Hyperbranched polymers with analogous structure as compared to dendrimers enjoy tremendous interest in both academia and industries because of their one-pot effortless synthesis to produce highly branched nanostructures with a large number of terminal groups.1−5 In addition to the development of various synthetic methods to produce hyperbranched polymers,6−31 the physical properties of hyperbranched polymers have also been intensively studies and compared to those of dendrimers.1,4,32−37 Various structural parameters, including the molecular weights, polymer compositions, terminal end groups, degree of branching (DB), and segmented architectures, have been investigated, and their influence on the glass transition temperature (Tg) was broadly reported. Back in the 1990s, Fréchet, Hawker, and Wooley34,38,39 studied the polyester and polyether dendrimers and found that the Tg was significantly affected by the composition of chainend groups and the dendrimer molecular weights but little affected by the polymer architecture and DBs. When studying hyperbranched polymers from one-pot synthesis, several groups have reported similar dependence of Tg on these structural parameters.40,41 Meanwhile, copolymerization of AB2 with AB or AB(BP) monomers (BP representing a protected B group that does not react during the polymerization) has been applied to synthesize hyperbranched polymers with tunable DBs, although the reported effects of DB on the Tg were not always © XXXX American Chemical Society

Received: May 29, 2016 Revised: June 14, 2016

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DOI: 10.1021/acs.macromol.6b01144 Macromolecules XXXX, XXX, XXX−XXX

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showed monomodal peaks with a clean shift toward higher molecular weight direction (Figure 1B). The absolute numberaverage molecular weights (Mn,MALLS) increased linearly with conversion, and the values were always higher than the Mn,RI, indicating the compact structure of the hyperbranched polymers (Figure 1C). Literature reports indicated that the Tg of benzyl ether-based dendrimers increased with molecular weight until around Mn ∼ 10 000.38 To systematically study the dependence of Tg on molecular weights in our polytriazole-based hyperbranched polymers, three polymers with Mn,MALLS = 41 300, 70 300, and 102 500 (HB-Bn-3 to HB-Bn-5, Table 1) were purified from samples withdrawn at different conversions in the reaction of [AB2-Bn]0/[CuSO4·5H2O]0/[ascorbic acid]0 = 100:1:5. Two other hyperbranched polymers with lower molecular weights (Mn,MALLS = 4900 and 9800, HB-Bn-1 and HB-Bn-2, Table 1) were separately prepared from a step-growth CuAAC polymerization of AB2-Bn monomer in the presence of 1 equiv of N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA) ligand (Figure S2). All these five hyperbranched polymers had the same chemical compositions with similar DBs (Figure S3 and Table 1) but different molecular weights. They were subsequently characterized in differential scanning calorimetry (DSC) and showed similar Tg values between 17.9 and 18.6 °C (Figure 1D), indicating little influence of polymer molecular weights on Tg at least when molecular weight was Mn ≥ 5000. Meanwhile, two hyperbranched polymers HB-EO3 with different molecular weights (Mn,MALLS = 49 700 and 101 500 for HB-(EO3)-1 and HB-(EO3)-2, Table 1) were synthesized (Figure S4), and their DSC results (Table 1) confirmed that the Tg = −24.6 and −24.9 °C were little dependent on the molecular weights as well within our investigation. To demonstrate the effect of chemical composition on the Tg, hyperbranched polymers with HB-Bn25 as the core and HB(EO3)x as the shell were synthesized in one pot, in which the 25 and x represent the molar ratios of each reacted monomer to Cu catalyst in the one-pot polymerization, e.g., x = 25 and 100. Specifically, the CuAAC polymerization started with [AB2Bn]0/[CuSO4·5H2O]0/[ascorbic acid]0 = 25:1:5 in DMF at 45 °C and reached complete AB2-Bn conversion after 2.5 h, producing the hyperbranched polymer HB-Bn25 with Mn,MALLS = 41 300. Without stopping the reaction, a second batch of AB2-EO3 monomer (100 equiv to the initial amount of Cu) was added, and the conversion of AB2-EO3 increased with time before reaching complete after another 18 h. During the polymerization, hyperbranched copolymers at around 25% and 100% AB2-EO3 conversions were withdrawn out in large amount for purification and further characterization (Figure S5 and Figure 2A). The mole fractions of AB2-Bn structural units ( f Bn) in both segmented hyperbranched copolymers, HBBn25@(EO3)25 and HB-Bn25@(EO3)100, were determined in 1H NMR spectroscopy by integrating the peaks of Ha and Hb as f Bn = 0.51 and 0.19, respectively (Figure 3A). The mole fractions were very similar to the ratios of reacted monomers in the polymerization, indicating very few primary CuAAC cyclizations of these two monomers in the reaction. The DSC characterization showed that the Tg of hyperbranched copolymers decreased with the AB2-EO3 conversion (decreased f Bn). The final copolymer HB-Bn25@(EO3)100 had a Tg = −22.3 °C, very close to the Tg = −24.6 °C of HB-EO3 homopolymer (Figure 2B and Table 1), indicating possible shield effect of the outer layer to the inner segment due to the three-dimensional structures of the hyperbranched polymers.

contrast to the extensive studies of the influence of DB, molecular weights, and chain ends on the Tg of hyperbranched polymers, there is so far no report to discuss the effect of monomer sequence (e.g., random or segmented structure) in hyperbranched copolymers on the Tg, partially because of the lack of facile synthetic techniques that could produce segmented hyperbranched polymers in one pot. Very recently, our group developed a novel copper-catalyzed azide−alkyne cycloaddition (CuAAC)50−57 polymerization of an AB2 monomer in a one-pot process. The in situ formed polytriazole polymers complexed Cu catalysts and segregated them into polymer domains, resulting in a selective polymer− monomer reaction and a “living” chain-growth polymerization with linear increase of molecular weight with conversion and clean chain extension via sequential monomer additions.58,59 In addition, modular AB2 monomers were designed to carry a dangling dock and can be easily modified to introduce various functional groups with tunable reactivity and polarities. This technique thus provides synthetic robustness to synthesize hyperbranched copolymers with segmented structures with different monomer sequences in one pot. Herein, we report our first study to explore the effect of monomer sequence on the Tg of segmented hyperbranched copolymers. Three AB2 monomers with the same structural platforms but different dangling groups, including benzyl (Bn) and oligo(ethylene oxide) (EOx, x = 3 or 7.5), were synthesized (Scheme 1). Variation of the addition sequences and feed ratios Scheme 1. Illustrative Structures of Hyperbranched Copolymers with Segmented Structure, e.g., HBBn25@(EO3)x (x = 25 or 100) on the Left and HB-(EO3)25@ Bny (y = 25 or 100) on the Right

of these monomers produced a library of hyperbranched polymers with tunable compositions, segmented structures, and terminal groups, which were directly used for exploration of the dependence of Tg on each structural parameter.

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RESULTS AND DISCUSSION CuAAC polymerization of the AB2-Bn monomer was carried out in DMF using CuSO4·5H2O/ascorbic acid as the catalyst with feed ratios of [AB2-Bn]0/[CuSO4·5H2O]0/[ascorbic acid]0 = 100:1:5 at 45 °C and [AB2-Bn]0 = 0.5 mol/L. The monomer conversion determined by monitoring the disappearance of alkynyl groups in the 1H NMR spectroscopy increased with reaction time and reached >98% at 14 h, as shown in Figure 1A. The molecular weights of hyperbranched polymers HB-Bn at different conversions were determined using THF size exclusion chromatography (SEC) with both refractive index (RI) detector and multiangle laser light scattering (MALLS) detector. The elution chromatograms in the RI detector B

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Figure 1. (A) Dependence of AB2-Bn conversion on reaction time, (B) SEC traces in RI detector, (C) evolution of the number-average molecular weight (Mn,MALLS) and polydispersity index (Mw/Mn, RI detector) as a function of conversion in the one-pot polymerization: [AB2-Bn]0/[CuSO4· 5H2O]0/[ascorbic acid]0 = 100:1:5 at 45 °C, [AB2-Bn]0 = 0.5 mol/L, and (D) DSC curves of five hyperbranched polymers synthesized by CuAAC polymerization of AB2-Bn monomers.

Table 1. Summary of Hyperbranched (Co)polymers entries

Mn,RIa

Mn,MALLSb

Mw/Mna

DBc

f Bnd

HB-Bn-1 HB-Bn-2 HB-Bn-3 HB-Bn-4 HB-Bn-5 HB-(EO3)-1 HB-(EO3)-2 HB-(EO7.5)-1 HB-(EO7.5)-2 HB-Bn25-r-(EO3)25 HB-Bn25@(EO3)25 HB-Bn25@(EO3)100 HB-(EO3)25@Bn25 HB-(EO3)25@Bn100 HB-Bn25@(EO7.5)25 HB-Bn25@(EO7.5)50

3 500 8 300 22 400 34 400 48 600 27 400 46 600 31 500 55 700 29 400 38 500 63 300 34 000 49 200 51 600 61 700

4 900 9 800 41 300 70 300 102 500 49 700 101 500 65 700 138 700 62 700 99 100 229 500 86 200 152 300 136 900 213 500

1.56 2.18 1.29 1.28 1.30 1.21 1.15 1.09 1.10 1.25 1.33 1.33 1.25 1.22 1.35 1.35

0.81 0.82 0.79 0.80 0.79 0.76 0.77 0.71 0.72

1 1 1 1 1

0.49 0.51 0.19 0.49 0.78 0.49 0.32

Tg (°C) 18.3 18.6 17.9 18.1 18.6 −24.6 −24.9 −38.4 −38.7 −11.1 −15.3 −22.3 −8.9 5.1 −29.8 −37.4

a

Apparent number-average molecular weight and molecular weight distribution measured by THF SEC with RI detector, calibrated with linear PMMA standards. bThe number-average molecular weight measured by THF SEC with a MALLS detector. cDB was determined by 1H NMR spectroscopy using similar procedures as reported previously.58 dThe mole fractions of AB2-Bn structural units ( f Bn) in hyperbranched copolymers, determined by 1H NMR spectroscopy.

Figure 3. Overlaid 1H NMR spectra of segmented hyperbranched copolymers: (A) HB-Bn25 and HB-Bn25@(EO3)x, x = 25 and 100; (B) HB-(EO3)25 and HB-(EO3)25@Bny, y = 25 and 100.

Figure 2. (A) SEC traces and (B) DSC curves of HB-Bn25, HBBn25@(EO3)x (x = 25 or 100), HB-(EO3)25, HB-(EO3)25@Bny (y = 25 or 100), and HB-(Bn25-r-(EO3)25). C

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Macromolecules Hyperbranched copolymers with HB-(EO3)25 as core and HB-Bny as shell (y = 25 and 100) were also synthesized in a one-pot polymerization under similar conditions as those of HB-Bn25@(EO3)x (Figure S6 and Figure 2A). NMR spectroscopy confirmed that the composition of these hyperbranched copolymers was closely following the ratios of reacted monomers in the polymerization, as the mole fractions of f Bn in these two polymers were f Bn = 0.49 and 0.78 (Figure 3B and Table 1). DSC characterization determined that the Tg increased with the mole fraction of f Bn in the hyperbranched copolymers from Tg = −8.9 °C of HB-(EO3)25@Bn25 to Tg = 5.1 °C of HB-(EO3)25@Bn100. It is interesting to note that the Tg difference between the HB-(EO3)25@Bn100 and HB-Bn25 was 12.8 °C, much larger than the Tg difference of 2.3 °C between HB-Bn25@(EO3)100 and HB-(EO3)25 (Table 1), indicating a less effective shielding of the HB-Bn shell on the HB-EO3 core, probably due to the larger size of AB2-EO3 monomer than AB2-Bn. As a comparison, a hyperbranched random copolymer, HB(Bn25-r-(EO3)25), was synthesized in a one-batch monomer addition at feed ratios of [AB2-Bn]0/[AB2-EO3]0/[CuSO4· 5H2O]0/[ascorbic acid]0 = 25:25:1:5. The chemical composition of the random copolymer calculated based on its 1H NMR spectrum was f Bn ≈ f EO3 ∼ 0.5, similar to the monomer feed ratio (Table 1 and Figure S7). The DSC characterization of the HB-(Bn25-r-(EO3)25) hyperbranched copolymer showed a Tg = −11.1 °C, which was between the Tg values of segmented hyperbranched copolymers HB-Bn25@(EO3) 25 and HB(EO3)25@Bn25, although all of these polymers had the same chemical composition and shared similar molecular weights (Figure 2A). These results for the first time indicate that the segmented structure of hyperbranched copolymers could affect the glass transition of copolymers. Our current synthetic technique that facilities the production of segmented hyperbranched structures via simply altering the monomer addition sequence provides great opportunities to tune the Tg of hyperbranched polymers. In the literature, both dendrimers and hyperbranched polymers showed significant dependence of Tg on the composition and polarity of terminal groups.40,60,61 In our study, the terminal monomer units (e.g., T units) included not only two azido terminal end groups but also the dangling dock group, all of which could affect the glass transition of the hyperbranched polymers. For instance, modification of the HBBn25 using excess alkynyl-terminated mEO3 (i.e., ay-EO3) capped all azido groups (Figures S8 and S10) and decreased the Tg = 17.9 °C of HB-Bn25 to Tg = 1.9 °C of HB-Bn25-cap-EO3. Meanwhile, modification of the HB-(EO3)25 using excess phenylacetylene resulted in an increase of Tg = 24.1 °C in the produced HB-(EO3)25-cap-Ph because of the introduction of phenyl triazole moieties onto the terminal units (Figures S9 and S10). These results confirmed the significant effect of terminal end groups and the position of dangling groups on the Tg of hyperbranched polymers. In addition, the composition of the dangling dock group on the outer shell segment would also be important to affect the Tg. In the HB-Bn25@(EO3)x hyperbranched copolymers, a large fraction of HB-EO3 segments (mole fraction f EO3 = 0.80, weight fraction wEO3 = 0.84) was required to shield the HB-Bn25 core leading to an overall Tg = −22.3 °C of the hyperbranched copolymer, 2.3 °C different from that of HB-EO3 homopolymers. When an AB2-EO7.5 monomer with a longer EO7.5

dangling group was used as the second monomer (Figure S11), the produced hyperbranched copolymers HB-Bn25@(EO7.5)50 with f EO7.5 = 0.68 and wEO7.5 = 0.80 (determined by NMR, Figure S12) showed a Tg = −37.4 °C, only 1 °C different from the Tg = −38.4 °C of HB-(EO7.5)25 homopolymer, indicating a more significant shielding effect of outer segment HB-EO7.5 to the HB-Bn25 core (Figure 4).

Figure 4. (A) SEC trace and (B) DSC curve of hyperbranched polymers HB-Bn25 and HB-Bn25@(EO7.5)z (z = 25 and 50).

The “living” feature in this chain-growth CuAAC polymerization allows sequential monomer additions to produce multisegmented hyperbranched copolymers in one pot. In our investigation, a CuAAC polymerization that started with feed ratios of [AB2-EO3]0/[CuSO4·5H2O]0/[ascorbic acid]0 = 25:1:5 in DMF reached complete conversion (>99%) at 2.5 h and produced a hyperbranched polymer with Mn,MALLS = 49 700 and Mw/Mn = 1.20 (Figure 5A). Without stopping the

Figure 5. (A) SEC traces and (B) DSC curves of hyperbranched polymers HB-(EO 3 ) 2 5 , HB-(EO 3 ) 2 5 @Bn 5 0 , HB-(EO 3 ) 2 5 @ Bn50@(EO3)25, and HB-(EO3)25@Bn50@(EO3)25@Bn50.

reaction, a second batch of AB2-Bn monomer (50 equiv with respect to the initial amount of Cu) was added to the reaction system. The conversion of AB2-Bn reached >99% in 10 h and produced a segmented hyperbranched copolymer HB(EO3)25@Bn50 with Mn,MALLS = 103 000 and Mw/Mn = 1.22. The molar ratio of these two segments was calculated based on 1 H NMR spectra as f EO3:f Bn = 1:2 (Figure S13), and the Tg of this HB-(EO3)25@Bn50 hyperbranched copolymer was Tg = −3.7 °C, much higher than that of HB-(EO3)25. The processes of sequential monomer addition could be continued by adding a third batch of AB2-EO3 monomers (25 equiv) and a fourth batch of AB2-Bn monomers (50 equiv) to the reaction mixture after complete conversion of the monomers in the preceding batch. In these processes, the peaks in the SEC curves shifted smoothly to higher molecular weight region with the mole fraction of f Bn varied up and down (Figure 5A and Table 2). Accordingly, the terminal group compositions varied after the polymerization of each batch of monomer, which altered the overall Tg of these segmented hyperbranched polymers from Tg D

DOI: 10.1021/acs.macromol.6b01144 Macromolecules XXXX, XXX, XXX−XXX

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Table 2. Summary of Multisegmented Hyperbranched (Co)polymers entries

Mn,RIa

Mn,MALLSb Mw/Mna

HB-(EO3)25 HB-(EO3)25@Bn50 HB-(EO3)25@ Bn50@(EO3)25 HB-(EO3)25@ Bn50@(EO3)25@Bn50

27 400 39 600 48 700

49 700 103 000 151 000

60 700

211 000

f Bnc

Tg (°C)

1.21 1.22 1.20

0.67 0.49

−24.6 −3.7 −11.6

1.30

0.68

−1.8

Apparent number-average molecular weight and molecular weight distribution measured by THF SEC with RI detector, calibrated with linear PMMA standards. bThe number-average molecular weight measured by THF SEC with a MALLS detector. cThe mole fractions of AB2-Bn structural units ( f Bn) in hyperbranched copolymers, determined by 1H NMR spectroscopy.

= −3.7 °C of HB-(EO3)25@Bn50 to Tg = −11.6 °C of HB(EO3)25@Bn50@(EO3)25 and Tg = −1.8 °C of HB-(EO3)25@ Bn50@(EO3)25@Bn50. These results indicated that the Tg increased with the introduction of rigid HB-Bn segment as the outermost layer and decreased with the introduction of flexible HB-EO3 segment (Table 2).

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CONCLUSION A series of segmented hyperbranched copolymers with systematically varied compositions, molecular weights, and segmented structures were synthesized using a one-pot chaingrowth CuAAC polymerization. The effect of monomer sequence on the Tg of these hyperbranched copolymers was investigated. It was found that the last added AB2 monomers in the polymerization formed an outermost “shell” that could shield the contribution of inner segments to the glass transition of the segmented hyperbranched copolymers. These results demonstrated unique tunability of physical properties in segmented hyperbranched copolymers and may stimulate further interest to investigate the isolation properties of segmented hyperbranched polymers. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01144. Experimental details; Figures S1−S13 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Science Foundation (CHE1554519) and the ACS Petroleum Research Fund (PRF #54298-DN17) for financial support. Y. Shi and S. Luo acknowledge the partial financial support from the Center of Sustainable Energy at Notre Dame via the ND Energy Postdoctoral Fellowship Program. H. Gao thanks the support from the University of Notre Dame, the Center for Sustainable Energy at Notre Dame and NDnano. E

DOI: 10.1021/acs.macromol.6b01144 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b01144 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b01144 Macromolecules XXXX, XXX, XXX−XXX