Article pubs.acs.org/Macromolecules
Novel Hydroxyl-Functionalized Caprolactone Poly(meth)acrylates Decorated with tert-Butyl Groups Dorota Neugebauer,*,† Katarzyna Bury,† and Tadeusz Biela‡ †
Department of Physical Chemistry and Technology of Polymers, Faculty of Chemistry, Silesian University of Technology, M. Strzody 9, 44-100 Gliwice, Poland ‡ Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Łódź, Poland ABSTRACT: A series of new well-defined copolymers containing the hydroxyl-functionalized monomer, caprolactone 2-(methacryloyloxy)ethyl ester (CLMA), and tert-butyl acrylate/methacrylate (tBuA/tBuMA) were synthesized via atom transfer radical polymerization (ATRP). The composition of copolymer strongly depended on the selection of comonomer pair. The relative reactivity ratios of comonomers determined by the Jaacks method indicated significantly higher reactivity of CLMA in methacrylate/acrylate system (rCLMA = 2.53, rtBuA = 0.40), whereas tert-butyl monomer was faster polymerized in case of two methacrylates (rCLMA = 0.61, rtBuMA = 1.63). These results were compared with that obtained for the CLMA/MMA system, which was closer to CLMA/tBuMA, giving rCLMA = 0.83 and rMMA = 1.20. The copolymer composition influenced the glass-transition temperatures (Tgs), which decreased with an increase of CLMA content.
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methacrylate,19,21−24 but in these cases deprotection of hydroxyl groups is not available. The great of interest in the carboxyl- or hydroxyl-functionalized polymers is related to the polyelectrolyte character with sensitivity for pH and ionic strength in aqueous solutions or amphiphilic character leading to the formation of micelles with core/shell structure.25,26 Moreover, the presence of reactive −COOH or −OH groups that can be further modified in simple reactions (for example, esterification) gives a wider range of their future applications. Our present studies are focused on another hydroxylfunctionalized monomer poorly described in the literature, that is, 6-hydroxyhexanoic acid 2-(2-methacryloyloxy)ethyl ester commercially known as caprolactone 2(methacryloyloxy)ethyl ester (CLMA). Herein we report ATR copolymerization of CLMA with tert-butyl acrylate (tBuA) or tert-butyl methacrylate (tBuMA). Various catalytic systems and initial feeds of comonomers were used to achieve the hydroxyl-functionalized copolymers with narrow molecular weight distributions, proper polymerization degrees, and compositions. Two different monomer pairs were selected to prepare different polymeric structures, which were confirmed by the kinetics of both reaction systems and the determination of the relative reactivity ratios of comonomers by the Jaacks method. Additionally, the properties of P(CLMA-co-tBuA) and
INTRODUCTION The atom transfer radical polymerization (ATRP) as one of the controlled radical methods, including nitroxide-mediated and reversible addition−fragmentation chain transfer polymerizations, gives the ability to (co)polymerize a variety of monomers including (meth)acrylates, styrenics, and vinyl monomers yielding macromolecules with various architectures.1−4 Unfortunately, in the case of acidic monomers, like (meth)acrylic acid, the protection mostly by the tert-butyl group, but also 1-ethoxyethyl,5 as well as trimethylsilyl, benzyl, 2-tetrahydropyranyl, and p-nitrophenyl, is highly recommended.6 However, the controlled ATRP can be applied for functional monomers with unprotected hydroxyl groups, like 2hydroxyethyl (meth)acrylate (HEMA7−12/HEA13,14), where the presence of labile hydrogen atom makes limitation for polymerization via anionic mechanism, which is not tolerant for −OH, −COOH, and −NH2 functional groups. The ATRP examples of poly(ethylene glycol) methacrylate are also known.15,16 Direct polymerization of functional monomers enable the elimination of additional deprotection steps but can cause an increase of the molecular weight distribution, which was indicated for PHEMA (Mw/Mn ≤ 1.5) in comparison to that with masked hydroxyl groups (Mw/Mn = 1.1).7 The large group of the well-defined PHEMA/PHEA polymers was obtained by removing trimethylsilyl groups in P(HEMA− TMS),7,17−19 and P(HEA−TMS),14 or more stable tertbutyldimethylsilyl groups. 20 The literature also reports examples of ATRP of poly(ethylene glycol) methyl ether © 2012 American Chemical Society
Received: March 24, 2012 Revised: May 29, 2012 Published: June 12, 2012 4989
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increments of copolymers were calculated from the weight composition and dn/dc for PCLMA, PtBA, PtBMA, and PMMA, which were measured in CH2Cl2, and they reached values 0.060, 0.035, 0.044, and 0.058 mL/g, respectively. Proton nuclear magnetic resonance (1H NMR) spectra were recorded with UNITY/INOVA (Varian) spectrometer operating at 300 MHz using CDCl3 as a solvent and tetramethylsilane as an internal standard. Fourier transform infrared (FT-IR) analysis was conducted with BIORAD FTS 175 L spectrophotometer at room temperature using KBr tablets. Differential scanning calorimetry (DSC) was performed using METTLER-TOLEDO (DSC822e) apparatus for a temperature range from −60 to 150 °C, at a heating rate of 10 °C/min.
P(CLMA-co-tBuMA) were compared with earlier investigated P(CLMA-co-MMA) copolymers.27 The prepared copolymers could be transformed via esterification of hydroxyl groups with α-bromoisobutyryl bromide into ATRP macroinitiators for synthesis of graft copolymers as it was successfully performed by authors for P(CLMA-co-MMA), while the hydrolysis of tertbutyl units will be able to generate amphiphilic properties. These advantages make the CLMA copolymers as a promising candidates for biomedical applications in drug delivery systems. Previously, the literature reported CLMA based hydrogels with biocompatible and biodegradable properties, which were obtained by free radical copolymerization with HEA in the presence of a cross-linking agent and applied as homogeneneous random copolymer networks in tissue engineering.28,29
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RESULTS AND DISCUSSION The hydroxyl-terminated CLMA was copolymerized in a series of ATRP reactions with tert-butyl monomers tBuA and tBuMA using various initial feed of CLMA (5−50 mol %), which yielded copolymers with different compositions and different microstructures (Scheme 1).
EXPERIMENTAL PART
Materials. Caprolactone 2-(methacryloyloxy)ethyl ester (CLMA, Aldrich), tert-butyl methacrylate (tBuMA, Aldrich, 98%), tert-butyl acrylate (tBuA, Alfa Aesar, 99%), and methyl methacrylate (MMA, Aldrich, 99%) were dried over molecular sieves and stored in a freezer under nitrogen. Copper(I) bromide (CuBr, Fluka, 98%) and copper(I) chloride (CuCl, Fluka, 97%) were purified by stirring in glacial acetic acid followed by filtration and washing with ethanol and diethyl ether. After that, the solids were dried under vacuum. 4,4′-Dinonyl-2,2′dipyridyl (dNdpy, Aldrich, 97%), 1,1,4,7,10,10-hexamethyltriethylenetetraamine (HMTETA, Aldrich, 97%), N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA, Aldrich, 99%), ethyl 2-bromoisobutyrate (EtBriBu, Aldrich, 98%), and α-bromoisobutyryl bromide (BriBuBr, Aldrich, 98%) were used as received. All solvents were applied without purification. Polymer Synthesis: P(CLMA-co-tBuA) (Example for IV). The comonomers CLMA (1.5 mL, 6.83 mmol) and tBuA (3 mL, 20.48 mmol), anisole (1.5 mL), PMDETA (15 μL, 0.07 mmol), and catalyst CuCl (6.9 mg, 0.07 mmol) were placed in Schlenk flask and degassed by three freeze−pump−thaw cycles. The reaction flask was immersed in an oil bath at 60 °C. After 1 min, an initiator EtBriBu (10 μL, 0.07 mmol) was introduced to the reaction mixture and an initial sample was taken. During the reaction, samples were taken periodically to perform GPC and NMR analysis. The reaction was stopped by exposing the reaction mixture to air. Then it was dissolved in chloroform (CHCl3), passed through neutral alumina column to remove copper catalyst, and concentrated by a rotary evaporator. The copolymerizations of CLMA with tBuA, tBuMA, or MMA with various initial amounts of comonomers in the reaction mixture (M1:M2 = 5:95, 25:75, and 50:50 mol %; M1 = CLMA, M2 = tBuA, tBuMA, or MMA) were conducted according to the above procedure. Depending on the type of comonomer and the composition of copolymer, the viscous solution after evaporation was purified by precipitation in methanol or n-heptane (in the case of tBuMA copolymers with low content of CLMA) or it was dissolved in methanol and dialyzed against methanol for 24 h under gentle stirring (in the case of tBuMA with higher content of CLMA and all tBuA copolymers). Dialysis was performed using membrane tubing (Roth; MW cutoff, 14.0 kDa) from regenerated cellulose, while fresh methanol was replaced after 1 and 4 h. After purification copolymers were dried under vacuum at room temperature to a constant mass. Characterization. Molecular weights and dispersities were determined by gel permeation chromatography (GPC) equipped with an 1100 Agilent isocratic pump, autosampler, degasser, thermostatic box for columns, a photometer MALLS DAWN EOS (Wyatt Technology Corp., Santa Barbara, CA), and differential refractometer Optilab Rex. ASTRA 4.90.07 software (Wyatt Technology Corp.), which was used for data collecting and processing. Two PLGel 5 μm MIXD-C columns were used for separation. The calibration of the DAWN EOS was carried out by p.a. grade toluene and normalization with a polystyrene standard of 30 000 g/mol. The measurements were carried out in methylene chloride as the solvent at room temperature with a flow rate of 0.8 mL/min. The refractive index
Scheme 1. ATRP of CLMA and tBuA, tBuMA, or MMA
Various catalytic systems were applied to adjust activation/ deactivation rate of ATRP and prepare the proper polymers; i.e., copolymerizations of CLMA with tBuMA were catalyzed by CuCl complexed with dNdpy or other ligands (HMTETA and PMDETA), while the reactions with tBuA were carried out in the presence of both bromide and chloride complexes (CuBr/ PMDETA, CuCl/PMDETA, or CuCl/dNdpy). The reactions were performed at monomer to EtBriBu initiator ratio ([M]0: [I]0) equal to 300:1, 400:1, 600:1, or 800:1, at various temperatures (from rt to 70 °C). The detailed polymerization conditions are described in Table 1. The NMR analysis enabled determination of the conversion of both monomers (x), the degree of polymerization (DP), and the number-average molecular weight of polymer (Mn,NMR) as well as their content in the copolymer chain (F). The results for copolymerization of CLMA with tBuA are collected in Table 2, while with tBuMA in Table 3. The 1H NMR spectra of P(CLMA-co-tBuA) (Figure 1a) and P(CLMA-co-tBuMA) (Figure 1b) are presented for representative samples taken from the reaction mixtures, which means that the signals characteristic for resulted copolymer, unreacted monomers, and solvent can be identified. The CLMA conversion was determined using signals characteristic for vinyl protons CH2C(CH3)− at δ = 6.14 and 5.59 ppm (b) in CLMA 4990
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Table 1. ATRP Conditions for Synthesis of CLMA Copolymers
a
no.
M1:M2 (mol %)a
CuX/L
M/I/CuX/Lb
I II III IV V VI VII VIII−IX X XI XII XIII XIV
5:95 15:85
anisole (vol %)
temp (°C)
tBuA tBuA
CuBr/PMDETA CuBr/PMDETA
25:75 50:50
tBuA tBuA
5:95 25:75
tBuMA tBuMA
50:50
tBuMA
5:95 25:75
MMA MMA
CuCl/PMDETA CuBr/PMDETA CuCl/dNdpy CuCl/PMDETA CuCl/PMDETA CuCl/dNdpy CuCl/PMDETA CuCl/HMTETA CuCl/dNdpy CuCl/dNdpy
300/1/1/1 400/1/1/1 400/1/2/2 400/1/1/1 600/1/1/1 800/1/0.75/1.5 400/1/1/1 600/1/1/1 800/1/0.75/1.5 800/1/1/1 800/1/0.75/0.75 800/1/0.75/1.5 800/1/0.75/1.5
10 15 15 25 25 35 25 25 35 35 70c 35 35
70 70 70 60 60 70 60 60 rt 60 rt 70 70
M2
M1 = CLMA. bI = ethyl 2-bromoisobutyrate (EtBriBu). cAnisole:MeOH = 75:25.
Table 2. ATR Copolymerization of CLMA and tBuA 1
H NMR
no. I II III IVA IVB IVC IVD IVE VA VB VI
f CLMA [mol %] (wt %)
time [h]
x [%]
DP
DPCLMA
FCLMA [mol %]
5 (9) 15 (25)
24 24 24 1 3 4 8 11 1.5 2.2 24
29 21 28 13 17 21 26 28 24 36 17
86 82 113 51 67 82 104 111 145 219 136
9 16 28 22 28 31 41 44 104 132 99
11 19 25 43 42 38 39 40 72 60 73
25 (39)
50 (66)
GPC-MALLSa
GPC −3
Mn,th × 10 [g/mol] 11.8 12.4 17.7 9.3 12.1 14.4 18.3 19.5 30.6 43.3 28.9
−3
Mn × 10 [g/mol]
Mw/Mn
8.6 4.6 7.4 7.9 10.4 11.8 15.6 18.8 25.3 cross-linking 27.7
1.24 1.62 1.59 1.41 1.53 1.53 1.53 1.60 1.61 1.78
DSC
Mn × 10−3 [g/mol]
Mw/Mn
11.7 15.6 22.8 11.4 13.3 16.2 22.7 35.1 67.7 − 95.6
1.18 1.13 1.14 1.31 1.38 1.60 1.49 1.43 2.08 − 1.94
Tg [°C] 26 11
5
The refractive index increments of copolymers were calculated from the composition dn/dc = FCLMA × 0.060 + FtBuA × 0.035, which means it was in the range 0.060−0.035 mL/g.
a
Table 3. ATR Copolymerization of CLMA and tBuMA or MMA 1
H NMR
no. VII VIIIA VIIIB IXA IXB IXC IXD X XI XII XIII XIVb
f CLMA [mol %] (wt %)
time [h]
xM [%]
DP
DPCLMA
FCLMA [mol %]
5 (8) 25 (36)
4.5 0.25 0.75 0.25 0.75 1.25 2.75 24 0.75 24 24 28
51 20 42 19 38 45 56 8 41 26 52 38
202 119 249 113 225 267 332 63 324 206 415 304
6 31 50 22 42 51 64 5 124 74 25 62
3 26 20 20 19 19 19 8 38 36 6 20
50 (63) 5 (11) 25 (45)
GPC-MALLSa
GPC −3
Mn,th × 10 [g/mol] 29.5 20.2 40.7 18.5 36.5 43.3 54.0 9.7 59.0 37.0 45.4 40.3
−3
−3
Mn × 10 [g/mol]
Mw/Mn
Mn × 10 [g/mol]
Mw/Mn
20.8 14.5 26.4 13.5 23.2 32.6 54.3 7.9 45.6 20.5 31.6 35.2
1.35 1.29 1.36 1.27 1.28 1.37 1.63 1.46 1.36 1.63 1.29 1.40
38.3 20.2 39.9 18.3 36.7 49.9 114.5 30.4 94.7 40.2 46.2 49.5
1.15 1.30 1.34 1.24 1.24 1.38 1.86 1.93 1.66 1.56 1.26 1.45
DSC Tg [°C] 29
23
22 62 14
a
The refractive index increments of copolymers were calculated from the composition, which means dn/dc(CLMA/tBuMA) = 0.060−0.044 mL/g, and dn/dc(CLMA/MMA) = 0.060−0.058 mL/g. bReference 27.
and protons of methyl groups −CH2C(CH3)− at δ = 0.85− 1.20 ppm (k) in polymer backbone of P(CLMA-co-tBuA) (Figure 1a) or methylene groups −CH2(CH2)2OH at δ = 2.25−2.48 ppm (e) in the substituent of both CLMA monomer and P(CLMA-co-tBuMA) copolymer (Figure 1b). Additionally, in both series the signals of vinylic protons in monomer pairs,
these are 2H of CLMA (CH2C(CH3)− at δ = 6.14 and 5.59 ppm (b)) and 3H of tBuA (CH2CH− at δ = 6.28 and 5.72 ppm (b′) and CH2CH− at δ = 6.03 ppm (a′)) or 2H of tBuMA (CH2C(CH3)− at δ = 6.10 and 5.48 ppm (b)) can be compared with the multiplet at δ = 1.25−1.80 ppm, corresponding to 6 protons of methylene groups in 4991
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characteristic for CH stretching, that is, 3100−2800 cm−1, the bands from ν(CH), ν(CH2), and ν(CH3) were observed, while the band at 1450 cm−1 was recognized as coming from δ(CH2). The band at 3200−3500 cm−1 corresponding to O−H stretching confirmed the presence of hydroxyl groups in the copolymer structures. ATR copolymerizations were conducted in various reaction systems in order to optimize conditions that enable to reduce side reactions and yield copolymers with narrow molecular weight distributions (Table 2 for P(CLMA-co-tBuA) (I−VI) and Table 3 for P(CLMA-co-tBuMA) (VI−XII)). Copolymerization of 5 mol % of CLMA with tBuA catalyzed by CuBr/PMDETA resulted in copolymer P(CLMA-co-tBuA) (I) with narrow molecular weight distribution (Mw/Mn,MALLS = 1.19) at DP = 86, including 9 units of hydroxyl-functionalized CLMA. In the case to introduce larger amount of CLMA into the polymeric chain its initial amount was increased to 15 mol %, which yielded copolymer II containing 16 CLMA units along chain with similar length to the previous one. The addition of double portion of catalyst complex slightly increased polymerization degree without changes of dispersity index for copolymer III (M w/M n,MALLS = 1.14). The copolymerization with 25 mol % of CLMA was performed in the less active system with CuCl. It is well-known that the shorter Cu−Cl bond in comparison to the Cu−Br bond is proposed to be responsible for the slower exchange in the chlorine ATRP systems, which are useful especially in case of polymerizations with higher reaction rates. The rate of reaction was also decreased by a larger amount of solvent (25 vol % vs 15 vol %) and lower temperature (60 °C vs 70 °C). The polymer IVA with dispersity Mw/Mn,MALLS = 1.31 was reached at low conversion. When the reaction was continued, the polymerization degree was increased, but the molecular weight distribution of polymer was also broadened in range 1.31−1.49 (IVA−IVE). Although a copolymerization with an equimolar ratio of initial comonomer feeds ([CLMA]0:[tBuA]0 = 50:50 mol %) was performed at a higher ratio of monomer to initiator ([M]0:[EtBriBu]0 = 600:1), the reaction was fast yielding copolymer VA with DP ≈ 145 after 1.5 h and broad molecular weight distribution (Mw/Mn,MALLS ≥ 2). At slightly longer time above 2 h 15 min the reaction reached a relatively high conversion in comparison to other copolymers CLMA/tBuA, but unfortunately it was gelated (VB). When the milder conditions were applied (less activated catalyst complex and larger ratio [M]0:[EtBriBu]0), the reaction was drastically slower, yielding polymer VI with DP ≈ 135 within 24 h, but it did not enhance the dispersity, which was still high. It can be concluded that the content of hydroxyl groups reached critical level of reactive hydrogen atom, which easily generates the side reactions, and in consequence higher dispersity of hydroxylfunctionalized CLMA copolymers. Another series of ATR polymerizations of CLMA with tBuMA were performed at various initial comonomer ratios ([CLMA]0:[tBuMA]0 = 5:95, 25:75, or 50:50 mol %) using CuCl as catalyst. The studies on P(CLMA-co-tBuMA) showed that control on the copolymerization decreased with increasing amounts of CLMA feed as observed in polymer chains with similar DP having greater amounts of CLMA and higher dispersity index. This effect was observed, when the copolymers with DP = 200−250, that is, VII with Mw/Mn,MALLS = 1.15 ( f CLMA = 5 mol %), VIIIA−B, and IXB−C Mw/Mn,MALLS = 1.24−1.38 (f CLMA = 25 mol %), and XI Mw/Mn,MALLS = 1.56 ( f CLMA = 50 mol %), were compared. Moreover, when the
Figure 1. 1H NMR spectra of the final samples taken from the reaction mixture in synthesis of (a) P(CLMA-co-tBuA) (IV) and (b) P(CLMAco-tBuMA) (IX).
caprolactone substituent −C(O)CH2(CH2)3CH2OH (f, g, h) and 9 protons in the tert-butyl group −C(CH3)3 (j, j′) to calculate the total conversion. Moreover, in both spectra the signals characteristic for methylene groups adjacent to carbonyl and hydroxyl groups −C(O)CH 2 (CH 2 ) 3 CH 2OH (e and i, respectively) are identified, but in Figure 1a signal e at δ = 2.25−2.45 ppm is overlapped with signal coming from proton of methine group (n) in the polymer backbone. For both copolymers the signal at δ = 1.75−2.20 ppm (m) representing protons from −CH2− groups in poly(meth)acrylic main chain is partly covered in Figure 1a by signal a or in Figure 1b by signals a and a′ corresponding to protons of methyl groups in monomeric methacrylates. The structures of the resulting copolymers were confirmed by FT-IR analysis in the region of 4000−500 cm−1. In all spectra of P(CLMA-co-tBuA) and P(CLMA-co-tBuMA) the bands characteristic for analogical functional groups were identified at similar wavenumbers. The bands assigned to stretching vibrations of C−O and CO groups were located at 1175−1245 and 1730 cm−1, respectively. In the region 4992
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reaction was continued to higher conversion of monomer, as it was practiced for IX, the molecular weight distribution was broadened from 1.24 to 1.86 at conversion 19%, leading to 22 units of CLMA with hydroxyl group (IXA) and 56% related to 64 CLMA units (IXD), respectively. High value of Mw/ Mn,MALLS for IXD and unsymmetrical shape of the GPC signal suggest the occurrence of the side reactions, such as coupling, which resulted nearly 2 times higher Mn,MALLS than Mn,NMR. Similarly to the CLMA/tBuA system (V vs VI), the rate of propagation in CLMA/tBuMA copolymerization was decreased using dNdpy instead PMDETA, higher dilution (35 vol % instead of 25 vol %), and lowering the reaction temperature from 60 °C to rt, which yielded only 8% of monomer conversion after 24 h and in effect short chain copolymer with DP = 63 and high dispersity (IX vs X). However, the use of similar conditions with lower activity catalyst complex (CuCl/ HMTETA), but also admixture of polar solvent (anisole/ MeOH), suppressed undesired termination reactions at larger initial feed of CLMA, yielding Mn,NMR ∼ Mn,MALLS and lower Mw/Mn,MALLS (XI vs XII). Molecular weights and their distributions were established both by conventional GPC with linear polystyrene (PS) calibration and GPC-MALLS. The GPC traces of the resulting copolymers of CLMA with tBuA and tBuMA are presented in Figure 2. The monomodal peaks are shifted toward lower
Figure 3. Dependence of molecular weight Mn,th (⧫), Mn,GPC (▲), Mn,MALLS (■) on total monomers conversion for the polymerization of CLMA with (a) tBuA (IV) and (b) tBuMA (IX).
of CLMA/MMA copolymers. Reaction with 5 mol % of CLMA was enhanced replacing CuBr by CuCl in the complex with dNbpy, which led to copolymer XIII with DP = 415 and Mw/ Mn,MALLS = 1.26 (Table 3). It can be compared with the earlier described copolymers characterized by broader molecular weight distributions Mw/Mn,MALLS = 1.37 at DP = 336 or Mw/Mn,MALLS = 1.39 at DP = 496. The kinetic studies were performed for CLMA systems including also CLMA/MMA copolymerization (the last one was carried out using previously optimized conditions detailed in Table 1 as XIV). Figure 4 represents semilogarithmic kinetic plots of ln([M]0/[M]t) vs time for the copolymerization of CLMA with tBuA (Figure 4a), tBuMA (Figure 4b), and for comparison with MMA (Figure 4c). Each of the systems shows different behavior that is for monomer pair methacrylate/ acrylate the conversion is twice higher for CLMA (CLMA 22% vs tBuA 10% within 1 h), whereas for two methacrylates it is more or less lower for CLMA in comparison to the used methacrylate comonomer (CLMA 31% vs tBuMA 46% and CLMA 25% vs MMA 31% within 1 h). The monomer conversion data shown in Figure 4 were used to estimate monomer reactivity ratios r1 and r2 (where r1 is reactivity ratio of CLMA and r2 is reactivity ratio of comonomer tBuA, tBuMA, or MMA) by the Jaacks method.30 The value for comonomer was determined from the slope of linear relationship presented in Figure 5 as r2 = −ln(1 − XM2)/− ln(1 − XM1), while the reactivity ratio of hydroxyl-function-
Figure 2. GPC traces of (a) P(CLMA-co-tBuA) and (b) P(CLMA-cotBuMA) prepared with 25 mol % of initial feed of CLMA.
elution volumes with increasing molecular weight of copolymers. However, the shapes of GPC traces for samples IVE (Figure 2a) and IXC (Figure 2b) confirmed the participation of side reactions what results in discrepancy between Mn,MALLS and Mn,NMR. As shown in Figure 3, the molecular weights increase almost proportionally with conversion, but values obtained from conventional GPC are lower than Mn,NMR calculated on the basis of 1H NMR spectra. This is caused by the difference in hydrodynamic volume of copolymers with bulky 2-(6hydroxyhexanoyloxy)ethyl substituents and linear PS used as a standard. The molecular weights determined by GPC-MALLS are closer to NMR ones; however, the linear relationship starts to deviate with increasing dispersity index. Our previous studies on the copolymerization of CLMA with MMA indicated that polymers P(CLMA-co-MMA) with moderately low dispersity indices (Đ(DP/DPCLMA) = 1.20 (320/20), 1.34 (250/46), 1.36 (150/55), 1.43 (250/111), and 1.41 (280) for 5, 25, 50, 75, and 100 mol % of CLMA, respectively) could be prepared.27 In the present work the additional reactions were performed to improve the parameters 4993
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alized CLMA was evaluated by equation r1 = 1/r2. For the system where CLMA was incorporated into the copolymer chain more quickly than tBuA (Figure 4a), the values calculated from Figure 5 were rCLMA = 2.53 and rtBuA = 0.40, which means that methacrylate radical at the growing chain end reacts 2.5 times faster with CLMA than with tBuA. It is in agreement with the general tendency that methacrylates are more reactive than acrylates leading to copolymers with gradient structure. The reverse reactivity ratios (rCLMA = 0.61, rtBuMA = 1.63) were obtained in case when tBuA was replaced by tBuMA, indicating its slightly higher preference to react with tBuMA than with CLMA. It was in good correlation with slightly faster consumption of tBuMA showed in the kinetic plot for copolymerization (Figure 4b). In the system of another methacrylate pair CLMA/MMA the difference in the reactivity ratios was smaller (rCLMA = 0.83 and rMMA = 1.20), which confirmed almost simultaneous incorporation of both comonomers observed in Figure 4c, suggesting the formation of almost random copolymer under such conditions (Scheme 1). Additionally, instantaneous diagrams of copolymer composition versus feed composition of comonomer mixture for the studied systems are also detailed (Figure 6). It is evident from
Figure 4. Semilogarithmic kinetic plot for the synthesis of (a) P(CLMA-co-tBuA) (IV), (b) P(CLMA-co-tBuMA) (IX), and (c) P(CLMA-co-MMA) (XIV). [M]0 and [M]t represent the initial monomer concentration and the monomer concentration after time t, respectively. The initial composition of the reaction mixture is given in Table 1.
Figure 6. Instantaneous composition diagrams of various CLMA systems (FCLMA mole fraction of CLMA in copolymer vs f CLMA mole fraction of CLMA in monomer feed).
the plot that the system CLMA/MMA displayed azeotropic behavior at 0.35 mol % feed fractions of CLMA. Two other dependencies showed deviations from the diagonal (r1 = r2 = 1); the curve above (CLMA/tBuA) and curve below (CLMA/ tBuMA) mean the copolymer formed instantaneously was richer or poorer in CLMA than the monomer mixture it originated from. These observations led to the conclusion that the reverse gradient structures could be assigned to the copolymers with tert-butyl monomers (Scheme 1). Thermal analysis of amorphous copolymers P(CLMA-cotBuA) and P(CLMA-co-tBuMA) was performed by DSC. The glass transition temperature (Tg) determined at second heating run appeared as single step transition on DSC curve indicating no phase separation (Figure 7). Tg was dependent on the molecular weight and composition of the copolymers. Copolymerization of CLMA with tBuMA yielded more rigid copolymers with higher Tg (Figure 7b) than that of more flexible macromolecules obtained in the reaction with tBuA
Figure 5. Jaacks plot for copolymerization of CLMA with tBuA (IV), tBuMA (IX), and MMA (XIV) (M1 = CLMA, M2 = tBuA, tBuMA, or MMA).
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copolymers). The kinetics of copolymerization reactions of CLMA/tBuA and CLMA/tBuMA and the relative reactivity ratios of comonomers determined by the Jaacks method, rCLMA > rtBuA and rCLMA ≤ rtBuMA, suggested formation of strong gradient and slight gradient, respectively. The CLMA copolymers with tert-butyl groups were compared with CLMA/MMA copolymers, which presented almost random structures (rCLMA ≈ rMMA). Tg of copolymers containing tBuA (Tg = 26−5 °C for FCLMA ≈ 5−75 mol %) was changing in a broader range than for that with tBuMA (Tg = 29−22 °C for FCLMA ≈ 5−35 mol %), which can be related to different microstructures of polymeric chains and their molecular weight distributions. The prepared copolymers are a great potential for ATRP macroinitiators and amphiphilic grafted copolymers by transformation of hydroxyl groups into halogenoester groups and by hydrolysis of tert-butyl groups to hydrophilic acidic units.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Science Center (Grant No. N N204 122940) and the European Social Fund, Operational Programme Human Capital, Project UDAPOKL.04.01.01-00-114/09-00 (K.B.).
Figure 7. DSC thermograms of (a) P(CLMA-co-tBuA) (I, III) at DP ≈ 100 and (b) P(CLMA-co-tBuMA) (VII, XII) at DP ≈ 200 with various content of CLMA.
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(Figure 7a). For PtBuA copolymers Tg decreased from 26 to 5 °C with the increase in CLMA content. The same tendency was also observed for P(CLMA-co-tBuMA) copolymers, but in much narrower range (Tg = 29−22 °C for FCLMA = 3−35 mol %). Smaller difference of Tg with larger change in copolymer composition can be explained by significantly higher dispersity of sample XII (Mw/Mn = 1.56) vs VII, I, III (Mw/Mn = 1.15− 1.18), which gives the increased value of Tg for XII. The thermal properties of copolymers based on CLMA and tertbutyl monomer were compared with previously studied P(CLMA-co-MMA),27 where it was shown that the Tg depends on the composition of copolymers, and it was decreased from 52 to −24 °C for copolymers containing 5−50 mol % of CLMA. P(CLMA-co-tBuMA) and P(CLMA-co-tBuA), in opposite to P(CLMA-co-MMA), did not exhibit the predisposition to cross-linking during DSC analysis in the temperature range −70 to 150 °C.
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CONCLUSIONS ATRP of CLMA with tBuA and tBuMA yielded the functional copolymers containing both the reactive hydroxyl groups in CLMA and the acidic units masked by tert-butyl groups. The copolymer composition was controlled by initial ratio and selection of proper comonomer with good correlation to the catalytic system. The molecular weight distribution strongly depended on CLMA content; that is, in both cases the low amount 5−15 mol % of CLMA led to the well-defined copolymers with Mw/Mn < 1.2, and the increase in CLMA feed to 25 mol % gave a broader molecular weight distribution (with tBA: Mw/Mn < 1.5 at DP < 100, with tBMA: Mw/Mn < 1.4 at DP < 300 and ∼1.85 above 300), whereas with 50 mol % of CLMA it drastically increased up to ∼2.0 at DP < 100 (tBuA copolymers) or below 1.7 at DP = 200−330 (tBuMA 4995
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