Hyperbranched Polymers from Maleic Anhydride and Diethanolamine: Synthesis, Characterization and Solution Rheology

http://www.e-polymers.org e-Polymers 2006, no. 053 ISSN 1618-7229

Weiqiang Song,1,2 * Qinghuan Song3 Chengxun Wu1

1 Institute of Materials Science and Engineering, Donghua university, 1882 Yan’an western Road, Shanghai City 200051, People’s Republic of China; Fax +86 21 62193062; email: [email protected] 2* Isotope Institute of Henan Academy of Sciences, 7 Songshan Southern Road, Zhengzhou City 450052, People’s Republic of China ; Fax +86 371 68989652; email: [email protected] 3 Luohe Medical College, 148 Daxue Road, Luohe City 462002, People’s Republic of China; Fax +86 395 2127842; email: [email protected]

(Received: 11 May, 2006; published: 19 August, 2006)

Abstract: Novel hyperbranched polyesteramides have been synthesized from diethanolamine and maleic anhydride with ethylene glycol as a core monomer. No gelation occurred in the melt polymerization when diethanolamine and maleic anhydride were used in equimolar amount in the feed and when content of ethylene glycol was varied. Obtained products were soluble in water and in organic solvents such as NMF. The obtained polymers were characterized by using IR and NMR spectroscopies as well as GPC. It was suggested that the hyperbranched polyesteramides formed via a mechanism of a combination of esterification and addition reactions. The addition of a hydroxyl group to the - CH=CH- in maleic anhydride unit results in a formation of CH methine group. The ratio of -CH=CH- to CH in the polymers were estimated from 1H NMR spectrum as 0.140, 0.155, 0.175, 0.190 and 0.204 for polymers P1 to P5, respectively as the content of ethylene glycol charged decreased. The molecular weights and their polydispersities showed reverse dependences on the feed ratio of ethylene glycol to diethanolamine. The rheological properties of the polymers in aqueous solution were also examined. The polymers exhibit a steady increase of intrinsic viscosity with increasing molecular weight, and the Mark-Houwink exponent for these polymers is 0.26, which is much lower than 0.5 and suggested that these polymers possess a highly branched architecture. The polymer solutions showed a Newtonian behavior with steady shear viscosities independent of shear rate, which indicated the absence of a physical entanglement.

Introduction Dendritic polymers have received increasing attention in recent years due to their unique chemical and physical properties as well as their potential applications in coatings, additives, drug and gene delivery, macromolecular building blocks, nanotechnology, and supramolecular science [1]. In using the term dendritic polymers, we include both dendrimers and hyperbranched polymers. Fully branched, perfectly regular structures are referred to as dendrimers, while imperfectly branched or irregular structures are referred to as hyperbranched polymers (HPs). HPs are of more significance than dendrimers from the viewpoint of industrial applications, since

1 it can be easily prepared by direct “one-step” polymerization of multifunctional monomers. HPs can be applied as various coating resins, modifiers and additives for thermosets, thermoplastics and functional materials. Fundamental understanding of their behavior is therefore of general interest. The study of the effect of hyperbranched architecture on the flow behavior has been the subject of keen interest over the last few decades. Linear polymers generally obey the Mark-Houwink equation which exhibits a steady increase of intrinsic viscosity with increasing molecular weight. But for dendrimers, a maximum in the dependence of the intrinsic viscosity on the molecular weight has been observed [2, 3]. Hobson et al. reported the synthesis of poly(amidoamine) hyperbranched systems which display a viscosity/molecular weight profile similar to that exhibited by dendrimers [4]. Simulations of HPs showed that flexible HPs can exhibit a maximum in their intrinsic viscosity as a function of molecular weight [5, 6]. The maximum is, however, situated at a higher level of intrinsic viscosity and shifted to higher molecular weights than that for the equivalent dendrimers. Turner et al. reported the synthesis of all-aromatic hyperbranched polyesters with phenol and acetate end groups with a reported degree of branching of about 50% [7, 8]. Combination of size exclusion chromatography and different viscometry indicated that the Mark-Houwink exponent for these polymers was much lower than that for the linear polystyrene standard, and a maximum was not observed in the plot of intrinsic viscosity as a function of molecular weight. As a general rule, there is general agreement that HPs exhibit reduced viscosities compared with equivalent linear polymers, but the conditions for the existence of a maximum in the plot of intrinsic viscosity as a function of molecular weight are not clear [9]. Most of the previous studies on dendritic polymers focused on characterizing their intrinsic properties through dilute solution viscometry [2, 10, 11]. A few studies have examined the concentrated solution or melt properties of these polymers [3, 6, 12 - 14]. It has been found that dendrimer solutions with concentrations as high as 75 wt % exhibit typical Newtonian behavior [6], and that dendrimer melts also do not show abrupt slope change in zero-shear viscosity when plotted as a function of molecular weight on a logarithmic scale [3, 12]. These results indicate that no physical entanglements are present in these systems. Nunez et al. reported the solution rheology of hyperbranched polyesters and their blends with linear polymers [13]. Hyperbranched polyester solutions exhibited Newtonian behavior, even at high concentrations. Replacing linear polymers with hyperbranched polymers caused large reductions in the blend viscosities. Up to date, some HPs have found their field of applications, one of which is hyperbranched poly(ester amides) [15, 16]. The monomer for the polymer was obtained from the reaction of a cyclic anhydride with diisopropanolamine (DiPA), yielding a tertiary amide with one COOH and two OH groups [15]. Ester bonds were formed through an intermediate oxazolinium-carboxylate ion pair, and hyperbranched macromolecules were generated under reduced pressure (5 mbar) at around 180 °C without a catalyst. Nevertheless, DiPA as starting material is less economical than diethanolamine (DEA). Unfortunately, the polymerization experiments with a cyclic (e.g., phthalic) anhydride and DEA only led to strongly discolored, partially crosslinked, or even gelled products. And another problem was the inherent lack of molecular weight control in the polymerization. Van Benthem found that the gelation resulted from an undesired side product from the reaction of the OH group in DEA instead of the amine [17, 18]. By using DiPA instead of DEA, the reactivity of

2 secondary hydroxyl group was diminished, and the side reaction was suppressed in favor of the reaction of amine with the anhydride. An excess amount of DiPA was used to control the molecular weight of the formed polymer. In this work, we present a novel approach for the synthesis of hyperbranched polyesteramides. The hyperbranched polyesteramides were prepared by the polymerization of commercially available diethanolamine with maleic anhydride, and no gelation occurred in the polymerization process, which is different from the polycondensation of diethanolamine with other cyclic anhydrides. In addition, the rheological features of dilute and concentrated aqueous solutions of the polymers are investigated.

Results and Discussion

Polymerization

Fig. 1. IR spectrum of polymer P4.

Scheme 1. Mechanism for the esterification of hydroxyethyl amide via oxazolinium carboxylate ion pair formation and nucleophilic ring opening.

3 The strategy presented here is based on the principle of unequal reactivity of different functional groups. The ring-opening reaction of a cyclic anhydride with amine is much faster than that with alcohol at lower temperature. At the initial stage of polymerization, the amine group in diethanolamine reacted fast with maleic anhydride, predominantly generating an AB2-type intermediate, i.e. N, N-diethanol maleamic acid monoamide.

Fig. 2. NMR spectra of polymer P4. (A) 1H NMR; (B) 13C NMR; (C) 13C dept135 NMR.

4 Further polymerization of the compound formed in situ gave a soluble polymer rather than a gel, which is different from the polymerization of diethanolamine with other cyclic anhydrides in which a strongly discolored, partially crosslinked, or even gelled product was obtained. In the FTIR spectrum of the resulting polymer shown in Fig. 1, the absorptions at 1736 and 1175 cm-1 can be assigned to the C=O and C-O in the ester units (-COO-), indicating the presence of esterification of N,N-diethanol maleamic acid monoamide in the process of polymerization. Fast esterification, without any catalysts, that was observed in our study indirectly indicates formation of an oxazolinium intermediate (see Scheme 1) discovered earlier by Stanssens [18, 19]. Unexpectedly, in the 13C dept135 NMR spectrum shown in Fig. 2 (C), the positive signals at δ 73.53 to 74.00 is seen which can be reasonably assigned to a methine (CH) carbon. This methine group formation may be due to the fact that the -CH=CH- is in conjugating with the oxazolinium ring, which is in favor of a nucleophilic addition of a hydroxyl group to the -CH=CH- double bond (Scheme 2). The difference in chemical shifts for the signals at δ 73.53 to 74.00 resulted from the chiral nature of the methine (CH) carbon.

Scheme 2. Proposed mechanism for the addition reaction of OH in hydroxyethyl amides with C=C double bond.

Tab. 1. Contents of CH and CH=CH in polymers based on 1H NMR spectrum.

Integral area Area ratio Contents Polymers for CH=CHa for CHb CH=CH/CH CH=CH/CH P1 2.85 10.17 0.280 0.140 P2 4.10 13.23 0.310 0.155 P3 2.73 7.79 0.350 0.175 P4 4.89 12.88 0.380 0.190 P5 2.84 6.96 0.408 0.204 a Integral area of signals between δ 6.4 and 7.0; b integral area of signals between δ 4.0 to 4.6.

5 The structural perfection of hyperbranched polymers is generally characterized by the degree of branching (DB). Experimentally, DB was usually determined from 1H or 13C NMR spectroscopy by comparing the integral areas of the peaks for the respective units in the hyperbranched polymers [20]. The 1H and 13C NMR spectra (in DMSO-d6, Fig. 2) of the synthesized polymers in this paper are fairly complex, through which it is difficult to estimate the DB of the polymers, yet some information can be obtained. In 1H NMR spectrum, the signals between δ 6.4 and 7.0 can be assigned to CH=CH (including cis- and trans- conformations), and the signals between δ 4.0 to 4.6 can be assigned to methine CH resulted from the addition reaction. Comparison of the integral areas of these signals may determine the ratio of residual -CH=CH- groups to methine groups in the polymers, which is listed in Table 1. It can be found that most of the -CH=CH- groups of maleic anhydride were converted to ether groups in the resulting polymers through addition reaction, and suggest that the presence of ethylene glycol favors the addition.

Scheme 3. Mechanism for an acyl shift and ring opening of the oxazolinium ion by the secondary amine.

Scheme 4. Polymerization of A-C-B2 monomers into a polymer.

6 In addition, another side reaction may exist involving a nucleophilic ring-opening of the oxazolinium ion intermediate by a secondary amine as depicted in Scheme 3. Signals at around δ 2. 55 in the 1H NMR spectrum (Fig. 2 A) are attributed to NCH2CH2N. These signals are not so evidence, indicating a low probability of this side reaction at the experimental conditions in this paper. As revealed by the IR and NMR spectra shown above, the hyperbranched polyesteramides presented in this paper formed via a mechanism in a combination of esterification and addition reactions as depicted in Scheme 4. The fast ring-opening reaction of a cyclic anhydride with amine at lower temperature generates an N, N- diethanol maleamic acid monoamide that can be regarded as an A-C-B2 monomer, in which A, B and C represents the carboxylic acid, alcohol groups and the -C=C- double bond, respectively. The group B can react with groups A and C, resulting in the formation of an ester and an ether group, respectively. The addition of a hydroxyl group to the -CH=CH- in maleic anhydride unit results in a formation of CH methine group.

Molecular weight and its distribution

Fig. 3. GPC chromatograms for the resulting polymers.

Molecular weight of the resulting polymers was determined by GPC with an ultraviolet detector using DMF as eluent. As expected from the theoretical predictions [21, 22], the polymers have a broad molecular weight distribution (Fig. 3). The polymodal character of the chromatograms is not anticipated but in agreement with similar work reported by Mock et al [23]. The additional distribution mode with lower molecular weight can be explained by the presence of unreacted monomers or their low molecular weight derivatives. These extra peaks were deemed to represent only a very minor component and have been omitted from the calculation of average molecular weight listed in Table 2. If only the main distribution modes are considered, average molecular weight become higher but polydispersity become narrower with decreasing the ratio of ethylene glycol to diethanolamine (i. e., the ratio of core to monomer) as shown in

7 Fig. 4. This phenomenon is in disagreement with other work [21, 22, 24-29], in which the presence of a core molecule decreases the molecular weight of hyperbranched polymers with a concurrent reduction in the molecular weight distributions.

Tab. 2. Molecular weight, intrinsic viscosity and hydrodynamic radius of polymers.

4 4 3 Polymers Mn/10 Mw/10 Mw / Mn [η], cm /g Rh, nm P1 3.84 11.30 2.94 5.45 4.60 P2 4.60 12.76 2.78 5.58 4.83 P3 5.31 14.04 2.64 5.65 5.01 P4 5.95 15.11 2.54 5.88 5.21 P5 6.61 16.56 2.50 6.01 5.40

170000

160000

150000

w 140000 M 130000

120000

3.00

n 2.75 M / w

M 2.50

2.25 0.00 0.04 0.08 0.12 0.16 Core : monomer (molar ratio)

` Fig. 4. Dependence of Mw and Mw / Mn on the ratio of core to monomer.

Intrinsic viscosity Intrinsic viscosities of the resulting polymers were determined by using the Huggins equation and plotting ηsp/c as a function of c, where ηsp is the specific viscosity and c is the concentration. Extrapolation of the data to zero concentration yielded the intrinsic viscosity, [η]. Higher molecular weights and lower intrinsic viscosities indicate that the polymers possess highly branched architectures. Relationship between Mw and [η] is illustrated in the top image of Fig. 5. In the investigated Mw range in this study, [η] values showed a dependence on Mw, much like linear polymers which exhibit a steady increase of intrinsic viscosity with increasing molecular weight. A shape factor, α, which was defined by the Mark-Houwink equation ([η] = KMα) was calculated to be 0.26 from the slope drawn by linear regression. The α value was noticeably smaller than 0.5, which implies indeed that the resulting polymers have highly branched structures. Unexpectedly, this Mark-Houwink exponent is noticeably

8 similar to hyperbranched polyesteramides derived from bis(2-hydroxypropyl)amide determined in CH2Cl2, which is reported as 0.26±0.02 [29]. The relationship between dendrimer size and molecular weight has been examined by several experimental and computer simulation studies. One measure of size is the hydrodynamic radius (Rh) of a polymer in solution. Rh can be determined from the intrinsic viscosity by using Einstein’s equation for a hard sphere: [30] 1/3 -1/3 R =(3[η]M) (10π NA) (1) where [η] is the intrinsic viscosity, M is the molecular weight, and NA is Avogadro’s number. The calculated values are given in Table 3. Most studies have obtained power law relationships between the radius of gyration (Rg) and M. Since we have Rh instead of Rg values, we need to ensure that the two values are proportional to each other. This was determined to be the case for all generations [31] or for low generation numbers (G<6) [32]. So, we scale our Rh values with Mw as to compare them with literature values. The bottom image of Fig. 5 shows the power law 0.42 relationship between Rh and Mw, and the radius scales as Rh ~ Mw .

0.775

] 0.26 η

0.750

0.725 0.42 log[

h 0.700 log R 0.675

5.04 5.08 5.12 5.16 5.20 5.24 log M w

Fig. 5. Double logarithmic representation of Rh vs Mw and [η] vs Mw. Thick full lines represent the results of linear regressions.

Several studies relating radius to molecular weight have been reported. 0.37 Scherrenberg et al. [10] found that Rh ~ Mw for poly(propyleneimine) dendrimers with two different types of end groups, and Stechemesser et al. [33] found that Rh ~ 0.33 Mw for poly(amideamine) dendrimers in methanol using holographic relaxation spectroscopy. In a molecular dynamics study of dendrimers, Murat and Grest [34] 0.33 found that Rg ~ N for dendrimers under several solvent conditions, where N is the number of monomers in the dendrimers. Using Monte carlo simulations of dendrimers, Mansfield [35] found that R ~ M0.362 to R ~ M0.408 for generations 6 to 9, 0.39 respectively. In addition, Nunez et al. [13] found that Rh ~ Mw for hyperbranched polyesters in NMP. These studies involving dendrimers yielded a slightly smaller scaling exponent than 0.39 for hyperbranched polyesters and 0.42 obtained for polymers in this study. Although it may not be prudent to compare size scales of

9 dendritic polymers with different chemical compositions in different solvents, in a sense, the larger exponent values for hyperbranched polymers indicate less densely packed structures than dendrimers, due to their imperfect architectures. 0.61±0.04 Moreover, Rgz ~ Mw was obtained for bis(2-hydroxypropyl)amide-based hyperbranched polyesteramides with low molecular weights, based on values for the polymers fractal dimension yielded by SANS measurements and SEC-DV data, which were rationalized in the framework of percolation theory that was originally designed for randomly branched polymers [29]. Provided that Rh and Rg are 0.42 proportional to each other, Rg ~ Mw can be obtained in our study. And df = 2.38 can also be obtained since 1/d Rg ~ M f (2) where df is a fractal dimension [29]. This df is equal to the value calculated by using the Mark-Houwink exponent α according to [29,36] df = 3/(1+ α) (3) The better the solvent the more expanded a polymer coil is and the lower the corresponding df. For randomly branched polymers df = 2 or 2.28 for a good or a Θ solvent, respectively. Occasionally a fractal dimension of 3 was obtained for dendrimers [10], indicative for a spherical morphology and a uniform segment density distribution. More recently, Luca and Richards [37] demonstrated that the scaling relations of the various characteristic radii (Rg, Rh, RT, and Rη) with molecular weight, all had exponents less than 0.5, and that the exponent for Rg was interpreted as fractal dimension and had a value of 2.38 ± 0.25 determined in chloroform. In a sense, df = 2.38 in this study suggests that water is not a particularly good solvent for the resulting polymers and that the molecules are of little spherical nature.

Rheology of concentrated aqueous solutions

65%

0.1 50%

40% 30% 20% Steady shear viscosity (Pa (Pa s) viscosity shear Steady 0.01 10%

0 100 200 300 400 500 600 Shear rate (s-1)

Fig. 6. Viscosity as a function of shear rate for polymer P4. The concentrations are in terms of weight percent.

10 Rheological features of the polymers in aqueous solutions were examined by a rotary rheometer at 30 °C. All the samples in water exhibited similar Newtonian behavior within the shear rate range studied (up to 600 s-1), even at high concentrations. Typical plots of viscosity as a function of steady shear rate are shown in Fig. 6 for several concentrations of polymer P4. The viscosities are independent of shear rate, much like poly(amidoamine) (PAMAM) dendrimers in ethylenediamine solutions [16] and hyperbranched polyesters in N-methyl-2-pyrrolidinone(NMP) solutions [13]. Moreover, the solution viscosities show only a slight dependence on molecular weight as shown in Fig. 7, much like hyperbranched polyesters.

P1 P2

Pa s) P3 ( P4 0.1 P5 Steady shearviscosity 0.01

10 20 30 40 50 60 70 Weight % Polymer

Fig. 7. Zero-shear viscosities of polymers in aqueous solutions.

Experimental part

Synthesis of polymers In a 250 mL glass reactor, a calculated amount of diethanolamine (Shanghai No.4 Reagent & H.V. Chemical Co., Ltd., 98+%) and a calculated amount of ethylene glycol (distilled before use) were introduced under a nitrogen atmosphere. A calculated amount of maleic anhydride (Tianjin No.1 Chemical Reagent Factory, 99.5+%), which was equal to that of diethanolamine, was then slowly added, and the temperature of the mixture was controlled under 60 °C.

Tab. 3. The molar ratio of monomers for polymer synthesis.*

Number of Moles Polymers E D M E / D P1 0.1057 0.6340 0.6338 0.1667 P2 0.0456 0.6382 0.6374 0.0715 P3 0.0225 0.6766 0.6772 0.0333 P4 0.0107 0.6629 0.6624 0.0161 P5 0 0.6863 0.6867 0 *E = ethylene glycol, D = diethanolamine and M = maleic anhydride.

11 Then, the reaction mixture was heated to 140 °C with vigorous stirring. After 1 h, the evolving reaction water was removed by a 4 - 4.5 m3/min flow of nitrogen gas into the mixture for 4.5 h. The molar ratio of monomers is tabulated in Table 3.

Measurements NMR spectra were recorded on a Bruker Avance DPX-400 NMR spectrometer at 400 MHz. Samples was analyzed in DMSO-d6. In 1H and 13C NMR, the chemical shifts are expressed in ppm in the δ scale, compared to the singlet of tetramethylsilane (TMS), as the internal standard. Infrared (IR) spectra were recorded on a Nicolet 480 IR spectrometer, and the KBr pellets were used for the test. Gel permeation chromatography (GPC) was performed with a Shimadzu LC-10Avp HPLC fitted with a Shodex 806 MHQ column and a Shimadzu RF-10Axl Ultraviolet Detector in DMF (dimethyl formamide) as an eluent, and wavelength detected at 350 nm. The sample concentration is 1 wt %. For rheological characterization, two types of measurement were used depending on the viscosity of the sample. For low viscosities, a 0.57 mm inner diameter Ubbelodhe viscometer was utilized. For high viscosities, a HAAKE RS 150 rheometer and a Z 41 cylindrical rotor were utilized, and a HAAKE F6/8 was used to control the temperature of the rotor/sample/cup. Intrinsic viscosities of the samples were determined by using the Huggins equation and plotting ηsp/c as a function of c, where ηsp is the specific viscosity and c is the concentration. Extrapolation of the data to zero concentration yielded the intrinsic viscosity, [ η].

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