Adhesive RAFT agents for controlled polymerization of acrylamide: Effect of catechol-end R groups

Article abstract of DOI:10.1039/c5ra16193b

Synthesizing polyacrylamide (PAM) inorganic nanocomposites with stable tethering and controlled polymer length has been elusive. Herein, we report on the synthesis of trithiocarbonates with several catechol end R groups (as anchors) that differ in their carbonyl α-substituents. These so-called adhesive RAFT agents were subsequently examined in batch RAFT polymerization of the acrylamide (AM) monomer to study their living characteristics. The catechol-end trithocarbonates (Dopa-CTAs) and catechol-end PAM structures (≤46 kDa) were confirmed via 1D (¹H and ¹³C) and 2D (gHSQC, gHMBC) NMR. Subsequent anchoring of the end-functionalized PAM (grafting to) via catechol induced linkage to γ-alumina nanoparticles was successful, giving good correlation based on ATR-FTIR, DLS and TGA analyses. This unique methodology enables PAM-inorganic nanocomposites to be synthesized with stable tethering without significant rate retardation.

Full text of DOI:10.1039/c5ra16193b

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PAPER
Adhesive RAFT Agents for Controlled Polymerization of
Acrylamide: Effect of Catechol-end R Groups†
Olabode Oyeneye, William Z. Xu and Paul A. Charpentier*
Received 00th January 20xx,
Accepted 00th January 20xx
Synthesizing polyacrylamide (PAM) inorganic nanocomposites with stable tethering and controlled polymer length has
been elusive. Herein, we report on the synthesis of trithiocarbonates with several catechol end R groups (as anchors) that
differ in their carbonyl α-substituents. These so-called adhesive RAFT agents were subsequently examined in batch RAFT
polymerization of acrylamide (AM) monomer to study their livingness characteristics. The catechol-end trithocarbonates’
(Dopa-CTAs) and catechol-end PAM structures (≤ 46 kDa) were confirmed via 1D (1H and 13C) and 2D (gHSQC, gHMBC)
NMR. Subsequent anchoring of the end-functionalized PAM (grafting to) via catechol induced linkage to γ-alumina
nanoparticles was successful, giving good correlation based on ATR-FTIR, DLS and TGA analyses. This unique methodology
enables PAM-inorganic nanocomposites to be synthesized with stable tethering without significant rate retardation.
DOI: 10.1039/x0xx00000x
www.rsc.org/
29 potential for tailored materials with predetermined molecular
30 weight (MW), complex architectures, diverse functionalities
31 and narrow dispersity (Đ).23 In particular, the RAFT
32 polymerization technique has been shown to possess
33 advantages over both ATRP and nitroxide techniques because
34 of the ease of implementation and the wide range of
35 applicable monomers (functional and non-functional), solvents
36 and conditions. Under the “grafting to” approach, end-
37 functionalized polymers can be prepared utilizing a RAFT agent
1
Introduction
2
3
4
5
6
7
8
9
Polymeric inorganic nanocomposites (PNCs) using so-called
“smart” (co)polymers1-4 have shown potential by harnessing
the synergic effects of both the polymer and inorganic
components to enhance properties for end-use applications
such as water treatment flocculation.5-7 For linking the
polymer component to inorganic nanoparticles, various
coupling molecules have been investigated including
38 (ZC(=S)SR) that has a Z- or R- substituent bearing the required
4
carboxyls,8
catechol
derivatives,9-13
phosph(on)ates,1
39 end-group.8,24 However, selection of the substituents needs to
40 be suited for the specific monomer, as they influence the RAFT
41 agent reactivity, solubility and polymerization kinetics.25
42 Among the various classes of RAFT agents, trithiocarbonates
43 are more hydrolytically stable and offer better control over
44 polymer structure derived from more activated monomers,
45 such as acrylamide.26
10 silanes,15-17 and thiols.18-20 Catechol derivatives have been
11 shown to provide strong and stable chemisorption bonding
12 between the polymer component and inorganic
13 nanoparticles.11-13 To provide this catechol functionality,
14 dopamine is bifunctional with an amine moiety that can be
15 chemically modified for amide linkage formation to polymer,
16 while the catechol will promote mono- or bi-dentate bonding
17 to the inorganic nanoparticles (NPs).21,22
46 A number of studies have utilized a catechol moiety (as an
47 adhesive molecule) with RAFT polymerization techniques for
48 PNC syntheses, and catechol end-functionalization of polymers
49 is often achieved in-situ using catechol bearing RAFT agents for
50 polymerization13,27 or after polymer synthesis via post-
51 modification.28,29 However, to the best of our knowledge, no
52 studies have attempted to compare catechol bearing RAFT
53 agents having differing substituents at their alpha positions for
54 the most suited livingness characteristics with respect to
55 monomers. Herein, we investigate the influence of
56 trithiocarbonate RAFT agents bearing the same Z group but
18 Strategies commonly used for the synthesis of pre-defined
19 PNCs involve controlled radical polymerization (CRP) using
20 either “grafting from” or “grafting to” approaches, with the
21 latter entailing the immobilization of end-functionalized
22 polymer on NPs. The inorganic NPs being the core help to
23 define the final morphology of the PNC, in conjunction with
24 controlled polymerization to ensure uniform extension of the
25 polymeric chains from the NP core. Of the CRP techniques, the
26 reversible addition-fragmentation chain transfer (RAFT)
27 polymerization method has been given immense attention for
57 different catechol end
R groups on acrylamide (AM)
28 the synthesis of advanced materials. This is because of its
58 polymerization, and subsequent anchoring of the resulting
59 polymer to γ-alumina NPs. More specifically, the catechol RAFT
60 agents differ in the substituents on their trithiocarbonate α
61 carbon, and one of the RAFT agents being more bulky (see
62 Scheme 1). The catechol end R group affects the partitioning of
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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63 intermediate radicals, and should be a good homolytic leavin
64 group for preferential partitioning into new radical species
65 (derived from the R-group) which are capable of efficient re-
9
9
g
8
previously purged under argon atmosphere (ultra-high purity,
9 Praxair Inc. Canada).
100 Characterization: A brief and detailed description of the
66 initiation.30,31 We focused on end-functionalized polymers for
101 characterization methods can be found in the ESI.†
67 subsequent “grafting to” as opposed to surface-initiated
68 polymerization, because dense anchoring of the catechol-e
69 CTA on metal oxide NPs requires conditions that ca
70 hydrolytic decomposition of trithiocarbonate groups.10,32
71 AM monomer was chosen because of the wide utility of P
72 in applications as flocculants or additives in wastewa
73 treatment,5,33,34 while γ-Al2O3 was employed because of
1
1u0se3
T1h0e4
1A0M5
1t
1i
n
0
d
2
Synthesis of RAFT Agents with (2,5-dioxopyrrolidin-1-
yl)oxidanyl End Groups (Suc-CTAs, (2a-c)): The synthesis of
Suc-CTAs was performed based on a literature method13 by
varying R groups while using a simplified workup procedure.
NHS (0.40 g, 3.40mmol) and EDC (0.76 g, 3.43mmol) were
added to 2.68mmol each of CDSPA, DDMAT, and DoPAT
dissolved in dried DCM (30 mL, previously dried with
0er
0ts
0o
6
7
8
74 high OH density, high surface activity and propensity
1f
75 wastewater treatment.35,36
109 anhydrous Na2SO4), and allowed to react for 18 hr under
110 continuous stirring at room temperature. Each reaction
111 mixture was then washed with 150 mL of saturated NaHCO3
112 (aq) before collecting the DCM phase. Further extraction from
113 the aqueous phase was carried out with ethyl ether (5×30 mL),
114 and then combined with the DCM phase to give a single
115 organic phase, which was washed with deionized water (3×50
116 mL), brine (3×50 mL) and dried over anhydrous MgSO4 (7.0 g).
117 The hydrated MgSO4 was filtered off and the solvent removed
118 using a Rotavap to obtain yellowish solid products.
119 Suc-DDMAT: 1H NMR (600 MHz, CDCl3) δ (ppm): 0.89 (t, J = 7.0
120 Hz, 3 H, CH3CH2CH2), 1.23 - 1.34 (m, 16 H, CH3(CH2)8CH2), 1.36 -
121 1.42 (m, 2 H, CH2(CH2)2S), 1.66 - 1.72 (m, 2 H, CH2CH2S), 1.88
122 (s, 6 H, C(CH3)2), 2.82 (m, 4 H, (O=)C(CH2)2C(=O)), 3.31 (t, J = 7.0
123 Hz, 2 H, CH2S); 13C NMR (100 MHz, CDCl3) δ(ppm): 14.1
124 (CH3CH2CH2),
22.7
(CH3CH2CH2),
25.6
(C(CH3)2,
125 (O=)C(CH2)2C(=O)), 27.8 (CH2CH2S), 29.0 (CH2(CH2)2S),
126 29.1(CH2(CH2)3S), 29.3 (CH2(CH2)4S), 29.4 (CH3(CH2)2CH2), 29.5
127 (CH3(CH2)3CH2), 29.6 (CH3(CH2)4(CH2)2), 31.9 (CH3CH2CH2), 37.2
128 (CH2CH2S), 54.3 (C(CH3)2), 168.6 (N(C=O)2), 169.1 (C(=O)O),
129 218.7 (SC(=S)S). FTIR (cm-1): 2916 (υasCH2), 2847 (υsCH2), 1777
130 (υC=O, imide), 1734 (υC=O, ester), 1202 (υC-O, ester), 1073
131 (υC=S), 811(υasS-C-S).
76 Experimental Section
1
132 Suc-DoPAT: H NMR (600 MHz, CDCl3) δ (ppm): 0.88 (t, J=7.0
77 Materials: γ-alumina (dTEM ≤ 50 nm, surface area > 40 m2/g
133 Hz, 3 H, CH3CH2CH2), 1.22 - 1.33 (m, 16 H, CH3(CH2)8CH2), 1.39
78 (BET), acrylamide (AM, ≥ 98%), 4,4’-azobis(4-cyanovaleric acid)
134 (quin, J=7.2 Hz, 2 H, CH2(CH2)2S), 1.62 - 1.73 (m, 2 H, CH2CH2S),
79 (ACVA,
≥
98%),
Dopamine
hydrochloride,
N-
135 1.75 (d, J=7.4 Hz,
80 hydroxysuccinimide (NHS, 98%), N-(3-dimethylaminopropyl)-
3 H, CH(CH3)), 2.83 (br. s, 4 H,
136 (O=)C(CH2)2C(=O)), 3.37 (td, J=7.4 Hz ×2 and 3.1 Hz, 2 H, CH2S),
137 5.14 (q, J=7.4 Hz, 1 H, CH(CH3)); 13C NMR (100 MHz, CDCl3) δ
82 chloride (NaCl, ≥ 99%), 2-(dodecylthiocarbonothioylthio)-2-
81 N′-ethylcarbodiimide hydrochloride (EDC, ≥ 98%), sodium
138 (ppm): 14.1 (CH3CH2CH2), 16.7 (CH(CH3)), 22.6 (CH3CH2CH2),
83 methylpropionic
acid
(DDMAT,
98%),
2-
139 25.6 ((O=)C(CH2)2C(=O)), 27.8 (CH2CH2S), 28.9 (CH2(CH2)2S),
84 (dodecylthiocarbonothioylthio)propionic acid (DoPAT, 97%), 4-
140 29.0 (CH2(CH2)3S), 29.3 (CH2(CH2)4S), 29.4 (CH3(CH2)2CH2), 29.5
85 cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid
141 (CH3(CH2)3CH2), 29.6 (CH3(CH2)4(CH2)2), 31.9 (CH3CH2CH2), 37.5
86 (CDSPA, 97%) and methanol (MeOH, ≥ 99.9%) were purchased
142 (CH2CH2S), 45.0 (CH(CH3)), 167.2 (N(C=O)2), 168.5 (C(=O)O),
143 220.2 (SC(=S)S). FTIR (cm-1): 2914 (υasCH2), 2848 (υsCH2), 1786
88 organic solvents used were the highest purity available from
87 from Sigma Aldrich, Canada and used as received. All other
144 (υC=O, imide), 1736 (υC=O, ester), 1471, 1358, 1200 (υC-O,
89 the Caledon Laboratory Ltd., Canada. Sodium bicarbonate
145 ester), 1073 (υC=S), 813(υasS-C-S).
90 (NaHCO3, ≥ 95 %), anhydrous sodium sulfate (Na2SO4, ≥ 99 %)
146 Suc-CDSPA: 1H NMR (600 MHz, CDCl3) δ(ppm): 0.89 (t, J=7.0
91 and anhydrous magnesium sulfate (MgSO4) and sulfuric acid
147 Hz, 3 H, CH3CH2CH2), 1.23 - 1.33 (m, 16 H, CH3(CH2)8CH2), 1.35 -
92 (96.5%) were obtained from the Caledon Labs (CA).
148 1.44 (m, 2 H, CH2(CH2)2S), 1.66 - 1.73 (m, 2 H, CH2CH2S), 1.89
93 Triethylamine (Et3N, 99.5 %) and hydrogen peroxide (30.5%)
149 (s, 3 H, C(CH3)), 2.48 - 2.69 (m, 2 H, CH2CH2C(=O)O), 2.85 (br. s,
94 were procured EDM Chemicals (USA). Dialysis membranes (MW
150 4 H, (O=)C(CH2)2C(=O)), 2.94 (ddd, J=10.0, 6.2, 3.8 Hz, 2 H,
151 CH2C(=O)O), 3.34 (t, J=7.3 Hz, 2 H, CH2CH2S); 13C NMR (100
96 while 25 μm filters (Fischer Scientific) were obtained from
95 3,500 Da) were purchased from Spectrum Laboratories, Inc.,
152 MHz, CDCl3) δ (ppm): 14.1 (CH3CH2CH2), 22.7 (CH3CH2CH2),
97 VWR Canada. All batch polymerization reactions were
153 24.8 (C(CH3)), 25.6 ((O=)C(CH2)2C(=O)), 26.8 (CH2C(=O)O), 27.6
2 | J. Name., 2012, 00, 1-3
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154 (CH2CH2S), 28.9 (CH2(CH2)2S), 29.0 (CH2(CH2)3S),
155 (CH2(CH2)4S), 29.4 (CH3(CH2)2CH2), 29.5 (CH3(CH2)3CH2), 2
156 (CH3(CH2)4(CH2)2), 31.9 (CH3CH2CH2), 33.2 (CH2CH2C(=O)
157 37.1 (CH2CH2S), 46.0 ((CH3)C(C≡N)), 118.6 (C(C≡N)), 16
158 (C(=O)O), 168.8 (N(C=O)2), 216.5 (SC(=S)S). FTIR (cm-1): 29
159 (υasCH2), 2848 (υsCH2), 2235 (υC≡N), 1820, 1783 (υC=O, imid
160 1734 (υC=O, ester), 1423, 1383, 1293, 1199 (υC-O, ester), 10
161 (υC=S), 884, 803(υasS-C-S).
2
2
9
1
.6
.3
0
J=7.6 Hz, 2 H, CH2CH2S), 4.64 (q, J=7.0 Hz, 1 H, CH(CH3)), 6.39
(dd, J=7.9, 2.1 Hz, 1 H, ArC-H(m-OH)), 6.54 (d, J=2.4 Hz, 1 H,
ArC-H(o-OH)), 6.59 (d, J=7.6 Hz, 1 H, ArC-H(o-OH)); 8.60 (s) &
8.69 (s) (2H, Ar-OH), 8.31(t, J=5.6 Hz, 1 H, NHCH2CH2); 13C NMR
(100 MHz, CDCl3) δ (ppm): 14.1 (CH3CH2CH2), 16.1 (CH(CH3)),
22.7 (CH3CH2CH2), 27.8 (CH2CH2S), 28.9 (CH2(CH2)2S), 29.1
(CH2(CH2)3S), 29.3 (CH2(CH2)4S), 29.4 (CH3(CH2)2CH2), 29.5
29
2O1)2,
271.03
21164
2e1)5,
2
1
1
6
1
6
6
217 (CH3(CH2)3CH2), 29.6 (CH3(CH2)4(CH2)2), 31.9 (CH3CH2CH2), 34.6
218 (NHCH2CH2),37.7 (CH2CH2S), 41.3 (NHCH2CH2),47.8 (CH(CH3)),
162 Synthesis of Catechol End Group CTAs (Dopa-CTAs (3a-c)):
219 115.3 (ArC-H(o-OH)), 115.5 (ArC-H(o-OH)), 120.7 (ArC-H(m-
163 Typically, dopamine hydrochloride (0.50 g, 2.64mmol) and
220 OH)), 130.4 (CH2-ArC), 142.9 (ArC-OH), 144.0 (ArC-OH), 171.4
164 each of Suc-CDSPA, Suc-DDMAT and Suc-DoPAT (2.13 mmol)
221 (CHC(=O)NH), 223.4 (SC(=S)S). FTIR (cm-1): 3341 (υNH, amide),
165 were added to MeOH (30 mL) with Et3N (0.40 mL, 2.87mmol),
222 3238 (υOH, phenol), 2922 (υasCH2), 2848(υsCH2), 1633 and
166 and allowed to undergo dark reaction for 48 hr at room
223 1616 (υC=O, amide I & υC=C, aromatic), 1522 (υC-N & δNH,
167 temperature under continuous stirring. At the end of the
224 amide II), 1465, 1365, 1281, 1193, 1070 (υC=S), 813 (υasS-C-S).
168 reaction, the solvent was removed by rotary evaporation,
225 Dopa-CDSPA: H NMR (600 MHz, CDCl3) δ (ppm): 0.89 (t, J=6.8
169 followed by the addition of ether (20 mL) and washing of the
226 Hz, 3 H, CH3CH2CH2), 1.22 - 1.34 (m, 16 H, CH3(CH2)8CH2), 1.35 -
170 aqueous phase. Subsequently, the ether phase was washed
227 1.45 (m, 2 H, CH2(CH2)2S), 1.69 (quint, J=7.5 Hz, 2 H, CH2CH2S),
171 with deionized water (3×15 mL) and brine (3×15 mL). The
228 1.88 (s, 3 H, C(CH3)), 2.32 - 2.39 (m, 1 H, CH2 CH2C(=O)), 2.42 -
172 ether solvent was removed by vacuum evaporation, and then
229 2.47 (m, 2 H, CH2CH2C(=O)), 2.48 - 2.55 (m, 1 H, CH2 CH2C(=O)),
173 the viscous solute cooled (4oC) before precipitating in hexane
230 2.71 (t, J=7.0 Hz, 2 H, CH2-ArC), 3.33 (t, J=7.5 Hz, 2 H, CH2CH2S),
174 (except for Dopa-CDSPA) to give a bright yellow solid product,
231 3.43 - 3.54 (m, 2 H, NHCH2CH2), 5.63 (t, J=5.9 Hz, 1 H,
175 which was vacuum dried. In the case of Dopa-CDSPA, further
232 NHCH2CH2), 6.61 (dd, J=8.2, 1.7 Hz, 1 H, ArC-H(m-OH)), 6.72 (d,
176 purification was carried out via preparative column
1
a
b
233 J=1.8 Hz, 1 H, ArC-H(o-OH)), 6.83 (d, J=8.2 Hz, 1 H, ArC-H(o-
177 chromatography using silica gel (ethyl acetate: hexane= 3:1
234 OH)); 13C NMR (100 MHz, CDCl3) δ (ppm): 14.1 (CH3CH2CH2),
235 22.7 (CH3CH2CH2), 24.8 (C(CH3)), 27.7 (CH2CH2S), 29.0
178 v/v).
179 Dopa-DDMAT: 1H NMR (600 MHz, CDCl3) δ (ppm): 0.89 (t, J=7.0
236 (CH2(CH2)2S), 29.1 (CH2(CH2)3S), 29.3 (CH2(CH2)4S), 29.4
180 Hz, 3 H, CH3CH2CH2), 1.23- 1.32 (m, 16 H, CH3(CH2)8CH2), 1.36 -
237 (CH3(CH2)2CH2), 29.5 (CH3(CH2)3CH2), 29.6 (CH3(CH2)4(CH2)2),
181 1.41 (m, 2 H, CH2(CH2)2S), 1.66 (s, 8 H, CH2CH2S, C(CH3)2), 2.67
238 31.9 (CH3CH2CH2, CH2C(=O)NH), 34.5 (CH2CH2C(=O)), 34.6
182 (t, J=7.0 Hz, 2 H, CH2-ArC), 3.26 (t, J=7.6 Hz, 2 H, CH2CH2S),
239 (NHCH2CH2), 37.1 (CH2CH2S), 41.2 (NHCH2CH2), 46.6
183 3.41-3.49 (m, 2 H, NHCH2CH2), 6.56 (dd, J=8.0, 2.2 Hz, 1 H, ArC-
240 ((CH3)C(C≡N)), 119.2 (C(C≡N)), 115.5 (ArC-H(o-OH)), 115.7 (ArC-
184 H(m-OH)), 6.64 (t, J=5.5 Hz, 1 H, NHCH2CH2), 6.71 (d, J=2.0 Hz,
241 H(o-OH)), 120.8 (ArC-H(m-OH)), 130.7 (CH2-ArC), 142.9 (ArC-
185 1 H, ArC-H(o-OH)), 6.80 (d, J=8.2 Hz, 1 H, ArC-H(o-OH)); 13C
242 OH), 144.1 (ArC-OH), 171.4 (CH2C(=O)NH), 217.2 (SC(=S)S). FTIR
186 NMR (100 MHz, CDCl3) δ (ppm): 14.1 (CH3CH2CH2), 22.7
243 (cm-1): 3286 (overlap: υNH, amide & υOH, phenol), 2919
187 (CH3CH2CH2), 25.8 (C(CH3)2), 27.7 (CH2CH2S), 29.0 (CH2(CH ) S),
2 2244 (υasCH2), 2851(υsCH2), 2233 (υC≡N), 1640 and 1603 (υC=O,
188 29.1(CH2(CH2)3S), 29.3 (CH2(CH2)4S), 29.4 (CH3(CH2)2CH2), 29.5
245 amide I & υC=C, aromatic), 1519 (υC-N & δNH, amide II), 1442,
189 (CH3(CH2)3CH2), 29.6 (CH3(CH2)4(CH2)2), 31.9 (CH3CH2CH2), 34.5
246 1360, 1280, 1193, 1151, 1112 1065 (υC=S), 803 (υasS-C-S).
190 (NHCH2CH2), 37.2 (CH2CH2S), 41.7 (NHCH2CH2), 57.1 (C(CH3)2),
191 115.2 (ArC-H(o-OH)), 115.4 (ArC-H(o-OH)), 120.8 (ArC-H(m47
192 OH)), 130.8 (CH2-ArC), 142.9 (ArC-OH), 144.0 (ArC-OH), 17 .2
193 (CC(=O)NH), 219.9 (SC(=S)S). FTIR (cm-1): 3340 (υNH, amid
194 3186 (υOH, phenol), 2920 (υasCH2), 2850(υsCH2), 1622 a
195 1604 (υC=O, amide I & υC=C, aromatic), 1531 (υC-N & δN25
2
2348
2e4)9,
2n5d0
-
RAFT Polymerization of Acrylamide: All polymerization
experiments were performed at 2M monomer concentration
([M]0 = 0.049 mol AM) in 24.5 mL DMSO/DMF (97:3, vol%)
solvent (vol. of DMF is equivalent to 0.2[AM]0) and 70oC under
argon atmosphere. The DMF was added as an internal
2S5)2, reference for the determination of conversion of monomer
H1,
196 amide II), 1447, 1361, 1291, 1252, 1158, 1112, 1072 (υC=
197 813 (υasS-C-S).
253 using subsequent NMR analysis. The initial CTA to initiator
198 Dopa-DoPAT: 1H NMR (600 MHz, CDCl3) δ (ppm): 0.89 (t, J=
27
5
.0
4
ratio ([CTA]0/[I]0= 5) and the initial monomer to CTA ratio
([M]0/[CTA]0 = 500) were held constant to ensure controlled
polymerization. AM (3.554 g, 0.049 mol), ACVA (5.6 mg, 0.0196
mmol), 24.5 mL DMSO/DMF (97:3 vol%) solvent and the
catechol-end RAFT agent (0.098 mmol each, 3a-c) were added
to a 100-mL two-neck round-bottom flask equipped with a
magnetic stirrer, and a reflux condenser was connected to one
of its necks. The flask had its other neck sealed with a rubber
septum through which its content was purged with argon for
20 min, before immersing the flask into an oil bath for
temperature control as the experiment commenced. At
predetermined intervals, 2-3 drops of samples were taken for
199 Hz, 3 H, CH3CH2CH2), 1.21 - 1.35 (m, 16 H, CH3(CH2)8CH2), 1.3
200 1.45 (m, 2 H, CH2(CH2)2S), 1.55 (d, J=7.6 Hz, 3 H, CH(CH3)), 1.7
201 (quin, J=7.5 Hz, 2 H, CH2CH2S), 2.66 (t, J=6.8 Hz, 2 H, CH2-ArC
202 3.28 - 3.49 (m, 4 H, CH2CH2S, NHCH2CH2), 4.69 (q, J=7.6 Hz, 125H8,
2755-
2516
25)7,
203 CH(CH3)), 6.50 (t, J=5.6 Hz, 1 H, NHCH2CH2), 6.57 (dd, J=7.9,
2
2H
2
2
2(6d3,
2S6)4,
2
5
.1
))
SO61
(m62
9
204 Hz, 1 H, ArC-H(m-OH)), 6.67(d, J=1.8 Hz, 1 H, ArC-H(o-O
6
0,
205 6.80 (d, J=8.2 Hz, 1 H, ArC-H(o-OH));1H NMR (600 MHz, DM
-
206 d6) δ (ppm): 0.83 (t, J=6.8 Hz, 3 H, CH3CH2CH2), 1.16 - 1.27
207 16 H, CH3(CH2)8CH2), 1.28 - 1.35 (m, 2 H, CH2(CH2)2S), 1.42
208 J=7.0 Hz, 3 H, CH(CH3)), 1.60 (quin, J=7.5 Hz, 2 H, CH2CH2
209 2.48 (m, 2 H, CH2-ArC), 3.10 - 3.22 (m, 2 H, NHCH2CH2), 3.3326(t
,
5,
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266 monomer conversion analysis by 1H NMR while aliq
Journal Name
2
u
9
o
9
t
Since the trithiocarbonate moiety of RAFT agents is known to
decompose at elevated temperature,32 Dopa-CTAs (3a-c) were
synthesized via amide linkages under mild conditions (Scheme
3n0e2
1).13,42 This approach involved initial coupling of carboxyl CTAs
267 samples were quenched immediately in liquid nitrogen a
268 then purified prior to GPC analysis. The polymer samples w
269 purified by three cycles of precipitating in 20 times aceto
270 and re-dissolving in deionized H2O before freeze-drying30to
3n0d0
3e0re1
3
(1a-c) with a better leaving group (NHS) using EDC as the
271 obtain dried polymer. However, for NMR analysis of
272 structure of the synthesized polymer, further purification
3
3v
th
0
e
4
carboxyl activating agent before amidization. NHS esters allow
efficient coupling with amines to yield amide bonds.42 The
0ia
5
273 dialysis (3500 MWCO) against distilled water was carried ou3t.06 Dopa-CTAs ((3a-c), termed Dopa-DDMAT, Dopa-DoPAT and
307 Dopa-CDSPA respectively) were then prepared by reacting
274 Pre-treatment of γ-Al2O3 NP: Alumina NPs were pretreated by
308 dopamine with NHS-activated esters of the carboxylic RAFT
275 washing with acetone (twice) and immersed in piranha
309 agents (2a-c). The formed compounds (3a-c) were confirmed
276 solution for 30 min (96.5% H2SO4 and 30.5% H2O2 (4:1 vol.)) to
310 via 1D (1H and 13C) and 2D (gHSQC) NMR, ATR-FTIR, and UV-vis
277 remove organic contaminants and to enhance the
311 spectroscopy.
278 hydroxylation of the NP surface. Then, the NPs were extracted
312 1H NMR spectra of dopamine hydrochloride, DDMAT (1a), Suc-
279 by washing with water and ethanol, and then vacuum dried.
313 DDMAT (2a), and Dopa-DDMAT (3a) are compared in Fig. 1.
314 The spectra show all the 1H peaks for the four compounds
280 Preparation of γ-Al2O3-PAM Nanocomposite (Al-PAM):
281 Following ultrasonication of the pretreated alumina NP cores
315 (dopamine HCl, (1a), (2a) and (3a)), except the weak broad
282 (30 mg), a solution containing (5 mg/mL) of Dopa-PAM (Mn =
316 carboxylic acid peak which is located at 10.73 ppm (for full
283 26500, 42600, 53800 g/mol; synthesized from Dopa-CTA (3a)
)
317 spectra of DDMAT (1a), see ESI Fig. S1†). With DDMAT (1a
)
284 was dispersed in 15 mL deionized water at 50oC for 24 hr. Then
285 excessive polymer was removed via dissolution and
318 being converted into Suc-DDMAT
(
2a), this acid peak
319 disappears while a new peak 29, attributed to succinimidyl
286 centrifugation before freeze-drying to obtain the dried Al-PAM
320 protons, appears at 2.82 ppm. Further conversion of Suc-
287 nanocomposites. For preparing the control Al-PAM sample, a
321 DDMAT (2a) into Dopa-DDMAT (3a) was evident by the
288 similar procedure was employed except that the polymer used
322 absence of the peak 29 in the spectrum of Dopa-DDMAT (3a),
289 was a PAM synthesized with CTA (1a) (without catechol
323 and the presence of new peaks 17-19, 21, 22, and 25. The peak
324 17 is the characteristic signal for the secondary amide proton,
325 while peaks 21, 22 and 25 are ascribed to the catechol
290 moiety, Mn = 29600 g/mol).
326 moiety.13,40,43 It should be noted that the H peaks of phenol
1
291 Results & Discussion
327 hydroxyl groups 26 and 27 were absent when using CDCl3 as
328 solvent but would show when using DMSO-d6 as solvent. 13C
292 Synthesis and Characteristics of Catechol End Group RAFT Agents
293 (Dopa-CTAs (3a-c)): Though the carboxyl group of the CTAs (1a-
329 NMR spectra of dopamine HCl, DDMAT (1a), Suc-DDMAT (2a),
, Scheme 1) could be employed as anchor, we chose the
330 and Dopa-DDMAT (3a) are compared in ESI Fig. S2.† The
294
c
295 catechol moiety because of its ability to chelate various metal
331 synthesized Dopa-DDMAT (3a) was confirmed by the shifting
296 oxides (HfO2, ZrO2, MnO2, Y2O3, Al2O3, TiO2, Fe2O3),21,37-39
332 of the 13C carbonyl peak 16 and the presence of new 13C peaks
297 comparatively better pH stability of its complexes,40 and the
333 18 – 25 which are comparable to those of the dopamine HCl.
298 relatively mild procedure for its ligand exchange process.8,40,41
4 | J. Name., 2012, 00, 1-3
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334 All the correlation 1H/13C peaks for the synthesized Do
ARTICLE
3
p
6
a
9
-
Dopa-CDSPA (4c) giving the highest overshoot (Fig. 2a). Similar
overshoots have been observed in a number of studies
involving polymerization of acrylamide-based monomer
mediated with trithiocarbonate RAFT agents,26,32,44 with one of
the plausible reasons for such discrepancy as explained by
Thomas et al.23 being the limited extent of utilization of the
37s5. Dopa-CTAs.
335 DDMAT (3a) are clearly shown in ESI Fig S3†, confirming
336 peak assignments and molecular structure. Similarly,
337 conversions of (1b) to (2b) then (3b), (1c) to (2c) and (3c) w3e72
3i70
ts
th
re
in
TA
3a-c) was also confirmed by ATR-FTIR investigation (ESI Fig
341 S8 - S10†). Considering the UV wavelength range 320 to 2
342 nm for qualitative analysis, the trithiocarbonate group on 3th7e7
37e1
1
338 also confirmed by their H and 13C NMR spectra in support
37g3
339 information (ESI Figs. S4 – S7†). The synthesized Dopa-C37s4
340
(
3
8
7
0
6
The pseudo first order kinetic plots for AM polymerization
using the Dopa-CTAs shown in Figure 2b deviate from linearity,
343 Dopa-CTAs (3a-c) was confirmed by the presence of a stro3n7g8 approaching a polynomial distribution, thereby suggestive of
344 absorption peak centred at 308-310 nm13 while a shoulder
345 peak at 292-294 nm reveals the chromophoric effect of the
346 3,4-dihydroxyphenyl substituent (ESI Figs. S11 - S13†).
347 Batch RAFT Polymerization of the Dopa-CTAs: To investigate
348 the influence of the Dopa-CTAs over the growth of catechol
349 functionalized polyacrylamide (DPAM), RAFT polymerization of
350 acrylamide was carried out with a ratio of [AM]0:[Dopa-
351 CTA]0:[ACVA]0 = 2500:5:1 at 70oC for each of the synthesized
352 Dopa-CTAs (3a-c), with the results listed in Table 1. Due to
353 poor noise-to-catechol signal ratios as the DPAM Mw
354 increases, the number-average Mw values via NMR analysis
355 were only determined for 1hr DPAM samples and found to be
356 comparable with GPC measurements (ESI Table S1†). The RAFT
357 process was restricted to approximately 10 hr, since the
358 cumulative radical activity of ACVA in DMSO is known to drop
359 drastically beyond 10 hr.26 As seen in Fig. 2a, the number-
Figure 2. RAFT Polymerization of acrylamide mediated with Dopa-CTAs (3a-
c) using [M]0:[CTA]0:[I]0= 2500:5:1 ([M]0 = 2M), (a) evolution of molecular
weight (Mn) and dispersity with conversion (the theoretical Mn is
represented with broken line ---); and (b) pseudo-first order kinetics, where
P is AM conversion, P=1-[AM]/[AM]0). Note: the theoretical Mn line slightly
differs for each polymerization; however the lines overlap due to the
insignificant difference in the MW of the Dopa-CTAs (3a-c).
360 average molecular weights (Mn,GPC
)
of DPAMs
(
4a-c
)
379 the rate of propagation having non-steady state behaviour.
361 synthesized using the three Dopa-CTAs (3a-c) increase with
380 This non-linearity may be explained by the change in
362 increasing conversion of monomer AM while the dispersities
381 cumulative radical production from ACVA in DMSO at 70oC
363 (Đ) are very low, ≤ 1.21, showing the characteristics of
382 owing to its decay constant.26 Additional details on the
364 living/controlled polymerization. More so, increased molecular
383 cumulative radical production from ACVA in DMSO solvent as
365 weight was evidenced by the shift in the GPC DRI peaks toward
366 shorter retention times (ESI Fig. S14†). Nonetheless, the
367 number-average molecular weights (Mn,GPC) of the DPAM (4a-
368
c
) overshoot their predicted values (Mn,theo) with those of
a
Scheme 2b. R group radicals of the Dopa-CTAs (3)
384 related to its decomposition rate constant at 70oC can be
385 found in the literature.26 More so, as identified by Moad and
386 Barner-Kowollik,25 the causes of non-steady state
387 polymerization during the RAFT process include changing rate
388 coefficients with chain length, slow fragmentation of RAFT
389 adduct and large disparity in radical addition rates with respect
390 to monomer and CTA. Cognizant of these causes, we dislodged
391 the effect of the latter two by monitoring the rate of
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ARTICLE
Journal Name
392 propagation after the pre-equilibrium period (i.e after 1
393 indicative of when the initial Dopa-CTAs had been complet
394 consumed, Figure 2b) to address the steady state assumptio
395 of the propagating radicals [Pm·]. Overall, the Dopa-DDM
3a) RAFT agent appears to have the most preferred livin
397 characteristics based on its comparatively lower PDI valu
398 better linearity and lower extent of Mn overshoot (Fig. 2). T
399 is expected since the catechol R groups must be go
400 homolytic leaving groups and be capable of re-initiation, w
401 the ease of the former depending on the stability of th
4
4e2ly9
4
4A
43g2
4e3s3,
4h3is4
4o3d5
4i
h
2
r
8
,
is hardly seen in the 13C and gHMBC spectra in spite of an
extremely weak peak at 205 ppm which might be attributed to
it. Moreover, although there is no correlation 1H/13C peak of
carbon 3 of the Z' group in the 2D NMR spectra, this carbon
peak is clearly seen in the 13C NMR spectrum at 31.9 ppm (ESI
Fig. S16†). With dopamine group being chemically attached to
the end of polyacrylamide chains, it was expected that the
catechol moiety could induce chemisorption of the polymer
onto the γ-Al2O3 NP via covalent bonding or coordination
(mono- or bi-dentate bond).12,22 The catechol group acts as the
3
3
n
0
1
T
396
(
3th
3
6
7
4e
ir
402 corresponding expelled radicals (catechol R group derived).2443,485 adhesive moiety for mediating the nanocomposites formation
403 The expelled radicals for both Dopa-DDMAT (3a) and Do via the "grafting to" approach. The DPAM (4a) was selected for
404 CDSPA (3c) are tertiary, that of Dopa-DDMAT (3a) is stabilize anchoring to the pre-treated γ-Al2O3, since Dopa-DDMAT (3a
405 by two methyl groups and an electron donating carbo appeared to be the most preferred CTAs for mediating AM
406 carbon of amide group, while the other (3c derived) is l
407 stabilized owing to the electron withdrawing effect of th
408 cyano group on its radical carbon center (See Scheme 2b). T
409 expelled radical of Dopa-DoPAT (3b) is a secondary radic
410 stabilized by a methyl and an amide carbonyl. Steric effects44
4p3a9-
4
4
d
0
)
4n4y1l
app
4
e
4
ss
2
polymerization based on the estimated Ctr
and the
4
4
e
3
polymerization experiments. Fig. shows the ATR-FTIR
4
4
44a5l
h
4
e
4
spectrum of the dried γ-Al2O3-PAM PNC after extensive
washing, compared with those of the piranha-treated alumina
and DPAM (4a). While there is no significant peak in the
spectrum of the piranha-treated γ-Al2O3 in the range of 1000-
o6f
o
411 the catechol R groups were contributory to the stability44
7f
412 their corresponding expelled radicals.24
448 3500 cm-1, the synthesized DPAM (4a) shows strong amide
449 peaks at 3334 (asymmetric N-H stretching), 3188 (symmetric
413 Alumina-PAM Nanocomposite: The synthesized DPAM (4a
)
450 N-H stretching), 1652 (amide I C=O stretching), and 1606 cm-1
414 prepared via RAFT polymerization ([AM]0:[Dopa-CTA]0:[ACVA]0
415 = 2500:5:1 at 70oC; duration = 35 min) was characterized with
416 1D (1H and 13C) and 2D (gHSQC, gHMBC) NMR. For the 1D NMR
417 spectra, see ESI Figs. S15 - S16†). As shown in Fig. 3 (gHSQC
418 and gHMBC spectra), all the correlation H/13C peaks confirm
1
419 the peak assignments and the molecular structure of the
420 synthesized DPAM (4a). In addition to the major peaks (14, 16,
421 and 17) of the repeating unit of polyacrylamide, a few minor
422 peaks are present in the spectra of DPAM (4a). Peaks 1-12
423 suggest the presence of the Z' group (CH3-(CH2)11-) while peaks
424 19, 22-23, 25-26, and 29 indicate the presence of the
425 corresponding R group. The aromatic peaks 25, 26, and 29
426 confirm the catechol moiety in the synthesized DPAM. It
427 should be noted that the peak of trithiocarbonate carbon (13)
451 (amide II N-H deformation and C-N stretching) in addition to
452 three minor peaks at 2930, 1447, and 1414 cm-1 due to the C-H
453 stretching, CH2 bending, and C-N stretching vibrations,
454 respectively.46 The presence of these amide and C-H peaks in
455 the spectrum of the synthesized Al2O3-PAM PNC indicates
456 successful attachment of the DPAM to the Al2O3 NPs.
457 The attachment of DPAM on the surface of γ-Al2O3 NPs was
458 also confirmed by TGA and DLS. Fig. 5a compares the weight
459 loss versus temperature for piranha-treated Al2O3 NPs and
460 Al2O3-PAM nanocomposites prepared using DPAM of different
461 molecular weights and PAM (without catechol moiety, Mn,GPC
=
462 29600 Da) as a control. While the piranha-treated Al2O3 lost
463 1.7% of weight when being heated to 700 oC, the control
464 sample lost 5.7% of weight, indicating 4% of physically
465 absorbed PAM. The Al2O3-PAM nanocomposites prepared
466 using DPAM of different molecular weights (Mn = 26200,
6 | J. Name., 2012, 00, 1-3
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ARTICLE
467 33700, and 40700 Da) demonstrated significantly high wei groups bearing catechol polar ends provide good stability for
4
g
8
h
9t
468 losses of 23.0%, 58.9%, and 73.8%, respectively. The hig4h9er0 controlled polymerization.
469 sensitivity in TGA weight loss with increased MW may be due
470 to the shorter polymer chains having enhanced interactions
491 Conclusions
471 with the alumina NPs. The hydrodynamic size of the PNCs was
492 In order to produce stable tethering to metal oxide
493 nanoparticles, three novel catechol end trithiocarbonate CTAs
494 (Dopa-CTAs) which differ in their carbonyl α-substituents were
495 synthesized and their molecular structures were confirmed by
496 NMR, UV-Vis and ATR-FTIR. The Dopa-CTAs were all found to
497 mediate homo-polymerization of acrylamide in a controlled
498 and quantitative fashion, and the Dopa-DDMAT was found to
499 be the most preferred based on the livingness characteristics
500 and its structural constituents. As evident from the binding
501 studies with γ-Al2O3 NPs, catechol end-group CTAs provide a
502 suitable route for end-functionalization of PAM to allow post-
503 modification chemistry aimed at synthesizing PNCs.
504 Acknowledgements
505 The authors acknowledge Polyanalytik Inc. Ontario for
506 providing the PEO standards used for calibrating the GPC
507 instrument. We also thank Prof. John R. de Bruyn (Western
508 University) and his group for granting us access to use the
509 refractometer in their facility. Funding was provided by
510 Canada's National Science & Engineering Research Council
511 (NSERC) Discovery program.
512 Notes and references
513 1.
514
515 2.
516
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474 cummulant method was used as a criterion for comparison
525
475 because it is numerically stable and less sensitive to noise
526 7.
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527
h
477 Al2O3, Al2O3-PAM (26200 Da), Al2O3-PAM (33700 Da) a
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532
480 parameter ≤ 0.3 (Fig. 5b). Comparison of the hydrodynamic
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487 a suitable route for end-functionalization of PAM for post-
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