Article Cite This: Macromolecules XXXX, XXX, XXX-XXX
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Effect of Spacer Stiffness on the Properties of Hyperbranched Polymers Suresh Kumar Perala and S. Ramakrishnan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India S Supporting Information *
ABSTRACT: A series of peripherally clickable hyperbranched polyesters were prepared wherein the stiffness of the spacer segment between branching junctions was systematically varied. Three different four-carbon spacers were studied: one was fully saturated (HBPC4-S), another bears a double bond (HBPC4DBc), and a third carries a triple bond (HBPC4-TB); in the case of the sample with a double bond c indicates cis-rich isomer. The glass transition temperatures of the HB polyesters were seen to increase with the stiffness of the spacer segment from 27 to 60 °C. Furthermore, when the peripheral propargyl groups of the HB polyesters were quantitatively clicked with a crystallizable docosyl (C-22) azide, all the derivatives exhibited sharp melting/ crystallization peaks in the DSC traces, indicative of segregation and colocalization of the peripheral segments. WAXS data confirmed the crystallization of the alkyl segments in a paraffinic lattice, while SAXS profiles revealed the formation of a lamellar morphology; the interlamellar spacing reflected the compactness of the HB polymeric core and also the adaptability of the core toward the self-segregation. Whereas the effect of such structural variations in linear polyesters has been examined extensively, this represents the first study that reveals the implications chain stiffness and geometry on the properties of hyperbranched polymers.
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INTRODUCTION Hyperbranched polymers have been extensively studied because of the ease of synthesis and remarkably distinct properties compared to their linear counterparts.1−4 In the past two decades, a large variety of hyperbranched polymers (HBPs) have been synthesized using simple condensation methods as well as other chain polymerization approaches; these have led to the synthesis of numerous interesting systems. One of the interesting features of HBPs is that several of their molecular attributes can be readily varied, which in turn dramatically influences their properties. For example, the nature of the terminal groups governs their solubility,5−10 Tg,5−14,23 solution aggregate morphology,3,15,16 etc. Whereas the effect of chain stiffness on the properties of linear polymers has been wellunderstood, its effect on HBPs has been scarcely examined. Some years ago, we examined the effect of systematically varying the length of the spacer (molecular segment linking two branch points) on the solution and bulk properties of HBPs;17,18 further, we also showed that increasing the length of the spacer segment leads to a decrease in the compactness of the polymer chain as reflected by an increase in the Mark− Houwink exponent.19 Similarly, copolymerization strategies have also been used to decrease the branching density in a statistical manner, which, in turn, influences the solution and bulk behavior of hyperbranched polymers.20 More recently, we developed simple single-step methodologies to prepare peripherally “clickable” HB polyethers21,22 and HB polyesters;23,24 these polymers carry numerous © XXXX American Chemical Society
propargyl/allyl groups at their chain ends. We showed that these HB polymers can be used as scaffolds to quantitatively install a variety of peripheral segments, such as PEG or long chain alkyl segments that help tune their solubility in different types of solvents. We also showed that simultaneous installation of more than one type of segment at the periphery provided ready access to Janus25 and tripodal26 structures arising from immiscibility-driven self-segregation of the peripheral segments. In the present study, we have prepared a series of peripherally “clickable” HB polyesters, wherein the stiffness of the spacer segment is systematically varied, keeping its length almost constant; to achieve this, three different polyesters bearing fourcarbon (C4) spacers were prepared: one carried a butylene spacer, the second 2-butenylene, and the third a 2-butynylene segment. Whereas the effect of backbone stiffness on the properties of linear polymers has been extensively studied, such a study in the context of HBPs has not been reported. It may important here to recognize an intrinsic difference in the nature of this question in the two contexts; in HBPs the branch point breaks the continuity of the chain and serves to stiffen the chain, and hence the stiffness (or flexibility) being examined here is only that of the spacer segment. Furthermore, since we have utilized the peripherally clickable class of HBPs, we also examined another interesting question concerning the influence Received: June 30, 2017 Revised: September 29, 2017
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DOI: 10.1021/acs.macromol.7b01401 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Scheme 1. Synthetic Scheme for the HB Polyesters with Different Spacer Segments and Schematic Representation of HBP (HBPC4-DB)a
a
Typically, HBPs consist of three types of units, namely dendritic (D), linear (L), and terminal (T). The degree of branching (DB) is calculated by estimating the mole fraction of D and T units, which is usually done by NMR spectroscopy.
of the spacer segment flexibility on the conformational adaptability of the HB backbone, which in turn is reflected by the ability of the peripherally clicked docosyl segments to colocate and crystallize. The properties of these HB polyesters reveal interesting and subtle effects of the spacer stiffness on both the parent polymers and the peripherally functionalized derivatives.
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Preparation of Monomers. Synthetic procedures of all the monomers and yields are provided in the Supporting Information. Preparation of Parent HBPs. Synthesis of HBPC4-S (General Polymerization Procedure). The polymerization was done by melttransesterification polycondensation method. MONOC4-S (AB2 monomer) (500 mg, 1.51 mmol), dibutyl tin dilaurate (19.1 mg, 0.03 mmol), and a spin bar were taken in a long-necked small roundbottom flask. The flask was heated at 120 °C under dry nitrogen purge until the spin bar stopped rotating. It was then heated to 150 °C under vacuum (5 mbar) using a Kugelrohr apparatus with constant rotation. Bubbling of the melt indicated the formation of the condensate (propargyl alcohol). The condensate was removed under vacuum. Once the melt become highly viscous (∼4 h), the flask was cooled to room temperature; the polymer was dissolved in CHCl3 and precipitated out in methanol. The product was purified once more by reprecipitation using CHCl3 and methanol as the solvent and nonsolvent, respectively. After drying, the polymer was isolated as a fibrous brownish solid. Yield = 86%. 1H NMR (400 MHz, CDCl3, δ ppm): 1.99 (s, 4H, −OCH2CH2CH2CH2−); 2.54 (t, 1H, −COOCH2CCH); 4.11 (s, 2H, −OCH 2 CH 2 CH 2 CH 2 −); 4.42 (s, 2H, −OCH2CH2CH2CH2−); 4.92 (s, 2H, −COOCH2CCH); 7.71−7.72 (d, 2H, −ArH); 8.25 (s, 1H, −ArH). All other polymers, namely HBPC4-DBc (yield = 87%), HBPC4-TB (86%), LPC4-S (88%), LPC4-DBc (87%), and LPC4-TB (88%), were also prepared via the same procedure as for HBPC4-S. Alkyne−Azide Click Reaction. Preparation of HBPC4-S-C22. HBPC4-S (500 mg, 1.82 mmol) (here the number of moles was calculated by taking repeating unit molecular weight of HBP) and docosyl azide (704 mg, 2 mmol) were taken in a long-necked roundbottom flask, and 7 mL of THF solvent was added to it. Then dry nitrogen was purged for 10 min inside the reaction mixture. CuSO4· 5H2O (22.7 mg, 0.09 mmol) and sodium ascorbate (36.1 mg, 0.18 mmol) were dissolved in a small amount of water and was added to the reaction mixture, and nitrogen was purged for an additional 10 min. Then the round-bottom flask was sealed with a cap and stirred for 3 days at 50 °C. The reaction mixture was concentrated under reduced pressure and precipitated using cold pet ether and cold diethyl ether to get a white color powdery polymer. Yield = 91%.
EXPERIMENTAL SECTION
Materials and Methods. 5-Hydroxyisophthalic acid, 2-butyne-1, 4-diol, 1-hexadecanethiol, propargyl bromide (80 wt % in toluene), docosonol, tosyl choride, and DBTDL (dibutyl tin dilaurate) were purchased from Sigma-Aldrich and used as such. 2-Butene-1,4-diol and NaN3 were purchased from Spectrochem, and SOCl2 was purchased from SDFCL. All the remaining salts and reagents were purchased from commercial sources (Spectrochem, SDFCL) and used directly, unless mentioned otherwise. All the organic solvents were procured from SDFCL or Fischer Scientific, dried, and distilled if necessary. Pyridine was dried by using KOH; diethyl ether and toluene were dried by using Na, and SOCl2 was purified by using sulfur powder and distilled. All the products, monomers, and polymers were characterized by 1H NMR spectroscopy using a Bruker AV 400 MHz spectrometer in suitable deuterated solvents; TMS was used as an internal reference. Gel permeation chromatography measurements were performed using Viscotek instrument, and THF was used as the eluent. Differential scanning calorimetric studies were performed on a PerkinElmer instrument. Heating and cooling scans were performed at the rate of 10 °C/min; reproducibility of the scans was checked by ensuring that the third scan cycle was exactly identical to the second cycle. Wideangle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) measurements were performed on a PANalytical Empyrean instrument. Variable temperature IR studies were performed by using a PerkinElmer FTIR spectrometer. The sample was prepared by dissolving the polymer in CHCl3 (3 mg/mL) and then drop-casted on a KBr pellet. After thorough drying in a vacuum oven, the variable temperature measurements were carried out using a Specac setup; all samples were allowed to equilibrate at each temperature for ∼5 min before recording the spectrum. B
DOI: 10.1021/acs.macromol.7b01401 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. 1H NMR spectra of the clickable HB polyesters bearing three different spacer segments. The peaks marked by an asterisk (∗) are due to CDCl3 or DBTDL catalysts. Individual spectra with the integral intensities are available in the Supporting Information. All other polymers, namely HBPC4-DBc-C22 (89%) and HBPC4TB-C22 (88%), were also prepared via the same procedure as HBPC4S-C22.
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calibration; Mn was found to range from 5000 to 10 000 (Figure S2). Use of standard calibration is known to underestimate the molecular weights of HBPs, and hence these numbers represent the lower limit. One other critical structural parameter is the degree of branching (DB), which provides an estimate of the fraction of linear defects in the HBP. Often NMR spectra reveal well-resolved peaks that permit the estimation of DB; unfortunately, in the series of HBPs carrying a C4 spacer, the chemical shifts associated with the different subunits are not well-resolved (see Figure S11), but in an analogous HBP carrying C6 spacer the aromatic peaks are well-resolved to permit the estimation of DB, which was determined to be ∼0.5 (see Figure S34). On the basis of this and many other studies that have been done earlier in our lab on similar polymers, we can assume the DB in this case also to be ∼0.5, which is also to be expected based on a statistically random process, if the reactivity of the second B group of the AB2 monomer is not affected by whether the first B group has already reacted or not.27 The DSC thermograms of the three parent HB polyesters are shown in Figure 2a; all three samples are amorphous, and the Tg is seen to increase from 27 °C for HBPC4-S to 60 °C for HBPC4-TB. Clearly, with increase in stiffness of the spacer segment, the Tg of the HB polyester increases. In order to make a comparison, linear (meta-linked) analogues carrying the same three spacers were prepared starting from 3-hydroxybenzoic acid; based on end-group analysis using NMR spectral data, the DP of the polymers would be expected to lie in the range 21− 29 (Figures S26, S29, and S32), suggesting the formation of moderately high molecular weights (Mn ∼ 5000). The DSC scans of these are shown in Figure 2b; in all cases, the linear analogues exhibit a slightly lower Tg than their HB analogue. The presence of branching junctions in the HBP evidently increases the Tg; as elaborated by Frechet28 and Stutz,29 this
RESULTS AND DISCUSSION
The AB2 monomers were prepared starting with 5-hydroxyisophthalic acid as per Scheme 1; three different ω-halo alcohols (or their derivatives), namely 4-bromobutyl acetate, 4chloro-2-butenol, and 4-chloro-2-butynol, were used to prepare the monomers bearing different spacer segments. The dipropargyl ester derivatives were prepared in two steps from the corresponding dimethyl esters; these AB2 monomers were then polymerized under standard melt transesterification conditions using dibutyltin dilaurate (DBTDL) as the catalyst.23 The polymerization was first carried out under a dry N2 purge followed by a few hours under reduced pressure. The NMR spectra of the three polymers HBPC4-S, HBPC4DBc, and HBPC4-TB are shown in Figure 1 along with the peak assignments; in the label, C4 reflects the spacer length, while S stands for saturated, DB for double bond, and TB for triple bond. From the spectra, it is evident that both the internal olefin and the peripheral propargyl ester units remain intact during the melt polymerization as reflected by the relative intensities of the various peaks; in all cases the relative intensities of the propargyl ester group confirm that the 1 equiv has reacted leaving behind the second, as expected at high conversions for AB2 monomers. In the case of HBPC4-DBc, there is clear evidence for the presence of both cis and trans isomers, which is carried forward from the monomer without significant isomerization during the thermal polymerization (see Figure S15 for comparison with monomer spectra); roughly about 12−15 mol % of trans isomer is present in both the monomer and polymer. Thus, DBc in the label reflects the cis-rich nature of the double bond. The molecular weights of the HBPs were determined by GPC, using a standard PS C
DOI: 10.1021/acs.macromol.7b01401 Macromolecules XXXX, XXX, XXX−XXX
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Installing Crystallizable Segments on the Periphery. Core−shell dendrimers and HBPs have been extensively studied, primarily to understand their solution properties and aggregation behavior in segment-selective solvents; a variety of interesting aggregate morphologies have been revealed.3,15,16,30,31 When long alkyl chains are installed at the periphery of dendrimers, Meijer et al.32 showed that they selfsegregate to develop a lamellar morphology, wherein the dendrimers adopt either a jellyfish- or butterfly-type conformation that permits the alternate layering of the dendrimeric core and the peripheral segments; however, discriminating between these two possibilities could not be done, as the interlamellar spacing would be the same for both these models. More recently, we showed that similar behavior is also exhibited by analogous HBPs; importantly, if two immiscible segments, such as PEG and docosyl (C22 chain) units, are randomly installed on the periphery of HB polymers, they are able to selfsegregate to form Janus-type structures and subsequently form a bilayer-type lamellar morphology.25 Given that the peripheral segments on HBPs are able to self-segregate, we became interested in examining the effect of the conformational flexibility of the HB polymer backbone on the ability of the peripheral segments to self-segregate and colocate. To examine this, all three HBPs were reacted with docosyl azide (C22) under standard azide−yne click reaction conditions;33 docosyl chains were chosen because of their strong tendency to crystallize once colocated. This provides added stability to the self-segregated morphology, while at the same time it provides an opportunity to readily examine the microphase separation using tools, such as DSC, WAXS, and SAXS. All the three parent HB polyesters were clicked with dcosyl azide under standard conditions using CuSO4·5H2O/sodium ascorbate as the catalyst. The NMR spectra reveal complete conversion in all three cases (Figure 3); the complete
Figure 2. (a) DSC thermograms of the parent HB polyesters bearing three different spacer segments. (b) DSC thermograms of the linear polyesters bearing the different spacer segments.
effect is mitigated to an extent by the large number of terminal groups, which could have the exact opposite effect.
Figure 3. 1H NMR spectra of the three different HB polyesters bearing docosyl chains on their periphery; the peaks marked by an asterisk (∗) are due to CDCl3. D
DOI: 10.1021/acs.macromol.7b01401 Macromolecules XXXX, XXX, XXX−XXX
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similar weak high-temperature shoulder, which could be ascribed to the same process; this peak shifts from 106 to 86 °C for HBPC4-TB-C22 and HBPC4-DBc-C22, respectively, and presumably to an even lower temperature for HBPC4-SC22 and therefore merges with the main peak. In an effort to understand the structural origin of various transitions, variable temperature FT-IR measurements were carried out; it is well-known that the melting of linear alkanes is reflected by a shift in both the symmetric and asymmetric C−H stretching peaks.34,35 The variable temperature FT-IR spectra of the HBPC4-DBc-C22 (Figure 5) clearly revealed that the main
disappearance of the propargyl protons and the appearance of the triazole proton peaks, along with peaks due to the docosyl segment, confirm the completion of the reaction. The triazole proton peak merged with those of the aromatic protons in the case of HBPC4-S-C22, but it appears as a clear shoulder in the other two samples; in all three cases, however, the total intensity of this peak confirms the assignment. Thus, it is evident that in all cases the azide−yne click reaction has gone to completion generating three different HB polyesters bearing docosyl chains on their periphery. The DSC thermograms (Figure 4) reveal clear melting transitions in all the four samples; the main melting transition is
Figure 5. Variable temperature IR spectra of the C−H stretching region of HBPC4-DBc-C22. Inset: variation of C−H stretching frequency as a function of temperature.
peak in the DSC corresponds to the melting of the docosyl segmentsthe expected shift of both peaks to higher wavenumbers is seen nearly at the melting temperatures. However, no further change in the spectra was visible as one traverses the weak higher temperature transition. Similarly, WAXS measurements confirm the presence of a paraffinic lattice as reflected by an interchain packing distance of ∼0.42 nm (Figure 6); variable temperature studies also reveal that the
Figure 4. DSC thermograms of the HB polyesters bearing docosyl chains at their periphery.
assigned to the crystallization of the self-segregated docosyl chains, which is seen to vary between 60 and 70 °C. A weak low-temperature peak is also seen during melting in all three samples; this is probably a reflection of the presence of smaller crystallites. Interestingly, although the melting points varied to some degree, the weight fraction normalized intensity of the two peaks together (with respect to docosyl segments) changed only marginally, suggesting that the backbone stiffness exerts only a limited influence. A rather puzzling observation was the presence of a very weak higher temperature transition clearly seen for both HBPC4-DBc-C22 and HBPC4-TB-C22; this peak is seen during the heating and cooling scans and is completely reproducible during repeated scans. Careful examination of the cooling scan of HBPC4-S-C22 reveals a
Figure 6. Variable temperature WAXS profiles of HBPC4-DBc-C22.
melting of the lattice begins at 70 °C, as evident from the complete disappearance of the sharp peak, while at 80 °C the amorphous halo reflective of microphase separation also disappears. Thus, both variable temperature WAXS and FTIR measurements failed to reveal any change during this weak transition at 86 °C. We also examined the sample, while being heated, under a polarizing light microscope; however, here too there was no birefringence beyond the melting temperature of ∼70 °C, suggesting the absence of any liquid crystalline E
DOI: 10.1021/acs.macromol.7b01401 Macromolecules XXXX, XXX, XXX−XXX
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isomerize internal olefinic sites in vesicular bilayers.36,37 This approach relies on the reversible thiol−ene reaction with internal double bonds that leads to the isomerization of cis double bonds to trans ones; there is often a small residual thiol−ene adduct that remains.38,39 In an effort to isomerize the double bonds, HBPC4-DBc sample was treated with 0.5 equiv of a high boiling thiol, namely hexadecanethiol, in the presence of 0.2 equiv of AIBN; reaction was done in the presence of CHCl3 as a solvent at 71 °C in a sealed tube under N2 atmosphere for 60 h. Comparison of the NMR spectra clearly reveal the formation of the trans isomer; aliquots taken at intermediate time intervals in a control study reveals the different intermediate stages in the isomerization process (Figure S17). The trans double bond content in the final sample was estimated to be around 70 mol %; under these conditions, the extent of irreversible thiol−ene adduct was also estimated to be very low (
