Rational Synthesis of Hyperbranched Poly(ester)s - Industrial

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Rational Synthesis of Hyperbranched Poly(ester)s Tracy Zhang,† Bob A. Howell,‡ and Patrick B. Smith*,† †

Michigan State University − Saint Andrews Campus, 1910 W. St. Andrews Rd., Midland, Michigan 48640, United States Center for Applications in Polymer Science Department of Chemistry, Central Michigan University, Mt. Pleasant, Michigan 48859-0001, United States

‡

ABSTRACT: Hyperbranched poly(ester)s have a variety of unique properties as a result of their abundance of end groups which make them useful in a range of applications. The synthesis of hyperbranched polymers has generally been carried out using empirically derived procedures to avoid gelation. However, such synthetic strategies lead to materials with unpredictable molecular weight and properties. It has now been demonstrated that modeling may be utilized to direct the synthesis of hyperbranched poly(ester)s from a variety of monomers to form well-defined structures and terminal-group functionality while avoiding gelation. In particular, Macosko−Miller modeling permits the synthesis of hyperbranched poly(ester)s of specific molecular weights and end-group functionality even for multifunctional monomers for which functional groups are not equally reactive.

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INTRODUCTION Hyperbranched polymers (HBPs) have attracted considerable attention due to their unique structures, possessing a large number of end groups, low viscosity, good solubility, and facile synthesis procedures.1,2 Among hyperbranched systems, poly(ester)s offer attractive features for applications in a number of areas including coating formulations and platforms for the controlled release of actives.3,4 Traditionally, hyperbranched poly(ester)s (HBPEs) have most often been prepared by the polycondensation of either ABx monomers2 or two multifunctional monomers, Ax + By, where A and B are two types of mutually reactive functional groups, x and y are equal to or larger than 2, and one of them must be greater than 2. There is a much greater selection of Ax and By monomers. Further, the polymers from ABx monomers have only one possible composition from each monomer.5 Hyperbranched poly(ester)s from multifunctional alcohols such as glycerol, trimethylolpropane, and pentaerythritol with diacids such as succinic and adipic acid have been reported.6−15 The structure of these HBPEs were characterized in great detail. But synthetic strategies to avoid gelation were performed by stopping the reaction at a finite extent of reaction through measurement of viscosity or acid level or by controlling reaction time and temperature. The ability to target the molecular weight, composition, and degree of branching from © XXXX American Chemical Society

an initial predefined set of reactants and experimental conditions seemed to be lacking in these studies. This ability to target the structure of these HBPEs from initial conditions would save considerable effort and cost in their preparation and would greatly improve the consistency of the resulting polymer. The use of the gelation theories to prepare HBPEs of targeted structures has been demonstrated for polymers taken to a high extent of reaction. The synthesis of HBPEs from glycerol, trimethylolpropane, and adipic acid16,17 and also that of hyperbranched polycarbosiloxanes and polycarbosilanes18,19 used the Flory−Stockmayer (FS) strategy to prepare hyperbranched polymers by this approach. However, the general preparation of HBPEs using FS and Macosko−Miller (MM) theories20−26 to target specific molecular weights and end groups at high extents of reaction by proper choice of monomer stoichiometry has not been extensively reported. Stated differently, this formalism can be used to target specific Mw values and other structural features of hyperbranched polymers for reactions taken to completion, simply from the initial monomer stoichiometry. Received: Revised: Accepted: Published: A

November 14, 2016 January 19, 2017 January 19, 2017 January 19, 2017 DOI: 10.1021/acs.iecr.6b04435 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Scheme 1. Synthesis of Trimethylolpropane−Succinic Acid Hyperbranched Poly(ester) Containing Hydroxyl End Groups

Scheme 2. Synthesis of Trimethylolpropane−Succinic Acid Hyperbranched Poly(ester) Containing Carboxyl End Groups

Bimolecular Nonlinear Polymerization (BMNLP). Because of the limited number of ABx types of monomers, a biomolecular nonlinear polymerization (BMNLP) technique was developed to provide another approach for the preparation of hyperbranched polymers using step-growth polymerization techniques. In this approach, there are two reactive monomers, Ax and By, where A and B are two types of mutually reactive functional groups, x and y are equal to or larger than 2, and one of them must be greater than 2. BMNLP systems have significant advantages over the ABx systems including much greater selection of monomers, unlimited variety of polymer compositions, and compositionally identical polymers with different end groups (A or B). The essential feature of the BMNLP method is control of functional group stoichiometry to avoid gelation.5 The polymerization of Ax + By produces soluble polymers by adjusting the molar ratios of A/B to satisfy the following conditions:

hyperbranched polymers of high molecular weight and branching can be formed without gelation. Polymerization in dilute solution displayed no gelation even at stoichiometric ratios of monomers beyond the theoretical gel point due to a finite degree of cyclization. The results obtained from kinetic Monte Carlo simulations and experimental observations were in very good agreement. It has been demonstrated that the assumptions from Flory theory are not exact for real systems due to the presence of side reactions such as intramolecular cyclization. The experimentally determined molecular weight values for a given functional group stoichiometry tend to be lower than theoretically predicted.5 However, the theoretical stoichiometry provides a good starting point to avoid gelation for syntheses taken to high conversion. Several classes of hyperbranched polymers have been prepared using this theory as a guide.1,16−19,30,31 Macosko−Miller Conditional Probability Model. The prediction of the molecular weight of hyperbranched systems with the Flory−Stockmayer model32 assumes equal reactivity of functional groups. For monomers containing functional groups of unequal reactivity such as glycerol, this model fails to correctly predict the observed molecular weight values and gel point. This is also probably true for multifunctional monomers as a result of substituent effects on the rate of reaction. A conditional probability model developed by Macosko and

(x − 1)(y − 1) < [A]/[B] < 1/[(x − 1)(y − 1)]

This equation is applicable in the absence of intramolecular reactions, such as cyclization. Modeling and experimental studies have been performed to reflect the influence of the polymerization procedure on polymer topology and other structural properties of hyperbranched polymers obtained from BMNLP systems.27−29 These studies demonstrated that B

DOI: 10.1021/acs.iecr.6b04435 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Infrared spectra of a TMP−SA hyperbranched poly(ester) containing carboxyl end groups (top) and hydroxyl end groups (bottom).

end groups is shown in Figure 2. Multiple resonances for the quaternary carbon atom and the methylene carbon atom of

Miller can be used to predict the weight-average molecular weight as a function of extent of reaction for monomers of unequal reactivity. This has been demonstrated for a series of hyperbranched polymers made from glycerol.16

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RESULTS Hyperbranched poly(ester)s derived from trimethylolpropane (TMP) with succinic acid (SA) (Schemes 1 and 2) and pentaerythritol (PE) with adipic acid (AA) containing different end groups (hydroxyl or carboxyl) were synthesized and characterized. The infrared spectra of a TMP-SA HBPE are presented in Figure 1. The spectrum of the hydroxyl-terminated polymer contains a hydroxyl absorption at 3462 cm−1, aliphatic C−H absorptions at 2952 and 2874 cm−1, ester carbonyl absorption at 1737 cm−1, and C−O absorption at 1173 cm−1. The infrared spectra of the polymer-containing carboxyl terminal groups display a broad carboxyl absorption centered at 3225 cm−1, aliphatic C−H absorptions at 2965 and 2883 cm−1, ester carbonyl absorption at 1740 cm−1, carboxyl carbonyl absorption at 1708 cm−1, and C−O absorption at 1171 cm−1. The synthesis of hyperbranched poly(ester)s of pentaerythritol and adipic acid was conducted at 180 °C in absence of a catalyst. Products from the attempted poly(esterification) in the presence of a catalyst, such as toluenesulfonic acid, gelled. The poly(esterification)s involving pentaerythritol were heterogeneous reactions due to the fact that pentaerythritol is a high melting (260 °C) crystalline solid and is insoluble in adipic acid at the reaction temperature. During the reaction, the pentaerythritol slowly dissolves, apparently, as it is converted to the monoester. Therefore, the relative stoichiometry of reactive functional groups changes throughout the reaction. The rate of esterification in the presence of a catalyst is rapid and difficult to control to avoid gelation. The reaction is slower and better behaved in the absence of a catalyst. Since pentaerythritol is a very high melting solid with limited solubility, it apparently only dissolves as the free PE is removed by esterification with AA. The two parameters, Pa and Pb, have been defined elsewhere as the extent of reaction of adipic acid carboxyl units and the extent of reaction of the pentaerythritol alcohol units, respectively.16 The value of these parameters ranges from 0 (no reaction) to 1.0 (complete reaction). The functional group ratio ([−OH]/[−COOH]) at the gel point for the PE-AA HBPE is 3.0, such that when Pa is equal to 1.0, indicating that the carboxyl units have been completely converted to esters, and Pb is equal to 0.333. The quantitative 13C NMR spectrum of the pentaerythritol-adipic acid HBPE containing hydroxyl

Figure 2. Carbon-13 NMR spectrum of a pentaerythritol−adipic acid hyperbranched poly(ester) containing hydroxyl end groups.

pentaerythritol reflect the presence of unsubstituted, mono-, di-, tri-, and tetra-esterified structures. The mole ratio of pentaerythritol to adipic acid was determined from the ratio of the areas of carbon atoms 1 and 2 from pentaerythritol to those of carbon atoms A and B of adipic acid. An expansion of this region of the carbon-13 NMR spectrum is shown in Figure 3. The carbon resonances from free pentaerythritol and mono-, di-, tri-, and tetra-esters are labeled. The relative peak areas for these resonances reflect the mole ratios of the species present. The relative concentrations are listed in Table 1. The distribution of pentaerythritol structures was fit to a Bernoullian (random) statistical distribution by defining the variable “P” as the probability that a pentaerythritol hydroxyl unit was converted to an ester. Here, 1 − P is the probability that it was not converted. Therefore, the mole fraction of free pentaerythritol is (1 − P).4 The mole fraction of monoester is P(1 − P),3 and since there are four possible alcohol units to be converted, the probability is multiplied by 4. To be consistent, P is equivalent to Pb, the extent of reaction of the pentaerythritol alcohol units discussed earlier and is equal to 0.333 at the high extent of reaction for the carboxyl units when the functional group stoichiometry is equal to 3.0 (excess PE). The agreement between the observed mole ratios of the species present determined by 13C NMR and those predicted from a random statistical analysis is excellent and somewhat unexpected because of the heterogeneous nature of the reaction mixture. There are two potential explanations for the random substitution distribution. First, transesterification would cause C


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Figure 3. Pentaerythritol region of the carbon-13 NMR apectrum of a pentaerythritol−adipic acid hyperbranched poly(ester) containing hydroxyl end groups.

Table 1. Measured and Predicted Distributions of Different Species for Formation of Pentaerythritol−Adipic Acid Hyperbranched Poly(ester)s with Functional Group Stoichiometry [−OH]/[−COOH] = 3.0

a

species

probability

predicted distribution

experimental (NMR) distributiona

pentaerythritol monoester diester triester tetraester

(1-P)4 4P(1-P)3 6P2(1-P)2 4P3(1-P) P4

0.198 0.396 0.296 0.099 0.012

0.192 0.403 0.298 0.093 0.014

Estimated uncertainty is ±0.01. Figure 4. Carbon-13 NMR spectrum of a pentaerythritol−adipic acid hyperbranched poly(ester) containing carboxyl end groups.

the distribution to change continuously as the reaction proceeds until a random distribution is achieved. Transesterification is difficult to reconcile for this reaction, especially since the catalyzed process for this reaction mixture gels. A second, more likely explanation is that the rate of esterification is slow compared to the rate of dissolution of the pentaerythritol. Pentaerythritol has little solubility in the reaction mixture so as soluble monoester is formed; the pentaerythritol consumed is replenished by dissolution. The esterification pattern and stoichiometry would remain constant (and random) only if subsequent esterification is slow relative to the rate of pentaerythritol monoester formation. The 13C NMR spectrum of the pentaerythritol−AA HBPE containing carboxyl end groups (with stoichiometry of [−OH]/[−COOH] = 1/3) is given in Figure 4. The resonance for the quaternary carbon atom of pentaerythritol is a singlet at δ 62.57. Similarly, the resonance for the methylene carbons from pentaerythritol is a singlet at δ 41.91. The fact that these resonances are both singlets indicates that all the hydroxyl functional groups of pentaerythritol have been entirely converted to esters. Two carbonyl resonances are observed, one for acid functionality and the other for the ester moiety in this PE-AA HBPE containing carboxyl end groups.

The 1H NMR spectra for the pentaerythritol−adipic acid hyperbranched poly(ester) containing hydroxyl and carboxyl end groups are given in Figure 5. The corresponding 13C NMR spectra are presented in Figure 6. Both sets of spectra demonstrate that the reaction of pentaerythritol with adipic acid proceeds in accordance with BMNLP predictions: All carboxyl groups of adipic acid have been converted to ester functionality for the polymer containing hydroxyl end groups, and all the hydroxyl groups of pentaerythritol have been converted to ester functionality for the polymer containing carboxyl end groups. Gelation did not occur in either case. The SEC chromatograms for the PE−AA HBPEs containing hydroxyl end groups and those containing carboxyl end groups are given in Figure 7. The peaks for the monomers are identified on the chromatogram. Individual peaks for many of the oligomers are also resolved. It is not unusual to observe monomers and oligomers in hyperbranched polymers because they are often fairly low molecular weight systems. The low molecular weight fraction can be removed by ultrafiltration or solvent fractionation. D


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than the number-average values because the low molecular weight species are poorly detected by light scattering. Therefore, the absolute values of the dispersity index (DI) determined by this analysis are estimates only. However, the trends in the DI, which increase with increasing molecular weight, are consistent with expectations. HBPEs of TMP−AA with Different Functional Group Stoichiometries. In order to demonstrate the power of the conditional probability formalism, a series of HBPEs of glycerol−AA16 and TMP−AA were synthesized with [−OH]/ [−COOH] stoichiometries varying from the theoretical gel point (2:1) to stoichiometries far away from the gel point (2.4:1). These HBPEs were all viscous clear oils. The weightaverage molecular weight determined by SEC-MALS, weightaverage molecular weight calculated using the Flory−Stockmayer model, dispersity index (DI), degree of branching (DB %), and experimental glass transition temperature as a function of stoichiometry for the TMP series are collected in Table 2. The average degree of branching was calculated from the expression33

Figure 5. 1H NMR spectra (DMSO-d6) of pentaerythritol−adipic acid hyperbranched poly(ester) containing hydroxyl end groups (top) and carboxyl end groups (bottom).

Average degree of branching (%) =

2(triester concentration) 2(triester concentration) + (diester concentration) × 100

The relative concentration of the various species present was determined from integration of the appropriate resonances in the 13C NMR spectrum of the polymer. The Flory−Stockmayer model predicts that the functional group stoichiometry at the gel point for the A3 + B2 reaction is 2:1. In fact, the HBPE generated at this functional group stoichiometry is soluble and not cross-linked. The observed gel point is different from that theoretically predicted by the Flory−Stockmayer model because the assumption that there are no side reactions, such as cyclization reactions, is not valid. The gel point is observed at slightly lower stoichiometries. Even so, the agreement between predicted (Flory−Stockmayer model) and determined (SEC-MALS) molecular weight values is quite good. The Tg also increases only slightly with increasing Mw as shown in Table 2. A crude synthesized hyperbranched poly(ester) is a distribution of excess monomer, oligomers, and hyperbranched polymer. The lower the molecular weight of the HBPE is, the greater the proportion of monomer and oligomers is. These low molecular weight species could lead to plasticization of the polymer, lowering the Tg values of Table 2. This effect appears to be minimal since there is only a very slight molecular weight dependence of the Tg. Excess monomer and low-molecularweight oligomer can be removed, for example, by precipitation in water.1,30,31 For applications of controlled release, excess monomer and low molecular weight oligomers may be beneficial to the release rate. The Flory−Stockmayer model is not able to correctly predict the molecular weight values and gel points for monomers with unequal functional group reactivities, such as with glycerol. The Flory−Stockmayer model predicts the gel point for a stoichiometry of 2.0 at a high extent of reaction, Pb = 1.0. A series of HBPEs from glycerol and adipic acid of varying stoichiometry were synthesized to evaluate the ability of the Flory−Stockmayer and the Macosko−Miller models to predict their structures as shown in Table 3. According to the Flory−

Figure 6. 13C NMR spectra (DMSO-d6 solution) of pentaerythritol− adipic acid hyperbranched poly(ester) containing hydroxyl end groups (top) and carboxyl end group (bottom).

Figure 7. SEC chromatograms for the PE−AA HBPEs containing hydroxyl end groups (solid line) and containing carboxyl end groups (dotted line).

The molecular weight analysis was performed using a light scattering detector, which gives absolute molecular weight. These HBPEs have weight-average molecular weight values below 10,000 (Table 2) with dispersities ranging from 3 to 5 for the lower molecular weight materials and increasing for HBPEs with increasing molecular weight. The weight-average molecular weight values are determined with more accuracy E


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Table 2. Properties of TMP−AA Hyperbranched Poly(ester)s Produced from Different Functional Group Stoichiometries Mw (kDa)

a

ratio of [OH]/[COOH]

theoreticala

experimental

DIb

degree of branchingc

Tg (°C) ±0.5d

2:1 2.08:1 2.1:1 2.2:1 2.3:1 2.4:1

N/A 9.7 7.6 3.9 2.7 2.1

8.7 6.7 5.7 3.4 2.8 2.7

8.1 7.2 4.5 4.6 3.5 3.9

0.35 0.34 0.33 0.33 0.29 0.29

−29.6 −30.1 −30.7 −31.7 −32 −32.2

Flory−Stockmayer model. bSEC-MALS. cDetermined from quantitative 13C NMR measurements dDSC; second heat; 5 °C/min

Table 3. Determined and Predicted Mw as a Function of Stoichiometry for Glycerol−Adipic Acid Hyperbranched Poly(ester)

a

stoichiometry [−OH]/[−COOH]

Mw (Da) determineda

Mw (Da) predictedb

degree of branchingc

Pb from NMR

Pb, gel predictedd

2.16 2.00 1.93 1.74 1.69 1.52

1100 1250 2100 2400 4900 14000

1569 1454 2015 2938 3545 14492

0.24 0.23 0.32 0.30 0.32 0.36

0.96 0.93 0.91 0.92 0.91 0.90

no gel 1.00 0.98 0.93 0.92 0.54

SEC-MALS. bMacosko−Miller model. cDetermined from quantitative 13C NMR measurements dFlory−Stockmayer model

Stockmayer model, the glycerol−adipic acid HBPE with stoichiometry of 1.74 and 1.69 having Pb = 0.92 and 0.91, respectively, should be very near the gel point and that of stoichiometry 1.52 with Pb = 0.90 should be well past the gel point. In fact, all of the samples have finite molecular weight and do not exhibit gelation. The Macosko−Miller model, which accurately incorporates unequal monomer functional group reactivity, accurately predicts the molecular weight and gel behavior of all samples.16 The weight-average molecular weight determined by SEC-MALS, which was calculated using the Macosko−Miller model, dispersity index (DI), degree of branching (DB%), and predicted extent of reaction for gelation, Pb, gel, from the Flory−Stockmayer model as a function of stoichiometry for this series are collected in Table 3. Two HBPEs from trimethylolpropane, one with adipic acid and the other with succinic acid, were prepared under identical conditions with functional group stoichiometries [−OH]/ [−COOH] = 2:1. Both were clear viscous oils. Since SA has two fewer carbon atoms than AA, the hyperbranched poly(ester) from SA might be expected to be more rigid than that from AA, giving rise to materials possessing greater viscosity and higher Tg. The TMP−SA HBPE was indeed more viscous and possessed a higher Tg than TMP−AA HBPE, as expected. The weight-average molecular weight determined by SEC-MALS, dispersity, and glass transition temperature for TMP−AA HBPE and TMP−SA HBPE are listed in Table 4. The thermal stability of the two polymers is illustrated in Figure 8. The temperature of degradation onset for TMP−SA is slightly lower (324 °C) than that for TMP−AA (344 °C), but the temperature of maximum degradation rate is lower for TMP−AA (392 °C) than for TMP−SA (439 °C).

Figure 8. Mass loss versus temperature plots for TMP−AA and TMP−SA.

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CONCLUSIONS A range of hyperbranched poly(ester)s have been prepared under conditions predicted by various modeling approaches. These materials demonstrate that HBPEs can be prepared such that gelation is avoided, and the presence of either hydroxyl or carboxyl end groups is assured. Targeted polymer molecular weights at high monomer conversion may be achieved. For monomers containing functional groups of equal reactivity, the Flory−Stockmayer theory or an extended version of this approach, the bimolecular nonlinear polymerization methodology, may be used to determine conditions for a particular polymerization. When monomer functional group reactivity is not equal, this approach is not adequate. In this case, the Macosko−Miller theory may be used to define the appropriate synthesis and predict the product structure.

Table 4. Properties of Hyperbranched Poly(ester)s, TMP− AA, and TMP−SA

a

HBPE

Mw (Da)a

DI

Tg (°C)b

TMP−AA TMP−SA

8700 6100

8.1 6.1

−29 −12

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EXPERIMENTAL SECTION Polymerizations were carried out in dry glassware under a nitrogen atmosphere by first oligomerizing the reactants in the apparatus referred to as assembly I and then transferring them to apparatus assembly II to complete the reaction by removal of

SEC-MALS. bDSC; second heat; 5 °C/min. F


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Figure 9. Apparatus for poly(esterification).

For the TMP−AA HBPE with carboxyl end groups, the ratio of [−OH]/[−COOH] = 0.5, such that the mole ratio of TMP and SA for this stoichiometry was 0.33. TMP−SA Hyperbranched Poly(ester)s Containing Hydroxyl End groups. The procedure for the synthesis of the hyperbranched poly(ester) from TMP and SA was similar to that for the HBPE from TMP and SA. Into a dry 100 mL, three-necked, round-bottomed flask fitted with a magnetic stirring bar and a condenser bearing a gas-inlet tube was placed 5.00 g (37.4 mmol) of trimethylolpropane, 3.33 g (27.7 mmol) of succinic acid, and 0.24 g (1.39 mmol, 2.5 mol % of the reactive carboxyl groups present) of p-toluenesulfonic acid. The flask was mounted in an oil bath maintained at 140 °C. The mixture was stirred under a flow of nitrogen for about 1 h using reaction apparatus I and then for 2 h using reaction apparatus II. Periodically, aliquots of the mixture were removed for analysis by GPC. Typically, a molar mass of 5000 g mol−1 was achieved within 3 h. The product was clear and more viscous than the TMP HBPE from AA: 1H NMR (δ, CDCl3) 0.67− 0.79 (m, CH 3 ), 1.16−1.38 (m, CCH 2 CH 3 ), 2.55 (OCOCH2CH2COO), 3.35 (s, HOCH2 with triester substitution), 3.42 (s, HOCH2 with mono- or diester substitution), 3.49 (s, HOCH2, unreacted TMP), 3.79 (s, COOCH2 mono alcohol), 3.86−3.98 (m, COOCH2 with diester substitution or three −OH substitution); 13C NMR (δ, CDCl3) 6.91−6.99 (CH3), 21.26−22.47 (CCH2CH3), 28.47−28.78 (OCOCH 2 CH 2 COO), 41.76−42.58 (quaternary carbon atom), 61.47−64.51 (CH2 adjacent to −OH or alcohol ester substituted), 171.66−172.47 (carbonyl of mono-, di-, and triester group); IR (cm−1) 3298 (O−H stretch), 2961, 2864 (C−H saturated), 1733 (CO ester), 1175 (C−O stretch). TMP−SA Hyperbranched Poly(ester)s Containing Carboxyl End Groups. The procedure for the synthesis of a TMP hyperbranched poly(ester) containing carboxyl end groups from TMP and SA was similar to that for the synthesis of a HBPE with hydroxyl end groups. Into a dry, 100 mL, threenecked, round-bottomed flask fitted with a magnetic stirring bar and a condenser bearing a gas-inlet tube was placed 2.50 g (18.6 mmol) of trimethylolpropane, 6.59 g (55.84 mmol) of succinic acid, and 0.24 g (1.4 mmol, 2.5 mol % of the reactive hydroxyl groups present) of p-toluenesulfonic acid. The flask was mounted in an oil bath maintained at 140 °C. The mixture was stirred under a flow of nitrogen for about 1 h using reaction apparatus I and then for 2 h using reaction apparatus II. Typically, a molar mass of 6000 g mol−1 was achieved within 3 h. The crude product was dissolved in a minimum amount of

water by purging the reaction vessel with a stream of nitrogen (Figure 9). Infrared spectra were obtained using a Nicolet 20DXB FTIR spectrometer. Quantitative 13C NMR spectra were obtained using a 5−15% solution in deuterochloroform with a Varian Inova 500 MHz spectrometer operating at 125.7 MHz. A 90° pulse width was used, and the pulse repetition time was 10 s. Gated decoupling was used without NOE, and the sweep width was 31 kHz, number of points 13.1 K, 3.0 Hz line broadening. The relative areas of the resonances were compared to spectra obtained with a 5 s pulse repetition rate, yielding equivalent results, demonstrating that these conditions were quantitative. 1H NMR spectra were obtained using a Varian Inova 500 NMR spectrometer operating at 499.7 MHz for 1H observation. The pulse width was 8°, and the pulse repetition time was 5 s, sweep width 8000 Hz, number of points 65,536, 0.1 Hz line broadening, 16 scans. Proton and carbon chemical shifts are reported in parts-per-million (δ) with respect to tetramethylsilane (TMS) as an internal reference (δ = 0.00). Size exclusion chromatography (SEC) was performed using a Waters 1525 liquid chromatography equipped with two Agilent PL gel 3 μm MIXED-E columns in series and a Waters 410 refractive index detector in series with a Wyatt Technologies DAWN HELEOS II light scattering detector. The solvent was THF at a flow rate of 1 mL min−1. The sample concentration was roughly 5 mg mL−1. Common solvents and reagents were obtained from Thermo Fisher Scientific or the Aldrich Chemical Co. Trimethylolpropane (TMP), glycerol, pentaerythritol (PE), adipic acid (AA), succinic acid (SA), and other chemicals were from SigmaAldrich and used without further purification. Synthesis. The synthesis and characterization of HBPEs based on glycerol and TMP with adipic acid containing hydroxyl and carboxyl end groups have been described.1,16,17,30,31 The polyester was synthesized by melt polymerization at 140 °C in the presence of catalysts (TsOH, dibutyltin oxide, or without catalyst). The reaction was driven to completion by removing the evolved water by purging the reaction vessel with a continuous stream of nitrogen. Hyperbranched Poly(ester)s from TMP and Succinic Acid. For an A3 + B2 monomer system, a soluble HBPE is formed if the functional group stoichiometry, r > (x − 1)(y − 1), where x and y are the functionality of the monomers. For the HBPE containing hydroxyl end groups and mole ratio of end-group functionality equal to 2.0, the mole ratio of TMP to SA was 1.33. G


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reaction apparatus I and then for 2 h using reaction apparatus II. The product was a white, waxy, semisolid. The molar mass of product was about 5000 g mol−1. 1H NMR (δ, DMSO-d6) 1.51 (m, OCOCH2CH2), 2.27 (m, OCOCH2CH2), 3.35−4.05 (H from pentaerythritol moiety); 13C NMR (δ, DMSO-d6) 23.91 (OCOCH2CH2), 33.21 (OCOCH2CH2), 42.95−45.56 (quaternary carbon atom), 59.67−63.12 (CH2 adjacent to alcohol, ester substituted), 172.53−172.85 (carbonyl carbon atoms of the ester groups); IR (cm−1) 3462 (m, broad, O−H stretch), 2952 (s), 2874 (m, C−H aliphatic), 1737 (vs, ester CO), 1173 (s, C−O stretch). Pentaerythritol−AA Hyperbranched Poly(ester)s Containing Carboxyl End groups. Into a dry, 100 mL, three-necked, round-bottomed flask fitted with a magnetic stirring bar and a condenser bearing a gas-inlet tube was placed 0.78 g (5.71 mmol) of pentaerythritol and 5.00 g (34.21 mmol) of adipic acid. The flask was mounted in an oil bath maintained at 180 °C. The mixture was stirred under a flow of nitrogen for about 5 h in reaction apparatus I and then for 2 h in reaction apparatus II. The product was a white, waxy, semisolid with a molar mass of about 6000 g mol−1. 1H NMR (δ, DMSO-d6) 1.48 (m, OCOCH2CH2), 2.16−2.29 (m, OCOCH2CH2). 4.05 (s, COOCH2 with all four −OH substituted); 13C NMR (δ, DMSO-d 6 ) 23.82−24.17 (OCOCH 2 CH 2 ), 33.04−33.51 (OCOCH2CH2), 41.91 (quaternary carbon atom), 62.57 (CH2 adjacent to alcohol, ester substituted), 172.45 (carbonyl carbon atom of the ester group), 174.39−174.48 (carbonyl carbon atom of the acid end groups); IR (cm−1) 3221 (m), carboxyl of end groups, 2965 (s), 2883 (m), aliphatic C−H, 1737 (vs), ester carbonyl, 1708 (s), carboxyl carbonyl, 1174 (s), C−O stretch. Hyperbranched Poly(ester)s from TMP and AA Synthesized using Different Monomer Stoichiometries. A series of HBPEs from TMP and AA were synthesized from various functional group stoichiometries using procedures analogous to that used for the preparation of the hyperbranched poly(ester)s from TMP and AA. Functional group stoichiometries for of HBPEs synthesized from TMP and AA are listed in Table 3. The polymers are all viscous clear liquids. Hyperbranched Poly(ester)s from Glycerol and with Various Diacids. Glycerol−Succinic Acid Hyperbranched Poly(ester) Containing Hydroxyl End Groups. The glycerol− succinic acid poly(ester) containing hydroxyl end groups was prepared using melt polymerization. Into a dry, 100 mL, threenecked, round-bottomed flask fitted with a magnetic stirring bar and a condenser bearing a gas-inlet tube was placed 5.00 g (54.3 mmol) of glycerol, 4.81 g (40.7 mmol) of succinic acid, and 0.15 wt % dibutyltin oxide based on total monomer mass. The flask was mounted in an oil bath maintained at 150 °C. The mixture was stirred under a flow of nitrogen for about 3 h using reaction apparatus I and then for 9 h using reaction apparatus II. The product was obtained as a clear, colorless, very viscous liquid. The conversion was 80%. Typically the molar mass was 2000−,000 g mol−1: 1H NMR (δ, DMSO-d6) 2.56 (m, OCOCH2), 3.34−5.26 (H from glycerol moiety); 13C NMR (δ, DMSO-d6) 28.58 (OCOCH2), 59.43−75.91 (carbon atoms from glycerol), 171.91 (ester carbonyl carbon atom); IR (cm−1) 3462 (m, broad, O−H stretch), 2952 (s), 2874 (m, C− H aliphatic), 1737 (vs, ester CO), 1173 (s, C−O stretch). Glycerol−Sebacic Acid Hyperbranched Poly(ester) Containing Hydroxyl End Groups. A glycerol−sebacic acid poly(ester) containing hydroxyl end groups was prepared using a procedure similar to that used for the synthesis of

acetone. The acetone solution was slowly dropped into 1000 mL of rapidly stirred water to precipitate the product. Dissolution/precipitation was repeated twice. The final product was a clear, colorless, viscous liquid. The yield was 60%. 1H NMR (δ, DMSO-d6) 0.98−1.03 (m, CH3), 1.58−1.61 (m, CCH 2 CH 3 ), 2.62−2.77 (OCOCH 2 CH 2 COO), 4.17 (s, COOCH2 all three −OH substituted); 13C NMR (δ, DMSOd 6 ) 7.17 (CH 3 ), 22.44 (CCH 2 CH 3 ), 28.55−28.67 (OCOCH2CH2COO), 40.49 (quaternary carbon atom), 63.65 (CH2 adjacent to −OH or alcohol ester substituted), 171.77 (carbonyl ester group), 173.37 (carbonyl acid atom of acid end group); IR (cm−1) 3222 (carboxyl of end group), 2962 and 2883 (aliphatic C−H), 1735 (ester carbonyl), 1707 (carboxyl carbonyl), 1175 (C−O stretch). Hyperbranched Poly(ester)s Based on Pentaerythritol. The stoichiometry required to synthesize a soluble PE-based HBPE was determined using the same theory discussed earlier. For the pentaerythritol, AA system, an A4 + B2 system, an HBPE is formed if r > (x − 1)(y − 1) or r < 1/[ (x − 1)(y − 1)]. The stoichiometry for this synthesis was calculated to be just inside the gel point stoichiometry, with B as a minor component for which r is equal to 3.0 such that the mole ratio of PE and AA is 1.5. The stoichiometry for the synthesis of an HBPE with PE as the minor component which is just inside the gel point stoichiometry, has an r value equal to 1/(x − 1)(y − 1) or 0.333. This HBPE will have carboxyl end groups with the ratio of [−OH]/[−COOH] = 1/(4 − 1) × (2 − 1) = 0.33 or the mole ratio of PE to AA is 0.17. The synthesis of hyperbranched poly(ester)s of pentaerythritol and adipic acid containing either hydroxyl or carboxyl end groups was conducted at 180 °C in absence of a catalyst. Products from the attempted poly(esterification) in the presence of catalyst gelled. The poly(esterification)s involving pentaerythritol were heterogeneous reactions due to the fact that pentaerythritol has a high melting point of 260 °C and is an insoluble solid in adipic acid at the reaction temperature. During the reaction, the pentaerythritol slowly dissolves, apparently as it is converted to the monoester. Therefore, the relative stoichiometry of reactive functional groups changes throughout the reaction. The rate of esterification in the presence of a catalyst is rapid and controlled poly(esterification) to avoid gelation is difficult. In the absence of catalyst, the reaction is much more well behaved. The four hydroxyl groups of pentaerythritol have equivalent reactivity, such that the distribution of products formed would be expected to follow random statistics. Since pentaerythritol is a very high melting solid with limited solubility, it only dissolves as the free PE is removed by esterification with AA. Pentaerythritol−AA Hyperbranched Poly(ester)s Containing Hydroxyl End groups. Into a dry, 100 mL, three-necked, round-bottomed flask fitted with a magnetic stirring bar and a condenser bearing a gas-inlet tube was placed 5.00 g (36.72 mmol) of pentaerythritol, 3.58 g (24.49. mmol) of adipic acid, and 0.21 g (2.5 mol % based on the number of reactive carboxyl groups present) of TsOH. The flask was mounted in an oil bath maintained at 180 °C. The mixture was stirred under a stream of dry nitrogen for about 1 h. The reaction mixture appeared to cross-link into a brown gel. This reaction was repeated at 150 °C for about 4 h, and the reaction mixture appeared to partially gel. The reaction was repeated again but without catalyst. The flask was mounted in an oil bath maintained at 180 °C. The mixture was stirred under a flow of nitrogen for about 5 h using H



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glycerol−AA poly(ester)s. Into a dry 100 mL, three-necked, round-bottomed flask fitted with a magnetic stirring bar and a condenser bearing a gas-inlet tube was placed 5.00 g (54.3 mmol) of glycerol, 8.24 g (40.7 mmol) of sebacic acid, and 0.15 wt % dibutyltin oxide based on total monomer mass. The flask was mounted in an oil bath maintained at 150 °C. The mixture was stirred under nitrogen flow for about 3 h using reaction apparatus I and then for 9 h using reaction apparatus II. The product was a clear, colorless, viscous liquid but became an opaque, waxy material upon standing. The conversion was about 80%. The molar mass was 3000 g mol−1: 1H NMR (δ, DMSO-d6) 1.19 (s, OCOCH2CH2(CH2)4CH2CH2COO), 1.47 (s, OCOCH2 CH 2 (CH2 ) 4 CH 2CH 2 COO), 2.19−2.22 (m, OCOCH2CH2(CH2)4CH2CH2COO), 3.27−5.5 (H from glycerol, substituted and unsubstituted); 13C NMR (δ, DMSO-d6) 24.91 (OCOCH2CH2(CH2)4CH2CH2COO), 28.97−29.12 (OCOCH2CH2(CH2)4CH2CH2COO), 33.96 (OCOCH2CH2(CH2)4CH2CH2COO), 60.19−75.63 (carbon atoms from glycerol), 172.92−173.45 (ester carbonyl). Glycerol−Fumaric Acid Hyperbranched Poly(ester) Containing Hydroxyl End Groups. A glycerol−fumaric acid poly(ester) containing hydroxyl end groups was prepared using a procedure similar to that for the synthesis of the glycerol−AA polymer. Into a dry 100 mL, three-necked, roundbottomed flask fitted with a magnetic stirring bar and a condenser bearing a gas-inlet tube was placed 5.00 g (54.3 mmol) of glycerol, 4.73 g (40.7 mmol) of white, crystalline fumaric acid, and 0.15 wt % dibutyltin oxide based on total monomer mass. The flask was mounted in an oil bath maintained at 150 °C. The mixture was stirred under a slow flow of nitrogen for about 3 h using reaction apparatus I and then for 9 h using reaction apparatus II. The reaction mixture gelled presumably due to cross-linking through unexpected radical polymerization of the unsaturated carbon−carbon double bonds in fumaric acid. The experiment was repeated at 120°, and a light yellow solid was obtained. The conversion was about 80%. The molar mass was 3000 g mol−1: 1H NMR (δ, DMSO-d6) 3.47−4.17 (H from the glycerol moiety); 13C NMR (δ, DMSO-d6) 59.38−73.3 (carbon atoms from glycerol), 133.05−133.54 (unsaturated carbon atoms from fumaric acid), 164.25 (ester carbonyl carbon atom).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Patrick B. Smith: 0000-0003-1593-9451 Notes

The authors declare no competing financial interest.

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Article

ACKNOWLEDGMENTS

The authors thank Dr. Steven J. Martin for developing the MM conditional probability model for the HBPEs described in this manuscript. This contribution was identified by Session Chair Haifeng Gao (University of Notre Dame) as the Best Presentation in the session “Advances in Functional Polymers with Sophisticated Branched Structures” of the 2016 ACS Fall National Meeting in Philadelphia, PA. I


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Rational Synthesis of Hyperbranched Poly(ester)s - Industrial

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