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Synthesis and characterization of well-defined poly(tert-butyl acrylate) star polymers
European Polymer Journal 45 (2009) 1979–1993
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Synthesis and characterization of well-defined poly(tert-butyl acrylate) star polymers Barbara Mendrek a, Barbara Trzebicka b,* a b
University of Opole, Faculty of Chemistry, Oleska 48, 45-052 Opole, Poland Centre of Polymer and Carbon Materials, Polish Academy of Sciences, M. Curie-Sklodowskiej 34, 41-819 Zabrze, Poland
a r t i c l e
i n f o
Article history: Received 13 February 2009 Received in revised form 1 April 2009 Accepted 13 April 2009 Available online 24 April 2009
Keywords: Star polymers Poly(tert-butyl acrylate) Gel permeation chromatography Triple detection Intrinsic viscosity Branching parameter
a b s t r a c t Star polymers with different numbers and lengths of poly(tert-butyl acrylate) (PTBA) arms were obtained via atom transfer radical polymerization. Aliphatic alcohols with different number of hydroxyl groups varying from 3 to 6 and calix[4]arenes based on pyrogallol with 12 and 16 phenol groups were transformed to bromoester derivatives to prepare multifunctional ATRP initiators used as the cores of the stars. The star polymers were characterized by GPC with refractive index, multiangle laser light scattering and viscosimetric detectors. The molar masses of the stars reached 70,000 g/mol and the molar mass dispersities did not exceed 1.2. To elucidate the compact structure of the stars, their true molar masses were determined by GPC with triple detection (RI–MALLS–Visco) and compared with the apparent molar masses obtained from the calibration with linear poly(tert-butyl acrylate) standards. The intrinsic viscosities of the PTBA stars of the same molar mass decreased with the number of star arms but were always lower than the intrinsic viscosities of the analogue linear PTBA polymers. The values of the branching ratio g0 decreased with increasing number of arms indicating more compact structure of stars. The branching ratio g0 was correlated to the empirical predictions. Ó 2009 Published by Elsevier Ltd.
1. Introduction Polymers of nonlinear topology have been of interest to scientists for many years [1–5]. Studies have focused on the synthesis, characterization and application of branched polymers. Star polymers are a special kind of branched structure. They consist of a central core and different numbers of linear chains – arms connected to the core [1]. The properties of star polymers are different from those of linear analogues. Star polymers with exactly known numbers and lengths of arms have been useful tools in providing information how the number of arms influences the properties of polymers in solution and in the melt [1–3]. There are two main methods for the preparation of star polymers. The arm-first method involves the synthesis of * Corresponding author. E-mail address: [email protected] (B. Trzebicka). 0014-3057/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.eurpolymj.2009.04.016
living macromolecular chains – future arms of the star – and their reaction with linking agent, the core. In the core-first method a multifunctional compound, the core, initiates the polymerization of the monomer [6]. Many multifunctional compounds have been used as cores, amongst them dendrimers [7,8], hyperbranched polymers [9], microgels [10,11], cyclodextrins [12], calix[n]arenes [13,14], alcohols [15] and phenols [16]. The different numbers of initiating or terminating groups lead to stars with varying numbers of arms. Significant advancements in the field of controlled radical polymerization techniques, especially atom transfer radical polymerization (ATRP) [17], have opened new routes to the synthesis of well-defined star structures [13–15,18]. One of the methods used to obtain multifunctional initiators for ATRP is the transformation of the hydroxyl group in alcohols or phenols into a bromoester group [7,9,13–15,18]. Such initiators have been used in the ATRP of styrene and
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B. Mendrek, B. Trzebicka / European Polymer Journal 45 (2009) 1979–1993
various (meth)acrylic monomers. Adamante-based cores have been used for the preparation of 4-arm polystyrene and poly(methylmethacrylate) stars [19]. Resorcin-based calix[4]arene cores [14,20] and 4-tert-butylcalix[4,6,8]arenes cores [13] were used to obtain poly(methylmethacrylate) and poly(tert-butyl acrylate) stars, respectively, with eight or less arms. Poly(methylmethacrylate), polystyrene and block copolymer poly(methylmethacrylate)-poly(n-butyl methacrylate) 21-arm stars were synthesized via ATRP initiated with esterified cyclodextrin [12]. Three-, fourand six-arm poly(methylmethacrylate), polystyrene and poly(n-butyl acrylate) stars have been obtained by ATRP using esterified cyclotriphosphazenes and cyclosiloxanes as the cores [18]. Hyperbranched polymers used as cores lead to multiarmed star polymers. These cores have been used to synthesize poly(meth)acrylic star polymers with high numbers of arms via ATRP [9,21–24]. Physicochemical properties of branched macromolecules, amongst them star macromolecules, differ significantly from that of the linear chains of the same composition and molar mass and are the subject of numerous studies. The compact structures of the stars reflect their solution and solid-state properties: intrinsic viscosity [3,8–10,25], size in solution [2,3,8,16,25], glass transition temperature and mechanical properties [7,15,16,21]. In this work, we describe the synthesis of multifunctional initiators for ATRP and their use to obtain well-defined star polymers of tert-butyl acrylate with 3, 4, 6, 12 and 16 arms. The solution behavior of the stars is studied by gel permeation chromatography (GPC) with differential refractive index (RI), multiangle laser light scattering (MALLS) and viscosimetric detectors to obtain fundamental data on the hydrodynamics of these macromolecules. Viscosity data obtained from GPC for star-shaped and linear polymers are used to determine the parameters of the Kuhn-Mark–Houwink–Sakurada equation and the contraction factors resulting from them.
2. Experimental 2.1. Materials Trimethylolpropane (TMP, 97%), ditrimethylolpropane (DTMP, 97%), 2-bromopropionic acid (99%), 2-bromopropionyl bromide (97%), didecyldimethylammonium bromide (DDAB, 97%), 4-dimethylaminopyridine (DMAP, 99%), and N,N0 -dicyclohexylcarbodiimide (DCC, 99%), N,N,N0 ,N0 ,N00 pentamethyldiethylenetriamine (PMDETA, 99%), copper (I) bromide (CuBr, 99.999%) were purchased from Aldrich and used as received. Dipentaerythritol (DPENTA, technical grade, Aldrich) was recrystallized from water. Calix[4]arenes based on pyrogallol were synthesized and kindly provided by Wolf D. Habicher from the University of Technology, Dresden, Germany. tert-Butyl acrylate (tBuA, 98%), anisole (99%) and p-xylene (99%) were purchased from Aldrich and purified by distillation prior to use.
1,4-Dioxane (pure p.a.) was purified by distillation under nitrogen over sodium. N,N0 -Dimethylformamide (DMF, pure p.a.), chloroform (pure p.a.), dichloromethane (pure p.a.), acetone (pure p.a.), potassium carbonate anhydrous (pure p.a.), and sodium bicarbonate (pure) were purchased from POCh and used without purification. DOWEX MARATHON MSC cation exchange resin was transformed into the H+ type using 1.6 M HNO3. DOWEX MONOSPHERE 550A anion exchange resin was washed with water and dried in a vacuum before use. Both exchange resins were purchased from Aldrich. 2.2. Synthesis of the multifunctional initiators for ATRP of tert-butyl acrylate 2.2.1. Synthesis of trimethylolpropane tris(2-bromopropionate) (Br3TMP) TMP (1.50 g, 33.5 mmol) and DMAP (1.02 g, 8.38 mmol) were dissolved under nitrogen in dry 1,4-dioxane (38 mL) in a Schlenk flask equipped with a magnetic stirrer. 2-Bromopropionic acid (5.64 g, 36.85 mmol, 3.32 mL) was next added to the flask and cooled to 0 °C (ice bath). A solution of DCC (7.60 g, 36.85 mmol) in 1,4-dioxane (60 mL) was transferred to the flask dropwise. The solution was stirred at room temperature for 48 h. Dicyclohexylurea was filtered off and the 1,4-dioxane evaporated. The crude product of Br3TMP was dissolved in chloroform (25 mL). The solution was washed with water (2 25 mL), with saturated NaHCO3 (3 25 mL) and with water (1 25 mL). Chloroform was evaporated; the crude product was dissolved in acetone (20 mL) and passed through an anion exchange resin column. Acetone was then evaporated and the product was dried in a vacuum. Br3TMP was obtained in 85% yield (3.3 g) as a slightly yellow viscous liquid. The functionality as measured by 1 H NMR was 100%. 1 H NMR (600 MHz, CDCl3) dppm: 0.95 (t, 3H, CH3CH2–), 1.59 (q, 2H, CH3CH2–), 1.83 (d, 9H, CH3CHBrCOOCH2–), 4.07–4.26 (m, 6H, –CCH2O–), 4.40 (q, 3H, CH3CHBrCOOCH2–). MS(ESI): calculated for C15H23Br3O6 m/z = 557.09 [M+NH4+], found m/z = 556.90. Thin layer chromatography of Br3TMP was performed in chloroform (Rf = 0.65). 2.2.2. Synthesis of ditrimethylolpropane tetrakis(2-bromopropionate) (Br4DTMP) Br4DTMP was prepared in the same manner as that described above for Br3TMP using DMF as the solvent. Slightly yellow Br4DTMP was synthesized in 87% yield (5.49 g). 1 H NMR (600 MHz, CDCl3) dppm: 0.89 (t, 6H, CH3CH2–), 1.43–1.57 (m, 4H, CH3CH2), 1.83 (d, 12H, CH3CHBrCOOCH2–), 3.35 (br s, 4H, –CH2OCH2–), 4.00–4.22 (m, 8H, – CCH2O–), 4.40 (q, 4H, CH3CHBrCOOCH2–). MS(ESI): calculated for C24H38Br4O9 m/z = 808.17 [M+NH4+], found m/z = 808.00. Thin layer chromatography of Br4DTMP was performed in a mixture of the solvents chloroform/n-hexane (10:1) (Rf = 0.55).
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2.2.3. Synthesis of dipentaerythritol hexakis(2-bromopropionate) (Br6DPENTA) DPENTA (1.00 g, 4.0 mmol) and NaOH (1.00 g, 25.0 mmol) were dissolved in water (100 mL). DDAB (0.08 g, 0.2 mmol) was added and the solution was vigorously stirred. The solution of 2-bromopropionyl bromide (6.40 g, 36 mmol, 3.77 mL) in dichloromethane (75 mL) was added dropwise. The reaction mixture was stirred for 7 days. The organic phase was separated and washed multiple times with water to remove the DDAB residue. Dichloromethane was evaporated and the sample was dried under vacuum. The white powder of Br6DPENTA was obtained in 40% yield (1.67 g). 1 H NMR (600 MHz, CDCl3) dppm: 1.83 (d, 18H, CH3CHBrCOOCH2–), 3.52 (br s, 4H, –CH2OCH2–), 4.12–4.37 (m, 12H, –CCH2O–), 4.41 (q, 6H, CH3CHBrCOOCH2–). MS(ESI): calculated for C28H38Br6O13 m/z = 1103.13 [M+K+], found m/z = 1103.10. Thin layer chromatography of Br6DPENTA was performed in chloroform (Rf = 0.63). 2.2.4. Synthesis of calix[4]arene dodekakis(2-bromopropionate) (Br12CXA) Br12CXA was prepared in the same manner as described for Br3TMP. A yellow solid sample of Br12CXA was obtained in 73% yield (1.74 g). 1 H NMR (600 MHz, CDCl3) dppm: 0.79 (t, 12H, – C10H20CH3), 1.16 (s, 80H, –C10H20CH3), 1.6–2.0 (br m, 36H, CH3CHBrCOOCH2–), 3.9–4.8 (br m, 4H, CH3CHBrCOOCH2– and 4H, Ar–CH–Ar), 6.04 and 6.89 (br s, 4H, ArH). Thin layer chromatography of Br12CXA was performed in the mixture of chloroform/n-hexane (10:1) (Rf = 0.55). 2.2.5. Synthesis of calix[4]arene hexadekakis(2-bromopropionate) (Br16CXA) CXAOH16 (1.00 g, 1.09 mmol) was suspended in 1,4dioxane (30 mL). The solution of DMAP (0.53 g, 4.35 mmol) and 2-bromopropionic acid (2.92 g, 19.13 mmol, 1.72 mL) was added and cooled to 0 °C (ice bath). A solution of DCC (3.95 g, 19.13 mmol) in 1,4-dioxane (30 mL) was transferred to the flask dropwise. The mixture was stirred first for 2 h in an ultrasonic bath and next in a magnetic stirrer for 48 h. Purification was done in the same manner as for Br3TMP. A light orange solid of Br16CXA in 58% yield (1.94 g) was obtained. 1 H NMR (600 MHz, DMSO) dppm: 1.56, 1.69, 1.88 (br s, 48H, CH3CHBrCOOCH2–), 4.61–5.10 (br m, 16H, CH3CHBrCOOCH2–), 5.56, 5.74 (br s, 4H, Ar–CH–Ar), 6.05, 6.24 (br s, 4H, –ArH), 6.89 (br s, 16H, ArH of substituent). Thin layer chromatography of Br16CXA was performed in chloroform (Rf = 0.61). 2.3. Synthesis of the stars – ATRP of tert-butyl acrylate The appropriate multifunctional initiator (20 mmol), CuBr (1.43 g, 10 mmol), PMDETA (1.73 g, 10 mmol, 2.08 mL) and acetone or anisole (about 10% v/v related to monomer, sufficient to dissolve the initiator) were placed in a Schlenk flask. The solution was stirred until the Cu complex formed (the solution turned green) at which point the amount of tert-butyl acrylate correspond-
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ing to the chosen monomer to initiator ratio was added. The mixture was degassed using three freeze–vacuum– thaw cycles. The flask was placed in an oil bath and thermostated at 60 °C (acetone) or 80 °C (anisole). Samples were taken periodically for the kinetic analysis. The monomer conversion was measured by gas chromatography with p-xylene as the internal standard. After the desired conversion was obtained, THF (20 mL) was added and the solution was passed through a column with DOWEX-MSC-1 ion exchange resin to remove copper. The polymer was precipitated in methanol/water mixture (1:1) and dried. 2.4. Selective alkaline hydrolysis of ester bonds between the arms and the core of the stars The star polymer sample (0.3 g) was dissolved in THF (20 mL) in a round-bottomed flask fitted with a condenser and nitrogen inlet. KOH solution (2 mL, 1 M in ethanol) was added to the flask and the reaction mixture was refluxed at 60 °C for 10 min. The solvent was evaporated without heating under reduced pressure; the remaining polymer was dissolved in THF, precipitated in a methanol/water mixture (8:2) and dried. 2.5. Characterization NMR spectra were recorded using a Bruker Ultrashield 600 (600 MHz for 1H). The resonances are given in ppm referenced to the tetramethylsilane (TMS) peak. Gas chromatography was used to determine the conversion by measuring the residual monomer content with p-xylene as the internal standard, using a VARIAN 3400 gas chromatograph with the J&W Scientific DB-5 (30 m 0.32 mm) column. Mass spectra were run on a Bruker Esquire MS with an ion trap detector in combination with an Agilent/HP 1100 HPLC-device. Thin layer chromatography (TLC) was performed on commercial Merck plates coated with silica gel 60 F254 (0.25 mm thick). UV light with a wavelength of k = 254 nm was used for detection. The molar masses, the molar mass distributions and solution viscosities of the obtained polymers were determined by GPC with three detectors: a differential refractive index Dn-1000 RI WGE Dr. Bures, a viscosimetric detector g-1001 WGE Dr. Bures and a multiangle light scattering DAWN EOS from Wyatt Technologies. The column set containing four SDV columns from Polymer Standard Service (PSS): 1 105 Å + 1 103 Å + 2 102 Å was used. Measurements were performed in THF at 35 °C with a nominal flow rate of 1 mL/min. Conventional calibration was established with PTBA linear standards from PSS. Results were evaluated using the ASTRA software from Wyatt Technologies and the WINGPC software from PSS. The refractive index increment dn/dc for linear PTBA and for star polymers was measured independently in THF. The dn/dc of linear PTBA were determined for polymer of Mn = 53,200 g/mol and of Mn = 8700 g/mol. In both cases the values were equal to 0.0593 mL/g.
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3. Results and discussion 3.1. Synthesis of the multifunctional initiators for ATRP of tert-butyl acrylate The preparation of the atom transfer radical polymerization initiators is essential for obtaining polymers of well-defined structure. It is known that alkyl halides with substituents on a-carbon are good initiators for ATRP of (meth)acrylic monomers [13–15,18]. The ‘‘core-first” method was applied for the preparation of the star polymers [6]. The cores, which are initiators with a confirmed number of initiating sites were used for
the preparation of stars via atom transfer radical polymerization of tert-butyl acrylate (Scheme 1). Initiators with different numbers of initiating centers were obtained by the esterification of the hydroxyl group of the commercially available aliphatic alcohols (Scheme 2) and phenolic groups of the calix[4]arenes obtained by Habicher et al. [27] (Scheme 3). The esterification method of the 2-bromopropionylbromide or 2-bromoisobutyryl bromide in the presence of amine has been described previously [13–16,18,20,27]. However, the incomplete esterification of the OH groups is frequently a problem. The separation of the mixture of products is problematic. If the products do not crystallize
Scheme 1. Schematic representation of synthetic route to poly(tert-butyl acrylate) star polymers.
Scheme 2. Synthesis of the ATRP initiators by the esterification of hydroxyl groups of aliphatic alcohols.
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Scheme 3. Synthesis of the ATRP initiators based on pyrogallol[4]arenes.
or precipitate, the time consuming column chromatography must be used [14,20,27]. The aliphatic compounds trimethylolpropane (TMP) and ditrimethylolpropane (DTMP) were esterified using 2bromopropionic acid in the presence of N,N0 -dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) as described in [26]. The reaction leading to trimethylolpropane tris(2-bromopropionate) (Br3TMP), ditrimethylolpropane tetrakis(2-bromopropionate) (Br4DTMP) and dipentaerythritol hexakis(2-bromopropionate) (Br6DPENTA) is shown in Scheme 2. Thin layer chromatography confirmed that homogeneous products were obtained. The complete substitution of the 2bromoesters groups was confirmed by 1H NMR and MS(ESI) (Figs. 1 and 2). Dipentaerythritol (DPENTA) is not soluble in the majority of solvents suitable for the esterification. All attempts to obtain pure, single esterification products have failed: neither the esterification in DMSO using 2-bromopropionic acid, nor the use of pyridine as the solvent and proton acceptor, nor a heterophase reaction in 1,4-dioxane with 2-bromopropionic acid bromide yielded a single product
with all OH groups transformed to esters. DPENTA was finally esterified by 2-bromopropionic bromide in a twophase water/CH2Cl2 system in the presence of a phase transfer catalyst didecyldimethylammonium bromide (DDAB). This process yielded a single product in good yield. The 1H NMR spectrum (Fig. 2) shows that all hydroxyl groups were transformed into 2-bromoesters groups. Isotope distribution obtained from the MS/ESI spectrum confirmed that the DPENTA was esterified with a 100% yield. The thin layer chromatography of Br6DPENTA in chloroform confirmed that a single product was obtained. The pyrogallol[4]arenes CXAOH12 and CXAOH16 (Scheme 3) used in this work were obtained by Habicher et al. [27]. The same authors esterified CXAOH12 and CXAOH16 using 2-bromoisobutyryl bromide in the presence of pyridine, however they obtained a mixture of differently substituted products that had to be purified by column chromatography [27]. This effort verified that these difficulties could be overcome when 2-bromopropionic acid was used for the esterification of the calix[4]arenes CXAOH12 and CXAOH16.
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Fig. 1. MS/ESI and 1H NMR in CDCl3 spectra of Br3TMP (a) and Br4DTMP (b).
Multifunctional ATRP initiators based on pyrogallol[4]arenes: Br12CXA and Br16CXA were synthesized in the reaction shown in Scheme 3. Thin layer chromatography of Br12CXA and Br16CXA confirmed, for both cases, that only one product was obtained. The esterification degree of Br12CXA and Br16CXA were calculated from the 1H NMR spectra. These spectra are shown in Fig. 3. The broadening of the signals is caused by conformational effects [14,20,28] or by the hindered rotation of the substituents in the bridge [20]. The ratio of the integrals of the signals characteristic for protons from the CH3 group in the alkyl chain (a at d = 0.79 ppm) and the proton from the CH group (signal d + e at d = 4.22 and 4.59 ppm) in the 2-bromoester groups and methine bridges in Br12CXA (Fig. 3a) indicates that all
phenol groups were transformed into 2-bromoester groups. Complete esterification was also attained for Br16CXA initiator (Fig. 3b). This was confirmed by the ratio of the intensities of the signals of the CH group of the 2-bromoester groups (signal d at d = 4.91 ppm) and the CH group of the methine bridges (signal e at d = 5.56 and 5.74 ppm). 3.2. Synthesis of the tert-butyl acrylate star polymers 3.2.1. Stars with three, four and six arms The star polymers were synthesized using the ‘‘corefirst” method in which the initiating center localized in the core initiates the polymerization of the monomer [6]. ATRP of tert-butyl acrylate is shown in Scheme 1. The ali-
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Fig. 2. MS/ESI and 1H NMR spectra of fully substituted Br6DPENTA in CDCl3.
phatic initiators described above: Br3TMP, Br4DTMP and Br6DPENTA were used to obtain three-, four- and six-arm poly(tert-butyl acrylate) stars. CuBr/PMDTA was used as the catalyst. The activity of this system in the ATRP of acrylic esters is known [17]. The polymerization of tert-butyl acrylate using Br3TMP, Br4DTMP and Br6DPENTA as the macroinitiators was performed using [M]/[I] ratios of 300, 400 and 600, respectively, which corresponds to 100 moles of monomer per one mol of initiating sites. For Br3TMP and Br4DTMP the molar ratio of [I]:[CuBr]:[PMDTA] was [1]:[0.5]:[0.5], and the reaction was carried out in acetone at 60 °C. The ratio [1]:[5]:[5] with anisole as the solvent was used for Br6DPENTA. The samples were taken during the polymerization and analyzed using gas chromatography to check the monomer conversion. The plot of log([M]0/[M]t) versus time is nearly linear for three and four-arm stars (Fig. 4), indicating a constant number of propagating species throughout the polymerization. However, the kinetic plot deviated from linearity when Br6DPENTA was used as the macroinitiator, giving a relatively low conversion. The increase of the molar masses during polymerization was followed by GPC. Control of the polymerization process for three- and four-functional initiators is possible for up to 80% of monomer conversion, and for a six-functional initiator up to 40%. The symmetry of the refractive index and light scattering traces (Fig. 5) indicates that no star–star coupling reaction occurs. Molar masses for all obtained poly(tert-butyl acrylate) (PTBA) stars were measured using GPC with multiangle laser light scattering detection (GPC–MALLS) and calculated with dn/dc values (Table 1) taken from separate experiments. To show the compactness of the star structure, the apparent molar masses of all stars were calculated using
the calibration made with the linear poly(tert-butyl acrylate) standards. The theoretical molar masses of the star polymers were calculated from the monomer conversion using the below equation:
M theor star ¼ M i þ ½CtBuA 0 =½Ci 0 DC tBuA M tBuA
ð1Þ
where Mtheorstar is the theoretical molar mass of the star polymer; Mi is the molar mass of the initiator; [CtBuA]0 is the initial molar concentration of tert-butyl acrylate; [Ci]0 is the initial molar concentration of the initiator; DCtBuA is the consumption of tert-butyl acrylate; MtBuA is the molar mass of tert-butyl acrylate. The values of the measured molar masses, their apparent values from calibration with linear PTBA standards and the theoretical dependence are shown in Fig. 6. The molar masses of three-, four- and six-arm stars increased linearly with the conversion. The theoretical values of the molar mass are in good agreement with the molar masses obtained from GPC–MALLS (Fig. 6). As expected, the apparent molar masses are lower than the true values obtained using MALLS detection, which is caused by the lower hydrodynamic volume of stars in solution compared to their linear analogues of the same molar mass [14,21–23]. All the star polymers showed a monomodal and narrow molar mass distribution. The results of the measurements are provided in Table 1. Additional experiments for the [M]/[I] ratio corresponding to the monomer to initial sites ratios other than 100 were also performed. Polymerization was performed so the conversion level was low enough to ensure the avoidance of star–star coupling reaction and so that the molar masses distribution was narrow and monomodal. The realization of these requirements resulted in molar masses below 70,000 g/mol (Table 1).
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Fig. 3. 1H NMR spectra of (a) Br12CXA in CDCl3 and (b) Br16CXA in DMSO.
3.2.2. Stars with 12 and 16 arms Star polymers with 12 and 16 arms were obtained using esterified pyrogallol[4]arenes Br12CXA and Br16CXA as the cores. Polymerization kinetics were determined for
Br12CXA with the ratio [M]/[I] = 540 and the molar ratio of [I]:[CuBr]:[PMDTA] = [1]:[6]:[6], and for Br16CXA with [M]/[I] = 760 and the molar ratio of [I]:[CuBr]:[PMDTA] = [1]:[7]:[7]. Reactions were performed in anisole at
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Fig. 4. Kinetic plots for the polymerization of tert-butyl acrylate, using Br3TMP (j), Br4DTMP (s), Br6DPENTA (D) initiators.
80 °C. The plots of log([M]0/[M]t) versus time for the polymerization of tert-butyl acrylate initiated with Br12CXA and Br16CXA cores are shown in Fig. 7. Both cases show deviation from linearity. The polymerization initiated with Br16CXA is slower than that for Br12CXA. The molar mass increases with conversion of the monomer. When the conversion exceeds 40% for 12-arm stars and 35% for 16-arm stars, the gel chromatograms showed a small peak in the lower molar mass region (Fig. 8, peaks denoted with x). This is probably caused by a transfer reaction, which leads to the presence of linear PTBA chains in the reaction mixture. The amount of the linear chains estimated from GPC did not exceed 10% for 12- and 16-arm stars.
All data obtained for 12- and 16-arm stars are given in Table 2. The molar masses measured using GPC–MALLS are verified by the values calculated from the conversion and [M]0/[I]0 ratio (Fig. 9). The differences between molar masses determined using the calibration made with the linear PTBA standards and from GPC–MALLS for 12- and 16-arm PTBA stars are considerable (Fig. 9). These differences are larger for 16arm stars, which is a result of their relatively smaller hydrodynamic volume in solution. The dispersity of molar masses of all stars measured by GPC–MALLS is low and did not exceed 1.08 (Table 2). The actual number of star arms was checked through the selective alkaline cleavage of ester bonds between the arms and the core (Scheme 4). This method of determining the number of arms was reported earlier [13–15]. Star polymers were hydrolyzed using KOH solution in ethanol. After reaction, the obtained polymers were isolated from the reaction mixture by precipitation and characterized using GPC–MALLS and 1H NMR. The 1H NMR spectrum of the polymer obtained after hydrolysis shows that tert-butyl ester groups in the PTBA were entirely preserved without transformation into carboxylic groups. The chromatograms of the 16-arm star polymer before hydrolysis and the polymers obtained after hydrolysis are shown in Fig. 10. The ratio of the molar mass before and after hydrolysis yields that the number of arms is equal to the number of groups initiating the polymerization:
fcalcd ¼ ðMn
star
Mn
core Þ=M n arm
ð2Þ
The results for the 12- and 16-arm stars are given in Table 3. The number of arms determined from the results of the hydrolysis is very close to the number of initiating sites in the macroinitiators (Table 3); this suggests that all of the 2-bromoester groups initiate the polymerization of tertbutyl acrylate. 3.3. Dilute solution properties of PTBA star polymers
Fig. 5. Chromatograms (RI traces) of the four-arm stars during polymerization of tert-butyl acrylate using Br4DTMP.
A very important aspect of the solution properties of nonlinear macromolecules is the proper determination of
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Table 1 Polymerization of tert-butyl acrylate using three-, four- and six-functional initiators. Initiator
Entry
Br3TMPa
1
300
2 3
450 537
Br4DTMPb
1
400
2
800
Br6DPENTAc
1
600
2
1200
a b c d
[M]/[I]
Conversion [%]
Mtheor
dn/dc [mL/g]
Mncalib [g/mol]
Mw/Mn
10 21 37 49 59 64 45
4400 8600 14,700 19,400 23,200 37,450 31,500
0.0559
MnGPC–MALLS [g/mol] 4500 7800 13,900 18,600 22,200 37,700 30,600
3000 5800 9900 12,400 14,000 20,400 21,700
1.18 1.26 1.14 1.05 1.05 1.07 1.04
32 57 62 66 75 67
17,200 30,000 32,600 34,600 39,200 60,500
0.0539
18,000 30,800 33,100 34,600 39,900 66,200
15,200 22,000 24,500 25,600 30,400 38,600
1.16 1.18 1.18 1.19 1.20 1.17
21 29.5 36 38.5 42.5 26.8
17,200 23,750 28,700 30,600 33,700 42,300
0.0593
17,600 23,300 27,700 30,400 36,800d 40,700
14,400 20,500 22,800 25,500 29,000 –
1.08 1.08 1.08 1.08 1.09 1.05
Molar ratio of [Br3TMP]:[Cu]:[PMDTA] = 1:0.5:0.5. Molar ratio of [Br4DTMP]:[Cu]:[PMDTA] = 1:0.5:0.5. Molar ratio of [Br6DPENTA]:[Cu]:[PMDTA] = 1:5:5. Shoulder in the high molar mass region.
their dimensions and shapes. A frequently applied method to characterize branched polymers is the comparison of their intrinsic viscosities with those of their linear homologues. Solution behavior was investigated for all of the star polymers, and is denoted as entry 1 in Table 1 and in Table 2. The total intrinsic viscosity of the PTBA stars and their linear analogues were measured using GPC with a viscosimetric detector connected online to the system. The integral of the measured slice viscosities (gsp)i over concentration yield the viscosity of the complete sample [g], which is called total intrinsic viscosity (equation below):
P ðgsp Þi DV ½g ¼ P c i DV
ð3Þ
The same value [g] is obtained when the intrinsic viscosity is measured without chromatographic separation i.e. using Ubbelohde viscometer [3,13]. The values of the total intrinsic viscosities of the stars were compared with those of the linear PTBA (Fig. 11). The value of the parameter a from the Kuhn–Mark– Houwink–Sakurada equation for the linear poly(tert-butyl acrylate) was measured in THF at 35 °C in the GPC system and equals 0.78 [24]. This value is similar to that obtained by Mrkvickova and Danhelka [29] for the linear poly(tertbutyl acrylates) in THF at 25 °C (a = 0.8). The results clearly show that viscosities are influenced by the differences in the investigated topologies. The solutions of the star samples exhibit considerably lower values of intrinsic viscosity than their linear counterparts. The total intrinsic viscosity decreases with increasing number of
arms. The viscosity of the three-arm stars does not differ from the values of the linear PTBA, which suggests that these stars behave in solution similarly to the linear chains. As can be seen in Fig. 11, the intrinsic viscosity of stars increases with molar mass. The a parameter in the Kuhn– Mark–Houwink–Sakurada equation calculated for stars based upon the linear fit of data shown in Fig. 11 is close to values obtained for the linear PTBA and varies from 0.76 to 0.79. Roovers observed a similar relation between the linear and star structures for polybutadiene [3] and PEO stars [8]. The investigation of the properties of the branched polymers in dilute solutions allows the determination of the branching ratio for the star macromolecules. These parameters were reported previously for different stars [2,3,8,13]. One of them is the branching ratio g0 defined by Zimm and Stockmayer [30] as the ratio of the intrinsic viscosities of branched to linear polymer with the same molar mass:
g0 ¼
½gbranched ½glinear at the same M
ð4Þ
The branching ratio g0 from Eq. (4) calculated for all PTBA stars from the data shown in Fig. 11 vary from 0.97 to 0.24 and are presented in Fig. 12. The values of g0 ratio decreases with increasing number of arms. But for the PTBA stars of the same functionality and the range of measured molar masses the values of factor g0 are nearly constant. The average values of g0 for stars with different functionality are given in Fig. 13. These results are compared with those obtained by Angot [13] for PS stars and with the data collected by Grest [2] for polybutadiene stars in
B. Mendrek, B. Trzebicka / European Polymer Journal 45 (2009) 1979–1993
Fig. 6. Molar masses of three-, four- and six-arm stars as a function of conversion.
Fig. 7. Kinetic plot for the polymerization of tert-butyl acrylate using Br12CXA (j) and Br16CXA (d) as initiators.
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good solvent. The value of g0 for PTBA stars with 28 arms obtained previously in our laboratory [24] is also added. Values of g0 were calculated using empirical Eq. (5) [31] valid for good solvent and lightly branched structures. In Fig. 13 this dependence of g0 as a function of the number of arms f is shown as a dashed line
g 0empirical ¼
0:58 3f 2 0:724 0:015ðf 1Þ f2 0:724
ð5Þ
As expected, the values of g0 decrease with increasing number of arms. However, when the number of arms reached 16 the value of g0 did not change significantly with the increase of the number of arms. Experimental results fit well to the empirical function up to stars with about 18 arms. Above this a difference between the empirical and experimental value is observed. Fig. 8. Chromatograms (RI traces) of the 12-arm star polymers.
Table 2 Polymerization of tert-butyl acrylate using 12- and 16-functional initiators. Conversion [%]
Mtheor
MnGPCa [g/mol]
Mncalib [g/mol]
Mw/Mn
Br12CXA
18.5 27 34.5 38.6 41 55.7
15,600 21,500 26,700 29,500 31,200 41,300
16,700 22,200 27,300 28,000 28,700d 39,200d
9600 14,600 19,200 21,600 21,900 26,400
1.05 1.07 1.04 1.08 1.08 1.06
Br16CXAc
17 24 28 31 35 39
19,600 26,400 30,300 33,200 37,200 41,000
21,200 27,000 28,900 33,500 35,000d 38,000d
8500 12,000 13,400 15,400 15,200 17,700
1.06 1.03 1.03 1.02 1.02 1.03
Initiator b
a b c d
Values obtained using measured dn/dc of 12 and 16-arm stars equal 0.0553 g/mL. Molar ratio of [Br12CXA]:[Cu]:[PMDTA] = 1:6:6. Molar ratio of [Br16CXA]:[Cu]:[PMDTA] = 1:7:7. Additional peak in the low molar mass region.
Fig. 9. Molar masses of 12- and 16-arm stars as a function of conversion.
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1991
Scheme 4. The selective alkaline hydrolysis of ester bonds between arms and the core of a 16-arm star.
of the growing radicals is observed. The theoretical molar masses calculated from the monomer consumption are in good agreement with the absolute molar masses obtained from GPC with light scattering detection. The intrinsic viscosity of the PTBA stars decrease with the number of arms when the molar mass is kept constant. The values of the branching ratio g0 is in good agreement with the theoretical values. The solution behavior reveals the compactness of the structure of the synthesized polymers
Fig. 10. Chromatograms of the 16-arm star polymer before hydrolysis (a) and after hydrolysis (b).
4. Conclusions ATRP initiated with bromoester derivatives of multifunctional alcohols and calix[4]arenes is reasonably well controlled and leads to well-defined stars with the number of PTBA arms varying from 3 to 16. When conversion is kept below a certain limit, no star coupling by combination
Table 3 The actual functionality of PTBA stars. Mn [g/mol]
Mw/Mn
f
Star
Arm
Star
Arm
52,100 91,800
4200 5700
1.06 1.05
1.1 1.35
theor
12 16
f
calcd from
11.7 15.6
(1)
Fig. 11. Log–log plot of intrinsic viscosities versus the weight average molar mass for the star PTBA and the linear PTBA.
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Fig. 12. The g0 values as a function of the molar masses of the PTBA star polymers.
Fig. 13. Values of g0 for different star polymers compared with the empirical function.
compared to their linear analogues having identical molar masses, and verifies the branched structure of the polymers. Acknowledgements Barbara Mendrek appreciates the support granted to the Ph.D. students of chemistry from the University of Opole by the European Social Fund. The authors wish to thank Wolf D. Habicher (University of Technology, Dresden, Germany) for the pyrogallol[4]arene samples. References [1] Burchard W. Solution properties of branched macromolecules. Adv Polym Sci 1999;143:113–94. [2] Grest S, Fetters L, Huang JS, Richter D. Star polymers: experiment, theory and simulation. Adv Chem Phys 1996;XCIV:67–163. [3] Roovers J, Zhou LL, Toporowski PM, van der Zwan M, Iatrou H, Hadjichristidis N. Regular star polymers with 64 and 128 arms. Models for polymeric micelles. Macromolecules 1993;26:4324–31.
[4] Hult A, Johansson M, Malmstrom E. Hyperbranched polymers. Adv Polym Sci 1999;143:1–34. [5] Matthews OA, Shipway AN, Stoddart JF. Dendrimers-branching out from curiosities into new technologies. Prog Polym Sci 1998;23: 1–56. [6] Mishra MK, Kobayashi S, editors. Star and hyperbranched polymers. Basel, New York: Marcel Dekker, Inc.; 1999. p. 29–46. [7] Zhao Y, Chen Y, Chen C, Xi F. Synthesis of well-defined star polymers and star block copolymers from dendrimer initiators by atom transfer polymerization. Polymer 2005;46:5808–19. [8] Comanita B, Noren B, Roovers J. Star poly(ethylene oxide)s from carbosilane dendrimers. Macromolecules 1999;32:1069–72. [9] Chen Y, Shen Z, Barriau E, Kautz H, Frey H. Synthesis of multiarm star poly(glycerol)-block–poly(2-hydroxyethyl methacrylate). Biomacromolecules 2006;7:919–26. [10] Held D, Mueller AHE. Synthesis and solution properties of starshaped poly(tert-butyl acrylate). Macromol Symp 2000;157:225–37. [11] Beak KY, Kamigaito M, Sawamoto M. Star-shaped polymers by metal-catalyzed living radical polymerisation. 1. Design of Ru(II)based systems and divinyl linking agents. Macromolecules 2001;34: 215–21. [12] Ohno K, Wong B, Haddleton D. Synthesis of well-defined cyclodextrin-core star polymers. J Polym Sci Part A Polym Chem 2001;39:2206–14. [13] Angot S, Murthy KS, Taton D, Gnanou Y. Scope of copper halide/ bipyridyl system associated with calixarene-based multihalides for synthesis of well-defined polystyrene and poly(meth)acrylate stars. Macromolecules 2000;33:7261–74. [14] Stradman S, Tenhu H. Star polymers synthesised with flexible resorcinarene-derived ATRP initiators. Polymer 2007;48:3938–51. [15] Jankova K, Bednarek M, Hvilsted S. Star polymers by ATRP of styrene and acrylates employing multifunctional initiators. J Polym Sci Part A Polym Chem 2005;43:3748–59. [16] Kasko AM, Pugh C. Comparison of the thermotropic and solution behavior of six-arm star and comb poly[11-(40 -cyanophenyl-40 0 phenoxy)undecyl acrylate]s. Macromolecules 2006;39:6800–10. [17] Matyjaszewski K, Xia J. Atom transfer polymerization. Chem Rev 2001;101:2921–90. [18] Matyjaszewski K, Miller PJ, Pyun J, Kickelbick G, Diamanti S. Synthesis and characterization of star polymers with varying arm number, length and composition from organic and hybrid inorganic/ organic multifunctional initiators. Macromolecules 1999;32: 6526–35. [19] Huang CF, Lee HF, Kuo SW, Xu H, Chang FC. Star polymers via atom transfer polymerization from adamante-based cores. Polymer 2004;45:2261–9. [20] Stradman S, Luostarinen M, Niemela S, Rissanen K, Tenhu H. Resorcinarene-based ATRP initiators for star polymers. J Polym Sci Part A Polym Chem 2004;42:4189–201. [21] Kowalczuk-Bleja A, Sierocka B, Muszyn´ski J, Trzebicka B, Dworak A. Core–shell polyacrylate and polystyrene-block–polyacrylate stars. Polymer 2005;46:8555–64.
B. Mendrek, B. Trzebicka / European Polymer Journal 45 (2009) 1979–1993 [22] Shen Z, Chen Y, Barriau E, Frey H. Multi-arm star polyglycerolblock–poly(tert-butyl acrylate) and the respective multi-arm poly(acrylic acid) stars. Macromol Chem Phys 2006;207: 57–64. [23] Maier S, Sunder A, Frey H, Mulhaupt R. Synthesis of polyglycerolblock–poly(methyl acrylate) multi-arm star polymers. Macromol Rapid Commun 2000;21:226–30. [24] Kowalczuk A, Vandendriessche A, Trzebicka B, Mendrek B, Szeluga U, Cholewin´ski G, et al. Core–shell nanoparticles with hyperbranched poly(arylene oxindole) interiors. J Polym Sci Part A Polym Chem 2009;47:1120–35. [25] Frater DJ, Mays JW, Jackson C. Synthesis and dilute solution properties of divinylbenzene-linked polystyrene stars with mixed arm lengths: evidence for coupled stars. J Polym Sci Part B Polym Phys 1997;35:141–51.
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Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Synthesis and characterization of well-defined poly(tert-butyl acrylate) star polymers Barbara Mendrek a, Barbara Trzebicka b,* a b
University of Opole, Faculty of Chemistry, Oleska 48, 45-052 Opole, Poland Centre of Polymer and Carbon Materials, Polish Academy of Sciences, M. Curie-Sklodowskiej 34, 41-819 Zabrze, Poland
a r t i c l e
i n f o
Article history: Received 13 February 2009 Received in revised form 1 April 2009 Accepted 13 April 2009 Available online 24 April 2009
Keywords: Star polymers Poly(tert-butyl acrylate) Gel permeation chromatography Triple detection Intrinsic viscosity Branching parameter
a b s t r a c t Star polymers with different numbers and lengths of poly(tert-butyl acrylate) (PTBA) arms were obtained via atom transfer radical polymerization. Aliphatic alcohols with different number of hydroxyl groups varying from 3 to 6 and calix[4]arenes based on pyrogallol with 12 and 16 phenol groups were transformed to bromoester derivatives to prepare multifunctional ATRP initiators used as the cores of the stars. The star polymers were characterized by GPC with refractive index, multiangle laser light scattering and viscosimetric detectors. The molar masses of the stars reached 70,000 g/mol and the molar mass dispersities did not exceed 1.2. To elucidate the compact structure of the stars, their true molar masses were determined by GPC with triple detection (RI–MALLS–Visco) and compared with the apparent molar masses obtained from the calibration with linear poly(tert-butyl acrylate) standards. The intrinsic viscosities of the PTBA stars of the same molar mass decreased with the number of star arms but were always lower than the intrinsic viscosities of the analogue linear PTBA polymers. The values of the branching ratio g0 decreased with increasing number of arms indicating more compact structure of stars. The branching ratio g0 was correlated to the empirical predictions. Ó 2009 Published by Elsevier Ltd.
1. Introduction Polymers of nonlinear topology have been of interest to scientists for many years [1–5]. Studies have focused on the synthesis, characterization and application of branched polymers. Star polymers are a special kind of branched structure. They consist of a central core and different numbers of linear chains – arms connected to the core [1]. The properties of star polymers are different from those of linear analogues. Star polymers with exactly known numbers and lengths of arms have been useful tools in providing information how the number of arms influences the properties of polymers in solution and in the melt [1–3]. There are two main methods for the preparation of star polymers. The arm-first method involves the synthesis of * Corresponding author. E-mail address: [email protected] (B. Trzebicka). 0014-3057/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.eurpolymj.2009.04.016
living macromolecular chains – future arms of the star – and their reaction with linking agent, the core. In the core-first method a multifunctional compound, the core, initiates the polymerization of the monomer [6]. Many multifunctional compounds have been used as cores, amongst them dendrimers [7,8], hyperbranched polymers [9], microgels [10,11], cyclodextrins [12], calix[n]arenes [13,14], alcohols [15] and phenols [16]. The different numbers of initiating or terminating groups lead to stars with varying numbers of arms. Significant advancements in the field of controlled radical polymerization techniques, especially atom transfer radical polymerization (ATRP) [17], have opened new routes to the synthesis of well-defined star structures [13–15,18]. One of the methods used to obtain multifunctional initiators for ATRP is the transformation of the hydroxyl group in alcohols or phenols into a bromoester group [7,9,13–15,18]. Such initiators have been used in the ATRP of styrene and
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various (meth)acrylic monomers. Adamante-based cores have been used for the preparation of 4-arm polystyrene and poly(methylmethacrylate) stars [19]. Resorcin-based calix[4]arene cores [14,20] and 4-tert-butylcalix[4,6,8]arenes cores [13] were used to obtain poly(methylmethacrylate) and poly(tert-butyl acrylate) stars, respectively, with eight or less arms. Poly(methylmethacrylate), polystyrene and block copolymer poly(methylmethacrylate)-poly(n-butyl methacrylate) 21-arm stars were synthesized via ATRP initiated with esterified cyclodextrin [12]. Three-, fourand six-arm poly(methylmethacrylate), polystyrene and poly(n-butyl acrylate) stars have been obtained by ATRP using esterified cyclotriphosphazenes and cyclosiloxanes as the cores [18]. Hyperbranched polymers used as cores lead to multiarmed star polymers. These cores have been used to synthesize poly(meth)acrylic star polymers with high numbers of arms via ATRP [9,21–24]. Physicochemical properties of branched macromolecules, amongst them star macromolecules, differ significantly from that of the linear chains of the same composition and molar mass and are the subject of numerous studies. The compact structures of the stars reflect their solution and solid-state properties: intrinsic viscosity [3,8–10,25], size in solution [2,3,8,16,25], glass transition temperature and mechanical properties [7,15,16,21]. In this work, we describe the synthesis of multifunctional initiators for ATRP and their use to obtain well-defined star polymers of tert-butyl acrylate with 3, 4, 6, 12 and 16 arms. The solution behavior of the stars is studied by gel permeation chromatography (GPC) with differential refractive index (RI), multiangle laser light scattering (MALLS) and viscosimetric detectors to obtain fundamental data on the hydrodynamics of these macromolecules. Viscosity data obtained from GPC for star-shaped and linear polymers are used to determine the parameters of the Kuhn-Mark–Houwink–Sakurada equation and the contraction factors resulting from them.
2. Experimental 2.1. Materials Trimethylolpropane (TMP, 97%), ditrimethylolpropane (DTMP, 97%), 2-bromopropionic acid (99%), 2-bromopropionyl bromide (97%), didecyldimethylammonium bromide (DDAB, 97%), 4-dimethylaminopyridine (DMAP, 99%), and N,N0 -dicyclohexylcarbodiimide (DCC, 99%), N,N,N0 ,N0 ,N00 pentamethyldiethylenetriamine (PMDETA, 99%), copper (I) bromide (CuBr, 99.999%) were purchased from Aldrich and used as received. Dipentaerythritol (DPENTA, technical grade, Aldrich) was recrystallized from water. Calix[4]arenes based on pyrogallol were synthesized and kindly provided by Wolf D. Habicher from the University of Technology, Dresden, Germany. tert-Butyl acrylate (tBuA, 98%), anisole (99%) and p-xylene (99%) were purchased from Aldrich and purified by distillation prior to use.
1,4-Dioxane (pure p.a.) was purified by distillation under nitrogen over sodium. N,N0 -Dimethylformamide (DMF, pure p.a.), chloroform (pure p.a.), dichloromethane (pure p.a.), acetone (pure p.a.), potassium carbonate anhydrous (pure p.a.), and sodium bicarbonate (pure) were purchased from POCh and used without purification. DOWEX MARATHON MSC cation exchange resin was transformed into the H+ type using 1.6 M HNO3. DOWEX MONOSPHERE 550A anion exchange resin was washed with water and dried in a vacuum before use. Both exchange resins were purchased from Aldrich. 2.2. Synthesis of the multifunctional initiators for ATRP of tert-butyl acrylate 2.2.1. Synthesis of trimethylolpropane tris(2-bromopropionate) (Br3TMP) TMP (1.50 g, 33.5 mmol) and DMAP (1.02 g, 8.38 mmol) were dissolved under nitrogen in dry 1,4-dioxane (38 mL) in a Schlenk flask equipped with a magnetic stirrer. 2-Bromopropionic acid (5.64 g, 36.85 mmol, 3.32 mL) was next added to the flask and cooled to 0 °C (ice bath). A solution of DCC (7.60 g, 36.85 mmol) in 1,4-dioxane (60 mL) was transferred to the flask dropwise. The solution was stirred at room temperature for 48 h. Dicyclohexylurea was filtered off and the 1,4-dioxane evaporated. The crude product of Br3TMP was dissolved in chloroform (25 mL). The solution was washed with water (2 25 mL), with saturated NaHCO3 (3 25 mL) and with water (1 25 mL). Chloroform was evaporated; the crude product was dissolved in acetone (20 mL) and passed through an anion exchange resin column. Acetone was then evaporated and the product was dried in a vacuum. Br3TMP was obtained in 85% yield (3.3 g) as a slightly yellow viscous liquid. The functionality as measured by 1 H NMR was 100%. 1 H NMR (600 MHz, CDCl3) dppm: 0.95 (t, 3H, CH3CH2–), 1.59 (q, 2H, CH3CH2–), 1.83 (d, 9H, CH3CHBrCOOCH2–), 4.07–4.26 (m, 6H, –CCH2O–), 4.40 (q, 3H, CH3CHBrCOOCH2–). MS(ESI): calculated for C15H23Br3O6 m/z = 557.09 [M+NH4+], found m/z = 556.90. Thin layer chromatography of Br3TMP was performed in chloroform (Rf = 0.65). 2.2.2. Synthesis of ditrimethylolpropane tetrakis(2-bromopropionate) (Br4DTMP) Br4DTMP was prepared in the same manner as that described above for Br3TMP using DMF as the solvent. Slightly yellow Br4DTMP was synthesized in 87% yield (5.49 g). 1 H NMR (600 MHz, CDCl3) dppm: 0.89 (t, 6H, CH3CH2–), 1.43–1.57 (m, 4H, CH3CH2), 1.83 (d, 12H, CH3CHBrCOOCH2–), 3.35 (br s, 4H, –CH2OCH2–), 4.00–4.22 (m, 8H, – CCH2O–), 4.40 (q, 4H, CH3CHBrCOOCH2–). MS(ESI): calculated for C24H38Br4O9 m/z = 808.17 [M+NH4+], found m/z = 808.00. Thin layer chromatography of Br4DTMP was performed in a mixture of the solvents chloroform/n-hexane (10:1) (Rf = 0.55).
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2.2.3. Synthesis of dipentaerythritol hexakis(2-bromopropionate) (Br6DPENTA) DPENTA (1.00 g, 4.0 mmol) and NaOH (1.00 g, 25.0 mmol) were dissolved in water (100 mL). DDAB (0.08 g, 0.2 mmol) was added and the solution was vigorously stirred. The solution of 2-bromopropionyl bromide (6.40 g, 36 mmol, 3.77 mL) in dichloromethane (75 mL) was added dropwise. The reaction mixture was stirred for 7 days. The organic phase was separated and washed multiple times with water to remove the DDAB residue. Dichloromethane was evaporated and the sample was dried under vacuum. The white powder of Br6DPENTA was obtained in 40% yield (1.67 g). 1 H NMR (600 MHz, CDCl3) dppm: 1.83 (d, 18H, CH3CHBrCOOCH2–), 3.52 (br s, 4H, –CH2OCH2–), 4.12–4.37 (m, 12H, –CCH2O–), 4.41 (q, 6H, CH3CHBrCOOCH2–). MS(ESI): calculated for C28H38Br6O13 m/z = 1103.13 [M+K+], found m/z = 1103.10. Thin layer chromatography of Br6DPENTA was performed in chloroform (Rf = 0.63). 2.2.4. Synthesis of calix[4]arene dodekakis(2-bromopropionate) (Br12CXA) Br12CXA was prepared in the same manner as described for Br3TMP. A yellow solid sample of Br12CXA was obtained in 73% yield (1.74 g). 1 H NMR (600 MHz, CDCl3) dppm: 0.79 (t, 12H, – C10H20CH3), 1.16 (s, 80H, –C10H20CH3), 1.6–2.0 (br m, 36H, CH3CHBrCOOCH2–), 3.9–4.8 (br m, 4H, CH3CHBrCOOCH2– and 4H, Ar–CH–Ar), 6.04 and 6.89 (br s, 4H, ArH). Thin layer chromatography of Br12CXA was performed in the mixture of chloroform/n-hexane (10:1) (Rf = 0.55). 2.2.5. Synthesis of calix[4]arene hexadekakis(2-bromopropionate) (Br16CXA) CXAOH16 (1.00 g, 1.09 mmol) was suspended in 1,4dioxane (30 mL). The solution of DMAP (0.53 g, 4.35 mmol) and 2-bromopropionic acid (2.92 g, 19.13 mmol, 1.72 mL) was added and cooled to 0 °C (ice bath). A solution of DCC (3.95 g, 19.13 mmol) in 1,4-dioxane (30 mL) was transferred to the flask dropwise. The mixture was stirred first for 2 h in an ultrasonic bath and next in a magnetic stirrer for 48 h. Purification was done in the same manner as for Br3TMP. A light orange solid of Br16CXA in 58% yield (1.94 g) was obtained. 1 H NMR (600 MHz, DMSO) dppm: 1.56, 1.69, 1.88 (br s, 48H, CH3CHBrCOOCH2–), 4.61–5.10 (br m, 16H, CH3CHBrCOOCH2–), 5.56, 5.74 (br s, 4H, Ar–CH–Ar), 6.05, 6.24 (br s, 4H, –ArH), 6.89 (br s, 16H, ArH of substituent). Thin layer chromatography of Br16CXA was performed in chloroform (Rf = 0.61). 2.3. Synthesis of the stars – ATRP of tert-butyl acrylate The appropriate multifunctional initiator (20 mmol), CuBr (1.43 g, 10 mmol), PMDETA (1.73 g, 10 mmol, 2.08 mL) and acetone or anisole (about 10% v/v related to monomer, sufficient to dissolve the initiator) were placed in a Schlenk flask. The solution was stirred until the Cu complex formed (the solution turned green) at which point the amount of tert-butyl acrylate correspond-
1981
ing to the chosen monomer to initiator ratio was added. The mixture was degassed using three freeze–vacuum– thaw cycles. The flask was placed in an oil bath and thermostated at 60 °C (acetone) or 80 °C (anisole). Samples were taken periodically for the kinetic analysis. The monomer conversion was measured by gas chromatography with p-xylene as the internal standard. After the desired conversion was obtained, THF (20 mL) was added and the solution was passed through a column with DOWEX-MSC-1 ion exchange resin to remove copper. The polymer was precipitated in methanol/water mixture (1:1) and dried. 2.4. Selective alkaline hydrolysis of ester bonds between the arms and the core of the stars The star polymer sample (0.3 g) was dissolved in THF (20 mL) in a round-bottomed flask fitted with a condenser and nitrogen inlet. KOH solution (2 mL, 1 M in ethanol) was added to the flask and the reaction mixture was refluxed at 60 °C for 10 min. The solvent was evaporated without heating under reduced pressure; the remaining polymer was dissolved in THF, precipitated in a methanol/water mixture (8:2) and dried. 2.5. Characterization NMR spectra were recorded using a Bruker Ultrashield 600 (600 MHz for 1H). The resonances are given in ppm referenced to the tetramethylsilane (TMS) peak. Gas chromatography was used to determine the conversion by measuring the residual monomer content with p-xylene as the internal standard, using a VARIAN 3400 gas chromatograph with the J&W Scientific DB-5 (30 m 0.32 mm) column. Mass spectra were run on a Bruker Esquire MS with an ion trap detector in combination with an Agilent/HP 1100 HPLC-device. Thin layer chromatography (TLC) was performed on commercial Merck plates coated with silica gel 60 F254 (0.25 mm thick). UV light with a wavelength of k = 254 nm was used for detection. The molar masses, the molar mass distributions and solution viscosities of the obtained polymers were determined by GPC with three detectors: a differential refractive index Dn-1000 RI WGE Dr. Bures, a viscosimetric detector g-1001 WGE Dr. Bures and a multiangle light scattering DAWN EOS from Wyatt Technologies. The column set containing four SDV columns from Polymer Standard Service (PSS): 1 105 Å + 1 103 Å + 2 102 Å was used. Measurements were performed in THF at 35 °C with a nominal flow rate of 1 mL/min. Conventional calibration was established with PTBA linear standards from PSS. Results were evaluated using the ASTRA software from Wyatt Technologies and the WINGPC software from PSS. The refractive index increment dn/dc for linear PTBA and for star polymers was measured independently in THF. The dn/dc of linear PTBA were determined for polymer of Mn = 53,200 g/mol and of Mn = 8700 g/mol. In both cases the values were equal to 0.0593 mL/g.
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3. Results and discussion 3.1. Synthesis of the multifunctional initiators for ATRP of tert-butyl acrylate The preparation of the atom transfer radical polymerization initiators is essential for obtaining polymers of well-defined structure. It is known that alkyl halides with substituents on a-carbon are good initiators for ATRP of (meth)acrylic monomers [13–15,18]. The ‘‘core-first” method was applied for the preparation of the star polymers [6]. The cores, which are initiators with a confirmed number of initiating sites were used for
the preparation of stars via atom transfer radical polymerization of tert-butyl acrylate (Scheme 1). Initiators with different numbers of initiating centers were obtained by the esterification of the hydroxyl group of the commercially available aliphatic alcohols (Scheme 2) and phenolic groups of the calix[4]arenes obtained by Habicher et al. [27] (Scheme 3). The esterification method of the 2-bromopropionylbromide or 2-bromoisobutyryl bromide in the presence of amine has been described previously [13–16,18,20,27]. However, the incomplete esterification of the OH groups is frequently a problem. The separation of the mixture of products is problematic. If the products do not crystallize
Scheme 1. Schematic representation of synthetic route to poly(tert-butyl acrylate) star polymers.
Scheme 2. Synthesis of the ATRP initiators by the esterification of hydroxyl groups of aliphatic alcohols.
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Scheme 3. Synthesis of the ATRP initiators based on pyrogallol[4]arenes.
or precipitate, the time consuming column chromatography must be used [14,20,27]. The aliphatic compounds trimethylolpropane (TMP) and ditrimethylolpropane (DTMP) were esterified using 2bromopropionic acid in the presence of N,N0 -dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) as described in [26]. The reaction leading to trimethylolpropane tris(2-bromopropionate) (Br3TMP), ditrimethylolpropane tetrakis(2-bromopropionate) (Br4DTMP) and dipentaerythritol hexakis(2-bromopropionate) (Br6DPENTA) is shown in Scheme 2. Thin layer chromatography confirmed that homogeneous products were obtained. The complete substitution of the 2bromoesters groups was confirmed by 1H NMR and MS(ESI) (Figs. 1 and 2). Dipentaerythritol (DPENTA) is not soluble in the majority of solvents suitable for the esterification. All attempts to obtain pure, single esterification products have failed: neither the esterification in DMSO using 2-bromopropionic acid, nor the use of pyridine as the solvent and proton acceptor, nor a heterophase reaction in 1,4-dioxane with 2-bromopropionic acid bromide yielded a single product
with all OH groups transformed to esters. DPENTA was finally esterified by 2-bromopropionic bromide in a twophase water/CH2Cl2 system in the presence of a phase transfer catalyst didecyldimethylammonium bromide (DDAB). This process yielded a single product in good yield. The 1H NMR spectrum (Fig. 2) shows that all hydroxyl groups were transformed into 2-bromoesters groups. Isotope distribution obtained from the MS/ESI spectrum confirmed that the DPENTA was esterified with a 100% yield. The thin layer chromatography of Br6DPENTA in chloroform confirmed that a single product was obtained. The pyrogallol[4]arenes CXAOH12 and CXAOH16 (Scheme 3) used in this work were obtained by Habicher et al. [27]. The same authors esterified CXAOH12 and CXAOH16 using 2-bromoisobutyryl bromide in the presence of pyridine, however they obtained a mixture of differently substituted products that had to be purified by column chromatography [27]. This effort verified that these difficulties could be overcome when 2-bromopropionic acid was used for the esterification of the calix[4]arenes CXAOH12 and CXAOH16.
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Fig. 1. MS/ESI and 1H NMR in CDCl3 spectra of Br3TMP (a) and Br4DTMP (b).
Multifunctional ATRP initiators based on pyrogallol[4]arenes: Br12CXA and Br16CXA were synthesized in the reaction shown in Scheme 3. Thin layer chromatography of Br12CXA and Br16CXA confirmed, for both cases, that only one product was obtained. The esterification degree of Br12CXA and Br16CXA were calculated from the 1H NMR spectra. These spectra are shown in Fig. 3. The broadening of the signals is caused by conformational effects [14,20,28] or by the hindered rotation of the substituents in the bridge [20]. The ratio of the integrals of the signals characteristic for protons from the CH3 group in the alkyl chain (a at d = 0.79 ppm) and the proton from the CH group (signal d + e at d = 4.22 and 4.59 ppm) in the 2-bromoester groups and methine bridges in Br12CXA (Fig. 3a) indicates that all
phenol groups were transformed into 2-bromoester groups. Complete esterification was also attained for Br16CXA initiator (Fig. 3b). This was confirmed by the ratio of the intensities of the signals of the CH group of the 2-bromoester groups (signal d at d = 4.91 ppm) and the CH group of the methine bridges (signal e at d = 5.56 and 5.74 ppm). 3.2. Synthesis of the tert-butyl acrylate star polymers 3.2.1. Stars with three, four and six arms The star polymers were synthesized using the ‘‘corefirst” method in which the initiating center localized in the core initiates the polymerization of the monomer [6]. ATRP of tert-butyl acrylate is shown in Scheme 1. The ali-
B. Mendrek, B. Trzebicka / European Polymer Journal 45 (2009) 1979–1993
1985
Fig. 2. MS/ESI and 1H NMR spectra of fully substituted Br6DPENTA in CDCl3.
phatic initiators described above: Br3TMP, Br4DTMP and Br6DPENTA were used to obtain three-, four- and six-arm poly(tert-butyl acrylate) stars. CuBr/PMDTA was used as the catalyst. The activity of this system in the ATRP of acrylic esters is known [17]. The polymerization of tert-butyl acrylate using Br3TMP, Br4DTMP and Br6DPENTA as the macroinitiators was performed using [M]/[I] ratios of 300, 400 and 600, respectively, which corresponds to 100 moles of monomer per one mol of initiating sites. For Br3TMP and Br4DTMP the molar ratio of [I]:[CuBr]:[PMDTA] was [1]:[0.5]:[0.5], and the reaction was carried out in acetone at 60 °C. The ratio [1]:[5]:[5] with anisole as the solvent was used for Br6DPENTA. The samples were taken during the polymerization and analyzed using gas chromatography to check the monomer conversion. The plot of log([M]0/[M]t) versus time is nearly linear for three and four-arm stars (Fig. 4), indicating a constant number of propagating species throughout the polymerization. However, the kinetic plot deviated from linearity when Br6DPENTA was used as the macroinitiator, giving a relatively low conversion. The increase of the molar masses during polymerization was followed by GPC. Control of the polymerization process for three- and four-functional initiators is possible for up to 80% of monomer conversion, and for a six-functional initiator up to 40%. The symmetry of the refractive index and light scattering traces (Fig. 5) indicates that no star–star coupling reaction occurs. Molar masses for all obtained poly(tert-butyl acrylate) (PTBA) stars were measured using GPC with multiangle laser light scattering detection (GPC–MALLS) and calculated with dn/dc values (Table 1) taken from separate experiments. To show the compactness of the star structure, the apparent molar masses of all stars were calculated using
the calibration made with the linear poly(tert-butyl acrylate) standards. The theoretical molar masses of the star polymers were calculated from the monomer conversion using the below equation:
M theor star ¼ M i þ ½CtBuA 0 =½Ci 0 DC tBuA M tBuA
ð1Þ
where Mtheorstar is the theoretical molar mass of the star polymer; Mi is the molar mass of the initiator; [CtBuA]0 is the initial molar concentration of tert-butyl acrylate; [Ci]0 is the initial molar concentration of the initiator; DCtBuA is the consumption of tert-butyl acrylate; MtBuA is the molar mass of tert-butyl acrylate. The values of the measured molar masses, their apparent values from calibration with linear PTBA standards and the theoretical dependence are shown in Fig. 6. The molar masses of three-, four- and six-arm stars increased linearly with the conversion. The theoretical values of the molar mass are in good agreement with the molar masses obtained from GPC–MALLS (Fig. 6). As expected, the apparent molar masses are lower than the true values obtained using MALLS detection, which is caused by the lower hydrodynamic volume of stars in solution compared to their linear analogues of the same molar mass [14,21–23]. All the star polymers showed a monomodal and narrow molar mass distribution. The results of the measurements are provided in Table 1. Additional experiments for the [M]/[I] ratio corresponding to the monomer to initial sites ratios other than 100 were also performed. Polymerization was performed so the conversion level was low enough to ensure the avoidance of star–star coupling reaction and so that the molar masses distribution was narrow and monomodal. The realization of these requirements resulted in molar masses below 70,000 g/mol (Table 1).
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Fig. 3. 1H NMR spectra of (a) Br12CXA in CDCl3 and (b) Br16CXA in DMSO.
3.2.2. Stars with 12 and 16 arms Star polymers with 12 and 16 arms were obtained using esterified pyrogallol[4]arenes Br12CXA and Br16CXA as the cores. Polymerization kinetics were determined for
Br12CXA with the ratio [M]/[I] = 540 and the molar ratio of [I]:[CuBr]:[PMDTA] = [1]:[6]:[6], and for Br16CXA with [M]/[I] = 760 and the molar ratio of [I]:[CuBr]:[PMDTA] = [1]:[7]:[7]. Reactions were performed in anisole at
B. Mendrek, B. Trzebicka / European Polymer Journal 45 (2009) 1979–1993
1987
Fig. 4. Kinetic plots for the polymerization of tert-butyl acrylate, using Br3TMP (j), Br4DTMP (s), Br6DPENTA (D) initiators.
80 °C. The plots of log([M]0/[M]t) versus time for the polymerization of tert-butyl acrylate initiated with Br12CXA and Br16CXA cores are shown in Fig. 7. Both cases show deviation from linearity. The polymerization initiated with Br16CXA is slower than that for Br12CXA. The molar mass increases with conversion of the monomer. When the conversion exceeds 40% for 12-arm stars and 35% for 16-arm stars, the gel chromatograms showed a small peak in the lower molar mass region (Fig. 8, peaks denoted with x). This is probably caused by a transfer reaction, which leads to the presence of linear PTBA chains in the reaction mixture. The amount of the linear chains estimated from GPC did not exceed 10% for 12- and 16-arm stars.
All data obtained for 12- and 16-arm stars are given in Table 2. The molar masses measured using GPC–MALLS are verified by the values calculated from the conversion and [M]0/[I]0 ratio (Fig. 9). The differences between molar masses determined using the calibration made with the linear PTBA standards and from GPC–MALLS for 12- and 16-arm PTBA stars are considerable (Fig. 9). These differences are larger for 16arm stars, which is a result of their relatively smaller hydrodynamic volume in solution. The dispersity of molar masses of all stars measured by GPC–MALLS is low and did not exceed 1.08 (Table 2). The actual number of star arms was checked through the selective alkaline cleavage of ester bonds between the arms and the core (Scheme 4). This method of determining the number of arms was reported earlier [13–15]. Star polymers were hydrolyzed using KOH solution in ethanol. After reaction, the obtained polymers were isolated from the reaction mixture by precipitation and characterized using GPC–MALLS and 1H NMR. The 1H NMR spectrum of the polymer obtained after hydrolysis shows that tert-butyl ester groups in the PTBA were entirely preserved without transformation into carboxylic groups. The chromatograms of the 16-arm star polymer before hydrolysis and the polymers obtained after hydrolysis are shown in Fig. 10. The ratio of the molar mass before and after hydrolysis yields that the number of arms is equal to the number of groups initiating the polymerization:
fcalcd ¼ ðMn
star
Mn
core Þ=M n arm
ð2Þ
The results for the 12- and 16-arm stars are given in Table 3. The number of arms determined from the results of the hydrolysis is very close to the number of initiating sites in the macroinitiators (Table 3); this suggests that all of the 2-bromoester groups initiate the polymerization of tertbutyl acrylate. 3.3. Dilute solution properties of PTBA star polymers
Fig. 5. Chromatograms (RI traces) of the four-arm stars during polymerization of tert-butyl acrylate using Br4DTMP.
A very important aspect of the solution properties of nonlinear macromolecules is the proper determination of
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Table 1 Polymerization of tert-butyl acrylate using three-, four- and six-functional initiators. Initiator
Entry
Br3TMPa
1
300
2 3
450 537
Br4DTMPb
1
400
2
800
Br6DPENTAc
1
600
2
1200
a b c d
[M]/[I]
Conversion [%]
Mtheor
dn/dc [mL/g]
Mncalib [g/mol]
Mw/Mn
10 21 37 49 59 64 45
4400 8600 14,700 19,400 23,200 37,450 31,500
0.0559
MnGPC–MALLS [g/mol] 4500 7800 13,900 18,600 22,200 37,700 30,600
3000 5800 9900 12,400 14,000 20,400 21,700
1.18 1.26 1.14 1.05 1.05 1.07 1.04
32 57 62 66 75 67
17,200 30,000 32,600 34,600 39,200 60,500
0.0539
18,000 30,800 33,100 34,600 39,900 66,200
15,200 22,000 24,500 25,600 30,400 38,600
1.16 1.18 1.18 1.19 1.20 1.17
21 29.5 36 38.5 42.5 26.8
17,200 23,750 28,700 30,600 33,700 42,300
0.0593
17,600 23,300 27,700 30,400 36,800d 40,700
14,400 20,500 22,800 25,500 29,000 –
1.08 1.08 1.08 1.08 1.09 1.05
Molar ratio of [Br3TMP]:[Cu]:[PMDTA] = 1:0.5:0.5. Molar ratio of [Br4DTMP]:[Cu]:[PMDTA] = 1:0.5:0.5. Molar ratio of [Br6DPENTA]:[Cu]:[PMDTA] = 1:5:5. Shoulder in the high molar mass region.
their dimensions and shapes. A frequently applied method to characterize branched polymers is the comparison of their intrinsic viscosities with those of their linear homologues. Solution behavior was investigated for all of the star polymers, and is denoted as entry 1 in Table 1 and in Table 2. The total intrinsic viscosity of the PTBA stars and their linear analogues were measured using GPC with a viscosimetric detector connected online to the system. The integral of the measured slice viscosities (gsp)i over concentration yield the viscosity of the complete sample [g], which is called total intrinsic viscosity (equation below):
P ðgsp Þi DV ½g ¼ P c i DV
ð3Þ
The same value [g] is obtained when the intrinsic viscosity is measured without chromatographic separation i.e. using Ubbelohde viscometer [3,13]. The values of the total intrinsic viscosities of the stars were compared with those of the linear PTBA (Fig. 11). The value of the parameter a from the Kuhn–Mark– Houwink–Sakurada equation for the linear poly(tert-butyl acrylate) was measured in THF at 35 °C in the GPC system and equals 0.78 [24]. This value is similar to that obtained by Mrkvickova and Danhelka [29] for the linear poly(tertbutyl acrylates) in THF at 25 °C (a = 0.8). The results clearly show that viscosities are influenced by the differences in the investigated topologies. The solutions of the star samples exhibit considerably lower values of intrinsic viscosity than their linear counterparts. The total intrinsic viscosity decreases with increasing number of
arms. The viscosity of the three-arm stars does not differ from the values of the linear PTBA, which suggests that these stars behave in solution similarly to the linear chains. As can be seen in Fig. 11, the intrinsic viscosity of stars increases with molar mass. The a parameter in the Kuhn– Mark–Houwink–Sakurada equation calculated for stars based upon the linear fit of data shown in Fig. 11 is close to values obtained for the linear PTBA and varies from 0.76 to 0.79. Roovers observed a similar relation between the linear and star structures for polybutadiene [3] and PEO stars [8]. The investigation of the properties of the branched polymers in dilute solutions allows the determination of the branching ratio for the star macromolecules. These parameters were reported previously for different stars [2,3,8,13]. One of them is the branching ratio g0 defined by Zimm and Stockmayer [30] as the ratio of the intrinsic viscosities of branched to linear polymer with the same molar mass:
g0 ¼
½gbranched ½glinear at the same M
ð4Þ
The branching ratio g0 from Eq. (4) calculated for all PTBA stars from the data shown in Fig. 11 vary from 0.97 to 0.24 and are presented in Fig. 12. The values of g0 ratio decreases with increasing number of arms. But for the PTBA stars of the same functionality and the range of measured molar masses the values of factor g0 are nearly constant. The average values of g0 for stars with different functionality are given in Fig. 13. These results are compared with those obtained by Angot [13] for PS stars and with the data collected by Grest [2] for polybutadiene stars in
B. Mendrek, B. Trzebicka / European Polymer Journal 45 (2009) 1979–1993
Fig. 6. Molar masses of three-, four- and six-arm stars as a function of conversion.
Fig. 7. Kinetic plot for the polymerization of tert-butyl acrylate using Br12CXA (j) and Br16CXA (d) as initiators.
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good solvent. The value of g0 for PTBA stars with 28 arms obtained previously in our laboratory [24] is also added. Values of g0 were calculated using empirical Eq. (5) [31] valid for good solvent and lightly branched structures. In Fig. 13 this dependence of g0 as a function of the number of arms f is shown as a dashed line
g 0empirical ¼
0:58 3f 2 0:724 0:015ðf 1Þ f2 0:724
ð5Þ
As expected, the values of g0 decrease with increasing number of arms. However, when the number of arms reached 16 the value of g0 did not change significantly with the increase of the number of arms. Experimental results fit well to the empirical function up to stars with about 18 arms. Above this a difference between the empirical and experimental value is observed. Fig. 8. Chromatograms (RI traces) of the 12-arm star polymers.
Table 2 Polymerization of tert-butyl acrylate using 12- and 16-functional initiators. Conversion [%]
Mtheor
MnGPCa [g/mol]
Mncalib [g/mol]
Mw/Mn
Br12CXA
18.5 27 34.5 38.6 41 55.7
15,600 21,500 26,700 29,500 31,200 41,300
16,700 22,200 27,300 28,000 28,700d 39,200d
9600 14,600 19,200 21,600 21,900 26,400
1.05 1.07 1.04 1.08 1.08 1.06
Br16CXAc
17 24 28 31 35 39
19,600 26,400 30,300 33,200 37,200 41,000
21,200 27,000 28,900 33,500 35,000d 38,000d
8500 12,000 13,400 15,400 15,200 17,700
1.06 1.03 1.03 1.02 1.02 1.03
Initiator b
a b c d
Values obtained using measured dn/dc of 12 and 16-arm stars equal 0.0553 g/mL. Molar ratio of [Br12CXA]:[Cu]:[PMDTA] = 1:6:6. Molar ratio of [Br16CXA]:[Cu]:[PMDTA] = 1:7:7. Additional peak in the low molar mass region.
Fig. 9. Molar masses of 12- and 16-arm stars as a function of conversion.
B. Mendrek, B. Trzebicka / European Polymer Journal 45 (2009) 1979–1993
1991
Scheme 4. The selective alkaline hydrolysis of ester bonds between arms and the core of a 16-arm star.
of the growing radicals is observed. The theoretical molar masses calculated from the monomer consumption are in good agreement with the absolute molar masses obtained from GPC with light scattering detection. The intrinsic viscosity of the PTBA stars decrease with the number of arms when the molar mass is kept constant. The values of the branching ratio g0 is in good agreement with the theoretical values. The solution behavior reveals the compactness of the structure of the synthesized polymers
Fig. 10. Chromatograms of the 16-arm star polymer before hydrolysis (a) and after hydrolysis (b).
4. Conclusions ATRP initiated with bromoester derivatives of multifunctional alcohols and calix[4]arenes is reasonably well controlled and leads to well-defined stars with the number of PTBA arms varying from 3 to 16. When conversion is kept below a certain limit, no star coupling by combination
Table 3 The actual functionality of PTBA stars. Mn [g/mol]
Mw/Mn
f
Star
Arm
Star
Arm
52,100 91,800
4200 5700
1.06 1.05
1.1 1.35
theor
12 16
f
calcd from
11.7 15.6
(1)
Fig. 11. Log–log plot of intrinsic viscosities versus the weight average molar mass for the star PTBA and the linear PTBA.
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Fig. 12. The g0 values as a function of the molar masses of the PTBA star polymers.
Fig. 13. Values of g0 for different star polymers compared with the empirical function.
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