Letter pubs.acs.org/macroletters
Defect-Free Hyperbranched Polydithioacetal via Melt Polymerization Saptarshi Chatterjee and S. Ramakrishnan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India S Supporting Information *
ABSTRACT: Degree of branching (DB) describes the level of structural perfection of a hyperbranched polymer when compared to its defect-free analogue, namely the dendrimer. The strategy most commonly used to achieve high DB values, specifically while using AB2 type self-condensations, is to design an AB2 monomer wherein the reaction of the first B-group leads to an enhancement of the reactivity of the second one. In the present study, we show that an AB2 monomer carrying a dimethylacetal unit and a thiol group undergoes a rapid self-condensation in the melt under acid-catalysis to yield a hyperbranched polydithioacetal with no linear defects. NMR studies using model systems reveal that the intermediate monothioacetal is relatively unstable under the polymerization conditions and transforms rapidly to the dithioacetal; because this second step occurs irreversibly during polymer formation, it leads to a defect-free hyperbranched polydithioacetal. TGA studies of the polymerization process provided some valuable insights into the kinetics of polymerization. An additional virtue of this approach is that the numerous terminal dimethylacetal groups are very labile and can be quantitatively transformed by treatment with a variety of functional thiols; the terminal dimethylacetals were, thus, reacted with various thiols, such as dodecanethiol, benzyl mercaptan, ethylmercaptopropionate, and so on, to demonstrate the versatility of these systems as sulfur-rich hyperscaffolds to anchor different kinds of functionality on their periphery.
T
formed. In a more recent report, Segawa et al.13 showed that a suitably designed trifluoroacetophenone derivative polymerized via electrophilic aromatic substitution to yield polymers, wherein the DB could be varied from 0 to 100%; this was achieved by simply varying the mole ratio of monomer to trifluoromethane sulfonic acid, which was used as the catalyst. Most of these earlier studies required elaborate synthesis of fairly complex monomers and therefore were of limited general applicability. In another recent study, Huang et al.14 showed that polymerization of a suitable monomer via the Pd-catalyzed Heck-Miyaura coupling yielded a defect-free HBP; the authors attributed this to efficient catalyst migration from one aryl site on the AB2 monomer to the other. Even though it has been long-recognized that the absence of linear defects alone does not transform the hyperbranched polymer to a dendrimer,15 there continues to be a great deal of fundamental interest in developing interesting reaction schemes that permit the generation of defect-free HBPs. Some years ago, we reported a simple approach to prepare hyperbranched polyethers via an acid-catalyzed melt-condensation of a suitable AB2-type monomer carrying two methoxybenzyl groups and a hydroxyl group;16 this approach required the use of a per-alkylated aromatic ring to prevent cross-linking due to electrophilic aromatic substitution. However, recently we showed that when a fairly simple AB2 monomer, namely, 4hydroxymethyl benzaldehyde, dimethylacetal (B), was allowed to melt-condense under similar acid-catalyzed conditions, but at
he study of hyperbranched polymers (HBPs) that began over two decades ago continues to sustain a great deal of interest among researchers, primarily because of the enormous scope for structural variation using simple strategies1 and, more recently, because of the potential for generating a variety of fascinating structures by postpolymerization modification of the large number of peripheral functional groups.2 The globular conformation of HBPs and their small size provide some interesting opportunities for utilization as drug carriers, for cellular imaging, and many other such applications.3 Peripherally functionalized hyperbranched polymers exhibit several interesting properties, such as encapsulation of small molecules,4 tunable lower critical solution temperatures,5 reconfigurable Janus systems,6 and so on. One of the features that distinguishes hyperbranched polymers from the analogous structurally perfect dendrimers is the presence of a large number of branching defects;7 typically the degree of branching (DB) does not exceed the statistically expected value of 50%.8 Several attempts to overcome this problem of high defect levels have been made; some of the approaches utilize a reactive B3 core in combination with slow monomer addition,9 while the more successful approaches use a suitably designed monomer and polymerization process; wherein, after the reaction of the first B group in the AB2 monomer, the reactivity of the second one is substantially enhanced. This latter approach was first reported by Voit and co-workers10 and subsequently elaborated by several others.11−13 Some years ago, Sinananwanich et al.12 described the preparation of hyperbranched polymers using a monomer that carries a ketone and a thiol group; under strong acid catalysis, a defect-free hyperbranched polydithioketal is © 2012 American Chemical Society
Received: March 27, 2012 Accepted: April 6, 2012 Published: April 19, 2012 593
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of the hemithioketal that led to the formation of a defect-free hyperbranched polydithioketal, in their case, would also enable us to realize similar results; the intermediate monothioacetal would be labile under the acid-catalyzed polymerization conditions and, therefore, would rapidly and irreversibly transform to the more stable dithioacetal, which should lead to the generation of defect-free HBPs. In this report, we show that a readily accessible monomer A indeed polymerizes rapidly under fairly mild melt-polymerization conditions to yield a defect-free hyperbranched polydithioacetal; an interesting virtue of this polymer is the presence of a large number of terminal dimethylacetal groups that could be readily transformed to stable dithioacetals by a reaction with a variety of functional thiols, thereby, providing a useful approach to prepare defect-free HBP scaffolds for further postpolymerization functionalization. The synthesis of the AB2 monomer A was achieved using a fairly straightforward procedure depicted in Scheme 1. The polymerization of this monomer was done under previously optimized acid-catalyzed transetherification polymerization conditions in the melt, with the major difference that it occurred rapidly even at much lower temperatures. The earlier melt transetherification was typically carried out at 150 °C,19 while this polymerization occurred very rapidly even at 80 °C. The polymerization was carried out in the presence of 2 mol % of pyridinium camphor sulfonate (PCS),19 first, under a dry nitrogen purge for 45 min, followed by an additional 45 min under reduced pressure to drive the reaction to completion. The relatively simple structure of this monomer and the straightforward synthetic procedure makes this an attractive strategy to access defect-free HBPs. The NMR spectra of monomer A and the corresponding polymer are shown in Figure 1, along with the peak assignments. Several interesting
a significantly lower temperature, it yielded a soluble hyperbranched polyacetal (Scheme 1);17 interestingly, the electroScheme 1. Synthesis of the AB2 Monomer (A) and its Polymerization to Form Hyperbranched Polydithioacetal (Top); Below is a Depiction of the Formation of Previously Studied17 Hyperbranched Polyacetala
a
The repeat unit structure of the hyperbranched polydithioacetal is depicted so as to indicate the absence of linear defects.
philic substitution of the aromatic ring did not pose a problem of cross-linking in this case, evidently due to the lower reactivity of the intermediate carbocation. As anticipated, this hyperbranched polyacetal was susceptible to acid-catalyzed degradation, the rate of which was shown to be effectively regulated by the hydrophobicity of the terminal alkyl substitutents. While degradability has its virtues, developing alternate structures that do not degrade readily would be useful with regard to developing multiply functionalizable scaffolds. Hence, we designed a new monomer for the preparation of hyperbranched polydithioacetals, namely, 4-mercaptomethyl benzaldehyde, dimethylacetal (A), which we expected would polymerize under a similar acid-catalyzed melt-condensation process. Wilson et al. have recently shown that linear polydithioketals can indeed be formed under transthioketalization conditions wherein a dimethylacetal monomer was condensed with a α,ωdithiol using a solution polymerization process;18 this suggested that transthioketalization under melt-polymerization conditions should also be possible. Furthermore, based on the studies by Sinananwanich et al.,12 it was also clear that the high reactivity
Figure 1. 1H NMR spectra of the monomer A and the hyperbranched polydithioacetal. The expanded region shows the presence of several peaks in the methine region of the dendritic (D) subunits.
features are evident in the spectrum of the polymer: one is the splitting of the benzylic proton peak (d) into two sets of peaks of equal intensity because of the presence of a prochiral center (carbon c); and second, the relative intensities of the dimethylacetal groups (peak b) and the associated methine proton (peak a) clearly reveal the loss of 1 equiv of methanol, confirming the formation of a moderately high molecular weight polymer. 594
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Interestingly, the single methine proton of the dithioacetal unit (peak c) does not appear as a singlet but as a collection of several peaks; however, the total intensity of these peaks matches well with the expected value. In addition, peaks at 7.8 and 10 ppm reveal the presence of aldehyde units; this is possibly due to inadvertent hydrolysis of the terminal dimethylacetal linkages, the extent of which was estimated to be around ∼14%. The origin of the multiple peaks for the single methine proton of the dithioacetal unit appears to indicate that the chemical shifts may be sensitive to the adjacent repeat unit and to the nature of the remote functionality. The 13C NMR and the heterocorrelation spectra confirm that all the peaks in this region are indeed associated with the single methine carbon signal (see Figures S2 and S3). Complete transformation of the terminal acetal groups to aldehyde units simplifies the methine proton region; a similar simplification was also observed when the terminal groups were completely transformed to dithioacetals (see Figure S4). Both these findings support the hypothesis that the chemical shifts of the methine protons are indeed sensitive to the nature of the remote functionality; however, the presence of residual multiple peaks even after complete transformation of the terminal groups to a single type (aldehyde or dithioacetal), possibly reflects the sensitivity to the monomer sequence along the polymer chain similar to tacticity observed in linear polymers, like polypropylene.20 One interesting feature of this polymerization process is that the dithioacetal linkage is formed irreversibly under the acidcatalyzed polymerization conditions; more importantly, the intermediate monothioacetal wherein one of the methoxy groups has been displaced by the thiol is very labile and therefore is likely to be rapidly converted to the more stable dithioacetal. To test this hypothesis, we carried out model reactions using equivalent quantity of tolualdehyde dimethylacetal (TDMA) and benzyl mercaptan (BzSH), as depicted in Figure 2; the reaction was carried out in a NMR tube in the presence of 1 mol % of iodine as the catalyst21 using dry CDCl3 as the solvent. A stack plot of the variation of the complete proton NMR spectra (Figure S1) clearly revealed the formation of the intermediate species during the early stages followed by its rapid disappearance; at the end of the reaction only two species, namely, the dithioacetal and the starting dimethylacetal, were present, as depicted in Figure 2. A stack plot (Figure 2) of a selected region of the spectra, showing the methine proton region, clearly reveals this process. The methine proton region revealed three distinct peaks: one at ∼5.35 ppm corresponding to the starting dimethylacetal, the other, at ∼5.3 ppm, due to the intermediate monothioacetal, and the third that is considerably upfield (∼4.45 ppm) corresponds to the dithioacetal; this permitted us to directly follow the evolution of all the three species. The peak at ∼5.3 ppm grows rapidly initially, and then after about 15 min it begins to disappear, and at the end of the reaction it has almost completely disappeared. Figure 3 shows a plot of the variation of the peak intensities due to these different species; the variation of the concentrations is reminiscent of the classic sequential reaction process. The intermediate monothioacetal grows rapidly initially and goes through a maximum before decaying; the concomitant increase of the dithioacetal during this decay period clearly reflects the transformation of the monothioacetal intermediate to the final product. The disappearance of the starting BzSH was also monitored using the thiol peak at ∼1.78 ppm (Figure S1) and this also confirms the hypothesized reaction scheme. Because this reaction was carried out using a
Figure 2. Variation of proton NMR spectra (selected region) of the model reaction mixture as a function of reaction time; the numbers on the left indicates the times of reaction. The peaks marked by arrows represent the benzylic methine protons of the three different colorcoded species.
Figure 3. Variation of the mole-fraction of various species as a function of time in the reaction mixture of model reaction between TDMA and BzSH (1:1 mol-ratio), in the presence of 1 mol % of I2 as catalyst. The concentrations were estimated from their proton NMR spectra that were recorded in CDCl3 (Figure S1).
mole ratio of TMDA/BzSH of 1:1, the presence of only two species at the end of the reaction, namely, the dithioacetal and the starting dimethylacetal, clearly confirms that the intermediate monothioacetal is unstable and is completely transformed to the dithioacetal. Furthermore, since the intermediate monothioacetal represents the linear defect during the polymerization process, these observations clearly suggests the possibility of generating defect-free hyperbranched 595
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actual weight-losses were normalized with respect to the expected weight-loss for 100% conversion and replotted as percent conversion as a function of time (Figure S7); these plots also clearly reveal the increase in rate with temperature. In order to compare this polymerization with the polymerization leading to the hyperbranched polyacetals that we had reported earlier,17 we also carried out the isothermal TGA studies using 4-hydroxymethylbenzaldehyde, dimethylacetal (monomer B); as expected, the rate of this polymerization was significantly slower. Using the data from the early stages of the polymerization, one can estimate the rate constant for the polymerization from the second order kinetic plots (Figure 5);
polymers using the strategy outlined in Scheme 1. The reversibility of the first step leading to the formation of the monothioacetal intermediate under acid-catalyzed conditions may be inferred from earlier studies of the mechanism of hydrolysis of cyclic thiooxalane from a range of substituted benzaldehydes;22 these studies clearly suggested that the monothioacetals are indeed acid-labile with the protonation and cleavage of the C−S bond occurring preferably. Thus, clearly the polymerization scheme presented here meets all the prescriptions enumerated by Voit10 for achieving defect-free HBPs; namely a reversible first step between A and B and an irreversible reaction of the intermediate with the second B group. As with the model compounds, the melt polymerization process was also followed as a function of time; aliquots were taken at different time intervals and their NMR spectra were recorded. As evident from Figure 4, the variation of the NMR
Figure 4. Expanded benzylic methine region (showing only the L and T units) of the 1H NMR spectra of aliquots of polymer samples taken after different time intervals during the polymerization. The peak marked by an arrow indicates the linear defects, which increases initially and then completely disappears at longer times, clearly confirming the complete transformation of the intermediate monothioacetal units to the dithioacetals; the methine protons corresponding to these D units are not shown and appear upfield at ∼4.4 ppm.
spectra as a function of polymerization time showed that linear monothioacetal defects are indeed formed during the early stages, but subsequently disappears almost completely after 90 min leaving behind only dendritic (D) dithioacetal units (which appears as a collection of peaks at ∼4.4 ppm, see Figure S5) and the terminal (T) dimethylacetal units. As mentioned earlier, inadvertent hydrolysis of the terminal acetal groups also generates about 14% of aldehyde terminal groups. Kinetics of Polymerization: Because the polymerization to generate HBPs utilizes a single monomer, which is relatively nonvolatile, isothermal thermogravimetric analysis (TGA), can serve as useful tool to follow the kinetics of polymerization; the loss of the condensate, methanol, in the present study is a direct measure of the percent conversion.23 Typically, monomer A, along with 2 mol % PCS, is taken in an aluminum pan; the pan is heated rapidly at 15 °C/min to the desired temperature and the weight-loss is measured as a function of time under a flow of dry argon. The isothermal TGA scans of the polymerization of monomer A were carried out at different temperatures in order to estimate the activation energy for the polymerization process. All these scans (Figure S6) clearly indicate a rapid weight-loss in the beginning followed by a slow asymptotic tail approaching the expected weight-loss for 100% conversion. The
Figure 5. Second order kinetic plots for the formation of polydithioacetal (top) and polyacetal (bottom), where C represents the percent-conversion. Data from only the early stages of the polymerization process were utilized for this plot to minimize effects of slow diffusion. At longer times (∼85% conversion) deviation from linearity is seen (bottom), especially at 90 °C; reasons for which are unclear at this time.
the Arrhenius plots (Figure S8) of the rate constants at different temperatures yielded the respective activation energies for the two polymerizations. While the data for the HB polyacetal polymerization yielded a single rate constant, the dithioacetal formation exhibited a clear two-step variation (see Figure 5); the first faster step appears to reflect the formation of the monothioacetal, while the second slower one corresponds to the transformation of this intermediate to the dithioacetal. Needless to add that the analysis of data from the later stages of the polymerization, especially in the melt, is likely to be affected by several factors, such as increased viscosity and the consequent slower diffusion. Estimates of the of activation energies based on the rate constants retrieved from the early stages (∼ 60% conversion) yielded values of 88 and 74 kJ/mol, 596
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polydithioacetal. The presence of a large number of dimethylacetal terminal units on the periphery of the defectfree hyperbranched polydithioacetal provides a useful handle for further derivatization using a variety of thiols; complete transformation of the these terminal units using thiols, such as dodecane thiol, benzyl mercaptan, etc., was demonstrated. One interesting feature of these hyperbranched polymers is the high sulfur content within their core-region; these could serve to coordinate with various metal ions and provides some valuable functions to these systems. Some of these unique features are being presently explored in our laboratory.
for the polyacetal and polydithioacetal formation, respectively (Figure S8). To confirm that the two-step process does reflect the formation of the intermediate, a few other isothermal TGA experiments were conducted: (i) a model high boiling dimethylacetal (1-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy-4(dimethoxymethyl)-benzene) was treated with a simple dithiol, namely, 1,6-hexanedithiol, to generate a linear polydithioacetal (see Figure S9), and (ii) the same dimethylacetal was treated with two-equivalents of dodecanethiol (Figure S10). Both these experiments exhibited a similar two-step variation in the 2nd order kinetic plots; the inflection point occurs at about ∼60% conversion, which is only slightly higher than the expected value of 50%. This clearly suggested that, even though the second step is kinetically slower than the first one, its irreversibility under the acid-catalyzed polymerization conditions is crucial to the formation of defect-free hyperbranched structures. Thus, it is reasonable to ascribe the two-step kinetics, seen only in the case of the polydithioketal formation, to the distinct rates associated with the first step to generate the monothioacetal intermediate and the second to the transformation of this intermediate to the final dithioacetal. The solution NMR studies of the model systems is clearly in agreement with these observation (Figure 3), as it also reveals the fast accumulation of the intermediate monothioacetal before its relatively slower transformation to the final dithioacetal. Peripheral Functionalization: One other interesting feature of this transthioacetalization process is the irreversible formation of the dithioacetal linkages during the polymerization, unlike in the previously reported trans-acetalization process to prepare hyperbranched polyacetals.17 This aspect, in conjunction with the high reactivity of the intermediate monothioacetal, clearly leads to the formation of defect-free hyperbranched polymers; an important advantage is the large number of terminal dimethylacetal groups that are available for further functionalization without affecting the polymer backbone dithioacetal linkages. These dimethylacetal groups can be treated with any thiol-containing molecule to generate a range of peripherally functionalized systems; in the present study the pristine HB dithioacetal was treated with dodecanethiol, benzylmercaptan, and ethylmercaptopropionate to quantitatively transform the terminal dimethylacetal groups. The NMR spectra of the various derivatives reveal that the transformation is nearly complete (Figure S11). The molecular weights of the parent HBP and its derivatives were determined by GPC and were found to be in the range Mw ∼ 23000 (Figure S12). The DSC curves show that the Tg of the parent polymer is around 1.5 °C, while that of the dodecyl derivative is polymer −36 °C and for the benzyl derivative it is 6 °C (Figure S13); such a variation of the Tg of hyperbranched polymers with the nature of the terminal group has been reported by several others.24 In conclusion, we have developed a new approach for the preparation of a hyperbranched polydithioacetal using an acidcatalyzed melt transthioacetalization process; the unique feature of this process is that it generates a defect-free hyperbranched polymer. Kinetic studies of model reactions using NMR clearly revealed the rapid and presumably reversible formation of the monothioacetal intermediate; this intermediate is then irreversibly transformed to the dithioacetal in a relatively slower second step. This combination of a reversible formation of a relatively unstable monothioacetal intermediate and its irreversible transformation to the final dithioacetal is clearly responsible for the generation of defect-free hyperbranched
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EXPERIMENTAL SECTION
Materials: Terephthalaldehyde, sodium borohydride, trimethylorthoformate, sodium hydrogensulphide, dodecanethiol, 4-hydroxybenzaldehyde, 1,6-hexanediol, p-tolualdehyde, triethyleneglycol monomethyl ether, and 3-mercaptopropionic acid were purchased from SigmaAldrich Chemical Co. and used directly. Thioacetic acid, thiourea, and benzyl chloride were purchased from Spectrochem Chemicals Ltd., while common organic solvents and reagents were procured from Ranbaxy, Spectrochem or Nice Chemicals. Solvents were distilled prior to use and, if necessary, were dried and distilled following the standard procedures. Methods: 1H, 13C, and 1H−13C-HSQC NMR spectra were recorded using a Bruker 400 MHz spectrometer using CDCl3 as the solvent and TMS as internal reference, unless otherwise mentioned. GPC measurements were carried out using Viscotek triple detector analyzer (TDA) model 300 system, that has a refractive index (RI), a differential viscometer (DV), and light scattering (LS) connected in series. The separation was achieved using a series of two PL gel mixed bed columns (300 × 7.5 mm) operated at 30 °C using THF as the eluent. Molecular weights were determined using a universal calibration curve based on the data from the RI and DV detectors using narrow polystyrene standards. The glass transition temperature of the sample was determined using a Mettler Toledo DSC instrument at a heating rate of 10 °C/min, under dry N2 atmosphere. The samples were first heated to 100 °C (to ensure that they melt and make good contact with the pan) and quenched prior to recording the Tg. Thermal gravimetric analysis (TGA) studies were carried out in a Netzsch TG209 F1 TGA instrument coupled with QMS 403 mass spectrometer under dry Argon atmosphere. Monomer Synthesis: Monomer A was synthesized from 4-hydroxymethylbenzaldehyde, which was first converted to 4-bromomethylbenzaldehyde by refluxing with aqueous HBr. The bromomethyl derivative was converted to the corresponding thioacetate by treatment with thioacetic acid/K2CO3. Subsequently, the aldehyde was converted to the dimethyl acetal by refluxing in dry methanol in the presence of trimethyl orthoformate, which upon hydrolysis yielded the required monomer (A). For further details of the synthesis of this monomer and the other model compounds refer to the Supporting Information. General Polymerization Procedure: Monomer A 1 g (5 mmol) was taken in a polymerization tube along with 2 mol % of pyridinium camphorsulphonate.19 The reaction mixture was degassed by purging N2 for 15 min and then heated to 60 °C under a N2 atmosphere to ensure homogeneous mixing of the catalyst and monomers. The polymerization was carried out at 80 °C for 45 min under N2 purging, followed by an additional 45 min at 80 °C under reduced pressure of 20 Torr using a Kugehlrohr apparatus. The polymer was dissolved in THF and the solution was neutralized using solid NaHCO3 and filtered. The filtrate was concentrated and poured into dry methanol to obtain the polymer. This polymer was further purified by dissolution followed by reprecipitation using THF-methanol. Yield is 0.5 g (60%). Model Kinetic Studies Using 1H NMR: A total of 12.5 mg (0.075 mmol) of 1-(dimethoxymethyl)-4-methylbenzene was taken along with 9.25 mg (0.075 mmol) of benzyl mercaptan and 0.6 mL of dry CDCl3 in an NMR tube; the 1H NMR spectra was recorded and this served as the starting spectra (0 min). 50 μL of an iodine solution in 597
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Chem. Ed. 2007, 45, 1177. (d) Saha, A.; Ramakrishnan, S. Macromolecules 2009, 42, 4956. (6) Samuel, A. Z.; Ramakrishnan, S. Macromolecules 2012, 45, 2348. (7) (a) Hawker, C. J.; Lee, R.; Frechet, J. M. J. J. Am. Chem. Soc. 1991, 113, 4583. (b) Kim, Y. H.; Webster, W. O. J. Am. Chem. Soc. 1990, 112, 4593. (8) (a) Hölter, D.; Burgath, A.; Frey, H. Acta Polym. 1997, 48, 30. (b) Hölter, D.; Burgath, A.; Frey, H. Acta Polym. 1997, 48, 298. (9) Hanselmann, R.; Hölter, D.; Frey, H. Macromolecules 1998, 31, 3790. (10) Maier, G.; Zech, C.; Voit, B.; Komber, H. Macromol. Chem. Phys. 1998, 199, 2655. (11) Smet, M.; Schacht, E.; Dehaen, W. Angew. Chem., Int. Ed. 2002, 41, 4547. (12) (a) Sinananwanich, W.; Ueda, M. J. Polym. Sci., Polym. Chem. 2008, 46, 2689. (b) Sinananwanich, W.; Higashihara, T.; Ueda, M. Macromolecules 2009, 42, 994. (c) Sinananwanich, W.; Segawa, Y.; Higashihara, T.; Ueda, M. Macromolecules 2009, 42, 8718. (13) Segawa, Y.; Higashihara, T.; Ueda, M. J. Am. Chem. Soc. 2010, 132, 11000. (14) Huang, W.; Su, L.; Bo, Z. J. Am. Chem. Soc. 2009, 131, 10348. (15) (a) Hobson, L. J.; Feast, W. J. Chem. Commun. 1997, 2067. (b) Haag, R.; Sunder, A.; Stumbe, J. F. J. Am. Chem. Soc. 2000, 122, 2954. (16) Jayakannan, M.; Ramakrishnan, S. Chem. Commun. 2000, 1967. (17) Chatterjee, S.; Ramakrishnan, S. Macromolecules 2011, 44, 4658. (18) Wilson, D. W.; Dalmasso, G.; Wang, L.; Sitaraman, S. V.; Merlin, D.; Murthy, N. Nat. Mater. 2010, 9, 923. (19) Behera, G.; Ramakrishnan, S. J. Polym. Sci., Polym. Chem. 2004, 42, 102. (20) A similar set of multiple peaks were also observed in the case of hyperbranched polyacetal. Reference 17. (21) Firouzabadi, H.; Iranpoor, N.; Hazarkhani, H. J. Org. Chem. 2001, 66, 7527. (22) (a) Fife, T. H.; Jao, L. K. J. Am. Chem. Soc. 1969, 91, 4217. (b) De, N. C.; Fedor, L. R. J. Am. Chem. Soc. 1968, 90, 7266. (23) (a) Thermal Analysis of Polymers: Fundamentals to Application; Menczel, J. D., Prime, R. B., Eds.; John Wiley & Sons: New Jersey, 2009. For previous reports that have utilized TGA for the study of polymerization kinetics, see: (b) Darda, P. J.; Hogendoorn, J. A.; Versteeg, G. F.; Souren, F. AIChE J. 2005, 51, 622. (c) Roy, R. K.; Ramakrishnan, S. J. Polym. Sci., Polym. Chem. 2011, 49, 1735. (24) (a) Kim, Y. H.; Webster, O. W. Macromolecules 1992, 25, 5561. (b) Wooley, K. L.; Fréchet, J. M. J.; Hawker, C. J. Polymer 1994, 35, 4489. (c) Uhrich, K. E.; Hawker, C. J.; Fréchet, J. M. J.; Turner, S. R. Macromolecules 1992, 25, 4583. (d) Miravet, J. F.; Fréchet, J. M. J. Macromolecules 1998, 31, 3461. (e) Ishizu, K.; Tsubaki, K.; Mori, A.; Uchida, S. Prog. Polym. Sci. 2003, 28, 27.
dry CDCl3 (which contains 1 mol % of iodine with respect to the reactants) was added to the NMR tube and the spectrum was recorded after regular time intervals. Isothermal TGA Experiments: All TGA experiments were done using aluminum pans. For the polymerization kinetics, the required AB2 monomer was homogenized with 2 mol % PCS and about 16−18 mg of the mixture was taken in the pan; the pan was placed in the TGA balance and rapidly heated at 15 °C/min to the required temperature and the weight loss was monitored as a function of time under a continuous purge of dry argon (20 mL/min). The model reactions were also carried out under similar conditions using required amounts of the appropriate reactants.
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ASSOCIATED CONTENT
S Supporting Information *
Detailed experimental procedures and spectral characterization of all monomers, polymers, and model systems. Isothermal TGA thermograms of the polymerization process, GPC chromatograms of the polymers, and DSC thermograms of various peripherally functionalized hyperbranched polydithioacetals. 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]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to thank the Department of Science and Technology, New Delhi, for the J.C. Bose fellowship (20112016) to S.R. and IISc central facilities for TGA and DSC data. S.C. would like to thank the Bristol-Meyers-Squib student fellowship for the year 2011−12.
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REFERENCES
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dx.doi.org/10.1021/mz300149t | ACS Macro Lett. 2012, 1, 593−598
