( Author for correspondence; Tel.: 886-35131512; Fax: 886-35131512; E-mail: [email protected]) Received 19 May 2004; accepted in revised form 1 November 2004
Key words: atom transfer radical polymerization (ATRP), block copolymers, polystyrene
Abstract
We aimed at the synthesis of well-define PS-b-P4VP by using atom transfer radical polymerization in two-step process. First, polystyrenes with benzyl bromide end group (PS-Br; by ATRP) were prepared as macroinitiator for the next ATRP of 4-vinyl pyridine and characterized these polymers from1H-NMR and MALDI-TOF. Comparing with MALDI-TOF-MS,1H-NMR and GPC analyses, this indicates that the formation of the block copolymer can be observed. During the polymerizations, molecular weight distribution and kinetics have been evaluated from GPC traces and1H-NMR analyses. We further characterized the thermal properties of these block polymers by DSC and TGA. DSC measurement on the PS-b-P4VP block copolymers exhibited two glass transitions, indicating that the resulting block copolymers are phase separated. Two maxima differential peaks were observed on the TGA trace for the PS-b-P4VP block copolymers might be assigned to the decomposition of the P4VP blocks at 380◦C and the PS blocks at higher temperature.
Introduction
The desire to control polymer properties through the synthe-sis of block copolymers and complex macromolecular archi-tectures is a continuing theme throughout polymer chemistry [1, 2]. Block copolymers are remarkable self-assembling systems that can assume a wide variety of morphologies including lamellar, hexagonal-packed cylindrical, and body-centered cubic micellar structures, depending on the relative volume fractions of the blocks [3, 4]. This clear picture of the morphology as a function of composition has primar-ily emerged from the investigation of diblock copolymers. The block copolymers with well-defined structures, such as molecular weight (MW) and molecular weight distribution (MWD), composition, architecture and end group function-ality, are very important, and this has been carried out by the following three methods [5]: (1) sequential monomer addition, (2) coupling reaction of “living” polymer chains, and (3) mechanism transformation. The development of ionic polymerization methods allowed for the preparation of well-defined polymers with controlled chain end func-tionalities and the synthesis of well-defined block and graft copolymers [6–9]. However, these polymerizations have to be carried out with nearly complete exclusion of moisture and air, and often at very low temperature. Moreover, only a few numbers of monomers can be polymerized, and the presence of functional monomers can cause undesired side reactions. Recently, Matyjaszewski has reported that atom transfer radical polymerization (ATRP) can be used to syn-thesize polymers with narrow molecular weight distribution (MWD) [10], well-defined block copolymers [11, 12], and
star polymers [13, 14]. The ATRP process uses an alkyl halide as initiator and a metal in its lower oxidation state with complexing ligands [15–21]. The process involves the successive transfer of the halide from the dormant poly-mer chain to the ligated metal complex, thus establishing a dynamic equilibrium between active and dormant species (Scheme 1). This controlled radical polymerization allows for the polymerization of a wide range of monomers such as styrenes [22, 23], acrylates [24, 25], methacrylates [26, 27], and various functional monomers.
It has been demonstrated that a block copolymer of P4VP has the ability to form self-assembly supramolecular structure [28], high complexibility with metal ion [29, 30], and electrical conducting property [31]. Polymerization of 4VP posses a very challenging problem for ATRP because both 4VP and P4VP are strong coordinating ligands that can compete for the binding the metal catalysts in these systems. Since the monomer is normally present in large excess over the employed ligand, there is a possibility of the formation of pyridine-coordinated metal ion complexes in the polymerization solution. Pyridine-coordinated copper complexes are not effective catalysts for ATRP. For ex-ample, addition of 5 vol% pyridine to the polymerization solution of styrene catalyzed by CuBr complexed by 4,4 -di(5-nonyl); 2,2-bipyridine (dNbpy) significantly slowed down the polymerization rate [21].
In this paper, we aimed at the synthesis of well-define PS-b-P4VP by using atom transfer radical polymerization in two-step process by using commercial available ligands. First, polystyrenes with benzyl bromide end group (PS-Br; by ATRP) is prepared as macroinitiator for the next ATRP
Scheme 1. Dynamic equilibrium that exists between the propagating and dormant species when a metal complex is used as the reversible halogen atom
transfer reagent.
of the 4-vinyl pyridine and characterized these polymers by1H-NMR and MALDI-TOF. During the polymerizations, molecular weight distribution and kinetics have been eval-uated by GPC traces and 1H-NMR analyses. We further characterize the thermal properties of these block polymers by DSC and TGA.
Experimental Materials
Styrene (S) and 4-vinylpyridine (4-VP) were distilled from calcium hydride before use. Copper(I) bromide (CuBr) was stirred in glacial acetic acid overnight, filtered, and then rinsed with absolute ethanol under a blanket of ar-gon and dried under vacuum at 80◦C for three days. All solvents were distilled prior to use. N,N,N,N,N -pentamethyldiethylenetriamine (PMDETA), 2,2-bipyridine (Bipy), 4,4-dinonyl-2,2-dipyridyl (dNBipy) were used as received. N,N-dimethylformamide (DMF) and toluene were distilled from magnesium sulfate and sodium/benzophenone immediately before use. All chemicals were purchased from Aldrich Chemical Co. (Milwaukee,WI).
Preparation of PS-Br Macroinitiator by the ATRP of Styrene
A typical polymerization is as follows: CuBr (0.1 mmol) was placed into a dry 25-mL round-bottom flask equipped with a stirring bar. Toluene (10 mL), styrene (30 mmol) and PMDETA (0.1 mmol) were added sequentially and the solution was stirred for 20 min to form the Cu com-plex. The initiator, 1-phenylethyl bromide, (0.1 mmol) was then added. This whole process was carried out in a nitrogen-filled dry box. The mixture was degassed with three freeze-thaw cycles. Polymerization was carried out at an ap-propriate temperature in an oil bath. The reaction mixture turned dark green immediately and became progressively more viscous. Upon completion of the reaction, the mixture was diluted five-fold with tetrahydrofuran (THF) and stirred with of Amberlite IR-120 (H form) cation-exchange resin (3–5 g) for 30–60 min to remove the catalyst. The mixture was then passed through a neutral alumina column and pre-cipitated into ten-fold of methanol. The resulting polymers were filtered and dried overnight at 60◦C under vacuum.
Preparation of PS-b-P4VP by the ATRP of 4-VP with PS-Br Macroinitiator
A typical procedure for the synthesis of PS-b-P4VP was as follows. Prior to the sequential polymerization, the PS-Br macroinitiator was dried overnight in a vacuum oven at 50◦C. In a flame-dried, two-necked flask, CuX (0.1 mmol) was placed into a dry 25-mL round-bottom flask equipped with a stirring bar. 4-VP (20 mmol), DMF(4 M) and a desired amount of ligand were added sequentially and the solution was stirred for 20 min to form the Cu complex. The macroinitiator (0.1 mmol) was then added. This whole process was performed in a nitrogen-filled dry box. The mix-ture was degassed with three freeze-thaw cycles. An aliquot of the solution (ca. 0.1 mL) was removed and then polymer-ization was carried out at an appropriate temperature in an oil bath. The reaction mixture turned dark green immedi-ately and became progressively more viscous. Periodically, aliquots (0.1 mL) were removed for analysis. Exotherms of 2–4◦C were typically observed, indicating that polymeriza-tion occurred. Upon complepolymeriza-tion of the reacpolymeriza-tion, the mixture was diluted five-fold with DMF and stirred with of Am-berlite IR-120 (H form) cation-exchange resin (3–5 g) for 30–60 min to remove the catalyst. The mixture was then passed through an alumina column and precipitated into ten-fold of 10% H2O/methanol. This purification protocol
resulted in the loss of up to 5∼10% of the polymer as a result of adsorption. The resulting polymers were redissolve in DMF and precipitated into ten-fold of ether. The resulting polymers were filtered and dried overnight at 60◦C under vacuum. These procedures were repeated twice to obtain the pure block copolymer.
Characterization
Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) spectra was per-formed on a PerSeptive Biosystems Voyager DE-STR equipped with 2-m linear and 3-m reflector flight tubes and a 337-nm nitrogen laser (pulse width, 3 ns), along with a delayed extraction capability. All experiments were carried out at an accelerating potential of 20 kV in both linear and re-flector modes. In general, mass spectra from 256 laser shots were accumulated summed to produce a final spectrum. An-giotensin I (human; Mw= 1296.5) (BACHEM) and insulin
PS solution, 90 µL of the matrix solution, and 30 µL of the cationizing agent were mixed in a glass vial. The weight ratio of polymer/matrix/cationizing agent was thus 1/9/1. Then 0.5 µL portions of the mixture were deposited onto 10–20 wells of the gold-coated sample plate and dried in air at room temperature. NMR spectra were recorded on a Brucker AM500 Spectrometer and were measured in DMSO-d6. Molecular weights and molecular weight
distri-butions were determined by gel permeation chromatography (GPC) using a Waters 510 HPLC – equipped with a 410 Differential Refractometer Index (RI) and a UV detector in series, and three Ultrastyragel columns (100, 500, and 103Å) connected in series in order of increasing pore size – using DMF as an eluent at a flow rate of 1.0 mL/min. The molecular weight calibration curve was obtained using polystyrene standards. Thermal analysis was carried out on a DSC instrument from DuPont (model 910 DSC-9000 con-troller) with a scan rate of 20◦C/min and temperature range of 20–200◦C in nitrogen atmosphere. Approximately 5– 10 mg sample was weighted and sealed in an aluminum pan. The samples were quickly cooled to room temperature from the first scan and then scanned between 30 and 280◦C at a scan rate of 20◦C/min. The glass transition temperature is taken as the midpoint of the heat capacity transition between the upper and lower points of deviation from the extrapolated glass and liquid lines. FTIR spectroscopy measurements were made from a NaCl disk using a Nicolet Avatar 320 FT-IR Spectrometer, with 32 scans collected at a resolu-tion of 1 cm−1. A DMF solution containing the sample was cast onto a NaCl disk and dried under conditions similar to those used in the bulk preparation. The sample chamber was purged with nitrogen to maintain the film’s dryness. Thermo-gravimetric analysis (TGA) experiments were performed by using a DuPont TGA-51 thermogravimetric instrument. The temperature was increased from 30 to 800◦C at a heating rate of 20◦C/min under nitrogen atmosphere. The degra-dation temperature was defined as the temperature at the maximum of the differential thermogravimetric curve.
Results and Discussion
The synthetic approach for the PS-b-P4VP block copoly-mers is depicted in Scheme 2. Initially, it was decided to grow a well-defined polystyrene chain from the phenylethyl bromide initiator and resulted chain end functionalized PS acted as a polymeric initiator for the next controlled poly-merization of 4-vinylpyridine. Figure 1 shows MALDI-TOF-MS and GPC curves of PS macroinitiators obtained from the different polymerization time interval. It can be
8000) to further polymerize with 4-vinylpyridine. The struc-ture of the block copolymer was characterized by1H-NMR spectroscopy. Figure 2 illustrates the NMR spectra of PS-Br, PS-b-P4VP, and P4VP polymers. The spectrum of the block copolymer shows signal superposition of PS segments with attached segments of 4-vinyl pyridine. Comparing with MALDI-TOF-MS and GPC analyses, the formation of the PS-b-P4VP block copolymer can be identified.
In a typical ATRP, the concentration of the active species remains unchanged throughout the reaction that can be veri-fied by a linear semilogarithmic plot of monomer conversion vs. time as shown in Figure 3 where conversion is calculated from1H-NMR of the feeding monomer initiated from PS-Br macroinitiator. If a stronger binding ligand was used, such as the tridentate PMDETA, a faster polymerization rate was ob-served. Under similar conditions, a PMDETA to CuBr ratio of 6 : 1 was needed to maintain a relatively fast polymer-ization rate to obtain a monomer conversion of 50% after 4 h. In contrast, at a PMDETA to CuBr ratio of 1 : 1, the polymerization was slower as shown in Figures 3(a) and 3(b). This result is quite similar with the earlier results on the polymerization of 4VP [31]. It is the evidence for the competitive coordination of 4VP monomer to copper. When polymerization of 4VP was carried out using macroinitiator and CuBr complexed by dNBipy as the catalyst in a ratio of 1 : 1 [Figure 3(d)], a very slow polymerization rate was ob-served. This is due to the same pyridine unit that functions as ligand and monomer, and hence, the dynamics equilibrium of the halogen atom transfer process will be disturbed. This undesired interaction will influence the polymerization rate and the molecular weight distribution during ATRP process. Even at a dNBipy to CuBr ratio of 6 : 1 as shown in Fig-ure 3(c), it still showed a slower polymerization rate than that of PMDETA to CuBr ratio of 1 : 1. Various ligands, contents, reaction time, molecular masses (Mw), yield and
PDI are summarized in Table 1. Overall, PMDETA sys-tem is the most efficient ligand for the polymerization of 4-vinylpyridine.
Figure 4 shows the dependence of the molecular weight of the block copolymers versus monomer conversion ini-tiated by macroinitiator. The drawn line represents the theoretical molecular weight, Mn(th), calculated from:
Mn(th)=
[M]
[I]0 × Mw(mon)+ Mw(init)
. (1)
We observe a near-linear increase in the measured num-ber average molecular weight (Mn) vs. monomer conversion
up to∼75% and the evolution of molecular weight agrees with the theoretical value, indicating that a living/controlled polymerization process proceeds in solution.
Scheme 2. Reaction scheme for the syntheses of block copolymers.
The polymerization of 4-vinylpyridine initiated by PS macroinitiator was examined with and without halogen ex-change. Figure 5 shows the kinetics plots of semilogarithmic of monomer conversion vs. time. The rate of polymerization without halogen exchange was faster than that with halogen exchange. The rate of polymerization in ATRP depends on the concentration of propagating radicals and is function of both the initiation efficiency and the concentration of de-activator in the system. For PS-Br/CuCl system, therefore, lower concentration of propagating radicals is obtained. The molecular weight also displayed a near-linear dependence on conversion (Figure 6). The molecular weight and poly-dispersity for the PS-b-P4VP block copolymers prepared with halogen exchange were lower than those without halo-gen exchange. The molecular weight distribution yielded by the PS-Br/CuX initiation system is worth to mention. The Cu(II)Br bond is relatively weaker than that of Cu(II)Cl, re-sulting in faster deactivation of the propagating radical and
lower polydispersity. This result is in accordance with the earlier results on the inhibition of radical polymerization of MMA by Cu(II)Br2and Cu(II)Cl2and trapping of alkyl
rad-icals [33, 34]. Thus, for chain extension from PS to 4VP, introduction of halogen exchange technique should yield a more precisely controlled block copolymer than that without the halogen exchange.
Thermal behavior of block copolymers was examined by differential scanning calorimetry (DSC) in the range of 0 to 200◦C. The temperature at the midpoint of the baseline shift was defined as the bulk glass transition temperature,
Tg. As shown in Figure 7, DSC measurement on the
PS-b-P4VP block copolymers exhibited two glass transitions, indicating that the resulting block copolymers are phase sep-arated. Typical Tg’s of PS, and P4VP are reported to be 100,
and 150◦C. Obviously, the two glass transition temperatures are quite similar to those of respective homopolymers. This result indicates that the block copolymer is in a
microphase-Figure 1. Comparing MALDI-TOF-MS and GPC of PS macroinitiators with the progress of polymerization: (a) 4h; (b) 8h; (c) 12h; (d) 16h.
Table 1. Block copolymerization conditionsafrom PSbto 4VP at 80◦C by ATRP Sample Ligandc I : C : Ld Time (h) Yield (%) Mw(GPC)e PDI
1 PMDETA 1 : 1 : 1 5 30 15300 1.33 2 PMDETA 1 : 1 : 1 9 48 18500 1.30 3 PMDETA 1 : 1 : 6 4 53 20300 1.32 4 PMDETA 1 : 1 : 6 9 74 23700 1.30 5 dNBipy 1 : 1 : 1 5 6 9400 1.43 6 dNBipy 1 : 1 : 1 9 11 10800 1.42 7 dNBipy 1 : 1 : 6 4 17 12000 1.43 8 dNBipy 1 : 1 : 6 9 48 17000 1.41 9 Bipy 1 : 1 : 6 24 8 10200 1.37
aMonomer: 4vinylpyridine (4VP); solvent: N,N-dimethylformamide (DMF); mole ratio of monomer/macro-initiator: 200 : 1.
bPS macroinitiator: M
n= 8000, Mw= 9600, PDI = 1.20.
c Ligand: PMDETA = N,N,N,N,N-pentamethyldiethylenetriamine; dNBipy =
4,4-dinonyl-2,2-dipyridy; Bipy= 2,2-bipyridine.
dMole ratio of initiator to copper bromide to ligand. ePolydispersity index from GPC traces.
Figure 3. Semilogarithmic kinetic plot for the ATRP of 4VP with various
amount of ligand at 80◦C [conditions: monomer (4 M), solvent: DMF, CuBr catalyst, PS macroinitiator Mn= 8000].
Figure 4. The dependence of the molecular weights and polydispersity
of the block copolymers on monomer conversion under different reaction conditions. The linear line represents the theoretical Mnvs conversion.
separated state. A large repulsion between the two different segments exists for the PS-b-P4VP system.
Thermal stability of block copolymers was examined by thermogravimetric analyzer (TGA) in the range of 30 to
Figure 5. Semilogarithmic kinetic plot for the ATRP of 4VP with various
amount of ligand at 80◦C by (•) with or () without halogen exchange [conditions: monomer (4 M); ligand: PMDETA; solvent: DMF, CuBr or CuCl catalyst, PS macroinitiator Mn= 8000].
Figure 6. The dependence of the molecular weights and polydispersity of
PS-b-P4VP copolymers on the monomer conversion with (•) and with-out () halogen exchange under different reaction conditions. [Conditions: same as in Figure 5.]
Figure 7. DSC thermograms of PS and PS-b-P4VP block copolymer.
Figure 8. TGA thermograms of PS, P4VP and PS-b-P4VP block
copoly-mer under N2atmosphere.
800◦C. As shown in Figure 8(c), TGA analysis of the block copolymer (sample 3, Table 1) shows a two-step thermal decomposition. In Figure 8(d), which was obtained by dif-ferentiating curve (c), we can observe two maximum points at 380 and 431◦C, respectively. The parent homopolymers have typically decomposed maximum points at 430◦C for PS (Mn = 8000) and 400◦C for P4VP (Mn = 13000)
under nitrogen atmosphere, respectively. Therefore, the two maxima observed on the TGA trace for the PS-b-P4VP block copolymer can be assigned to the decomposition of the P4VP blocks at 380◦C and the PS blocks at higher temperature.
Conclusion
The formation of block copolymers of styrene and 4-vinyl-pyridine was investigated by using ATRP. We used of ATRP with commercial available ligands to syntheses well con-trolled block copolymers from styrene and 4-vinylpyridine monomers. MALDI-TOF-MS,1H-NMR and GPC analyses verify that the successful synthesis of the PS-b-P4VP block copolymers. The kinetic study shows slower polymerization rate when using the CuBr/dNBipy catalyst system because the same pyridine unit functions as ligand and monomer. On
are found on the TGA trace for the PS-b-P4VP block copoly-mer as the result of the decomposition of the P4VP blocks at 380◦C and the PS blocks at higher temperature.
Acknowledgements
This research was financially supported by the National Sci-ence Council, Taiwan, Republic of China, under Contract Nos. NSC-90-2216-E-009-026.
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