Versatile synthesis of multiarm and miktoarm star polymers with a branched core by combination of menschutkin reaction and controlled polymerization

Article abstract of DOI:10.1021/ma301973v

Menschutkin reaction and controlled polymerization were combined to construct three types of star polymers with a branched core. Branched PVD was synthesized by reversible addition-fragmentation chain transfer (RAFT) copolymerization and used as a core reagent to synthesize multiarm and miktoarm stars with poly(ε-caprolactone) (PCL), polystyrene, poly(methyl methacrylate), poly(tert-butyl acrylate), and poly(N-isopropylacrylamide) segments. Effects of reaction time, feed ratio, and arm length on coupling reaction between PVD and bromide-functionalized polymer were investigated, and a variety of A_m-type stars (m ≈ 7.0-35.1) were obtained. Meanwhile, A_mB_n stars (m ≈ 9.0, n ≈ 6.1-11.3) were achieved by successive Menschutkin reactions, and A_mC_o stars (m ≈ 8.8-9.0, o ≈ 5.0) were generated by tandem quaternization and RAFT processes. Molecular weights of various stars usually agreed well with the theoretical values, and their polydispersity indices were in the range of 1.06-1.24. The arm number, chain length, and chemical composition of star polymers could be roughly adjusted by control over reaction conditions and utilization of alternative methods, revealing the generality and versatility of these approaches. These ion-bearing stars were liable to exhibit solubility different from normal covalently bonded polymers, and the chain relaxation and melting behaviors of polymer segments were strongly dependent on the macromolecular architecture.

Full text of DOI:10.1021/ma301973v

Article
pubs.acs.org/Macromolecules
Versatile Synthesis of Multiarm and Miktoarm Star Polymers with a
Branched Core by Combination of Menschutkin Reaction and
Controlled Polymerization
Meijing Zhang, Huanhuan Liu, Wei Shao, Chunnuan Ye, and Youliang Zhao*
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering,
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
S
* Supporting Information
ABSTRACT: Menschutkin reaction and controlled polymer-
ization were combined to construct three types of star
polymers with a branched core. Branched PVD was
synthesized by reversible addition−fragmentation chain trans-
fer (RAFT) copolymerization and used as a core reagent to
synthesize multiarm and miktoarm stars with poly(ε-
caprolactone) (PCL), polystyrene, poly(methyl methacrylate),
poly(tert-butyl acrylate), and poly(N-isopropylacrylamide)
segments. Effects of reaction time, feed ratio, and arm length
on coupling reaction between PVD and bromide-function-
alized polymer were investigated, and a variety of Am-type stars (m ≈ 7.0−35.1) were obtained. Meanwhile, AmBn stars (m ≈ 9.0,
n ≈ 6.1−11.3) were achieved by successive Menschutkin reactions, and AmCo stars (m ≈ 8.8−9.0, o ≈ 5.0) were generated by
tandem quaternization and RAFT processes. Molecular weights of various stars usually agreed well with the theoretical values,
and their polydispersity indices were in the range of 1.06−1.24. The arm number, chain length, and chemical composition of star
polymers could be roughly adjusted by control over reaction conditions and utilization of alternative methods, revealing the
generality and versatility of these approaches. These ion-bearing stars were liable to exhibit solubility different from normal
covalently bonded polymers, and the chain relaxation and melting behaviors of polymer segments were strongly dependent on
the macromolecular architecture.
and the “coupling onto” method sometimes suffers from
inefficient coupling reactions between polymer segments and
functional core.21−27 To further address these limitations, it is
necessary to explore new synthetic methodology to construct
star polymers, which potentially plays a crucial role in
promoting the development of polymer and materials science.
Meanwhile, self-condensing vinyl polymerization (SCVP)58
has been extended to controlled radical processes such as
nitroxide-mediated polymerization (NMP),59,60 atom transfer
radical polymerization (ATRP),61−65 and reversible addition−
fragmentation chain transfer (RAFT) polymerization66−75 to
prepare hyperbranched polymers with variable molecular
parameters. More importantly, part of branched polymers can
act as functional initiator or chain transfer agent (CTA) to
generate multiarm stars.53−55,76−79 For instance, RAFT
copolymerization via self-condensing vinyl polymerization
(RAFT SCVP) was used by us to synthesize multifunctional
branched copolymers and followed by chain extension
polymerization to prepare multiarm stars with predetermined
arm number and controlled arm length.78 More recently, Gao
and co-workers have successfully combined RAFT SCVP,
INTRODUCTION
■
Synthesis and properties of complex macromolecular archi-
tectures such as multiarm and miktoarm stars have attracted
much attention in the past decades due to their unique
topologies and physicochemical properties.1−20 Owing to their
compact structure and variable functionality, star polymers
usually exhibit bulk, solution, and interface properties different
from their linear analogues and have potential applications in
many fields.21−34 Moreover, miktoarm stars with at least two
kinds of polymer arms are liable to form higher order multiscale
self-assemblies involving multicompartment micelles originat-
ing from their branched architectures and heterophase
structures, and thus they can hold great promise as next-
generation advanced functional polymers.35−46
To date, some approaches involving “arm first”,47−52 “core
first”,53−55 and “coupling onto”56,57 methods based on linking
reactions and living/controlled polymerization techniques have
been efficiently used to construct numerous well-defined star
polymers although all of them have more or less disadvantages.
Generally speaking, the “arm first” approach using cross-linker
is liable to afford star polymers with poor structural
homogeneity and broadened molecular weight distribution in
which the formed core is of rather complex structure, the “core
first” approach to generate regular stars usually needs stopping
the reaction at suitable conversion to avoid star−star coupling,
Received: September 19, 2012
Revised: November 6, 2012
Published: November 16, 2012
© 2012 American Chemical Society
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Scheme 1. Synthetic Routes to Various Multiarm and Miktoarm Stars by Combination of Menschutkin Reaction and Controlled
Polymerization
Menschutkin reaction, and Cu(I)-catalyzed azide−alkyne
cycloaddition reaction to synthesize hyperbranched macro-
initiators, dendritic polymer brushes, and star-shaped poly-
mers.79 The Menschutkin reaction between alkyl halide and
amine is liable to quantitatively yield a quaternary ammonium
salt,80 which has been efficiently used to construct ion-
containing linear polymers, conjugated polymers, block
copolymers, branched, and dendritic polymers besides its
wide application in organic synthesis.79,81−86 This SN2 reaction
offering a facile method to N−C bond formation can be
accelerated by polar aprotic solvents, increased temperature and
pressure, and enhanced leaving-group ability, in which initial
polymer, alkylating agent, and modified polymer had better
dissolve in the solvent. To our knowledge, Menschutkin
reaction between polymers has been scarcely investigated
although it has a great potential in novel architecture
construction. Therefore, it is very timely to extend such
method to construct functional multiarm and miktoarm stars
with controlled compositions and precise microstructures.
The marriage of RAFT SCVP and Menschutkin reaction
potentially opens an avenue to address the synthetic difficulty
of multiarm and multicomponent stars. Herein we report on
facile synthesis of three types of novel multiarm and miktoarm
stars with a branched core by this combinatorial approach, in
which polymeric chains were grafted onto the branched scaffold
via Menschutkin reaction to construct complex macromolecular
architectures for the first time (Scheme 1). RAFT copoly-
merization of 4-vinylbenzyl dithiobenzoate (VBDB)75 and N,N-
dimethylaminoethyl methacrylate (DMA) was first performed
to generate branched poly(VBDB-co-DMA) (abbreviated as
PVD) with reactive dithiobenzoate moieties, couplable tertiary
amino groups, and relatively low polydispersity indices. On this
basis, Am- and AmBn-type stars (in which the subscript meant
the average number of arms) were obtained by Menschutkin
reaction(s) between branched PVD and as-prepared bromide-
terminated polymers, and AmCo-type miktoarm stars were
generated by tandem quaternization process and RAFT
polymerization. The isolated stars and their precursors were
1
characterized by H and 13C NMR, GPC, GPC-MALLS, IR,
and DSC. These approaches allow for versatile synthesis of the
target ion-bearing stars with tunable chemical composition,
well-defined arm length, and roughly controlled number of
arms due to relatively high grafting efficiency and mild reaction
conditions.
EXPERIMENTAL SECTION
■
Materials. All solvents, monomers, and other chemicals were
purchased from Sigma-Aldrich unless otherwise stated. Methyl
methacrylate (MMA, 99%), tert-butyl acrylate (tBA, 98%), N,N-
dimethylaminoethyl methacrylate (DMA, 99%), and styrene (St, 99%)
were passed through a basic alumina column to remove the inhibitor,
and ε-caprolactone (CL, 99%) was distilled over CaH2 under reduced
pressure before use. N-Isopropylacrylamide (NIPAM, 97%) was
recrystallized twice from mixtures of hexane and toluene. 2,2′-
Azobis(isobutyronitrile) (AIBN, 99%) was recrystallized twice from
ethanol. Dichloromethane (DCM) and dioxane were dried and
distilled over CaH2, toluene was distilled over sodium and
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Table 1. Results for Synthesis of PVD Branched Copolymers by RAFT Copolymerization of VBDB and DMA in Dioxane at 80
a
°C
b
c
c
d
e
f
g
g
run
DP0
t (h)
C (%)
Mn,MALLS
PDI
RB(th)
RB
P1
Fn
Fw
1
2
3
4
5
10
30
20
12
12
12
20
71.6
66.3
72.4
46.2
23 000
41 600
57 500
8 630
4.96
4.86
3.24
1.32
4.58
7.63
22.7
10.2
6.54
9.78
0.30
0.22
0.28
0.20
27.4
31.7
15.6
5.0
136
154
31.5
12.0
50.6
6.6
a
Polymerization conditions: [DMA]0:[VBDB]0:[AIBN]0 = DP0:1:0.2, [DMA]0 = 3.0 mol/L (runs 1−3); [DMA]0:[VBDB]0:[AIBN]0 = DP0:1:0.1,
b
c
[DMA]0 = 0.60 mol/L (run 4). DMA conversion determined by 1H NMR. Number-average molecular weight and polydispersity determined by
GPC-MALLS. Theoretical repeat units per branch calculated by equation RB(th) = DP0 × C% + 1. Repeat units per branch determined by H
NMR, RB = (I7.98 + I4.06)/(I7.98 − I4.53), where I meant peak area at various chemical shifts. Proportion of dithiobenzoate functionality connecting
with benzyl moiety. Number-average (Fn) and weight-average (Fw) CTA functionalities per branched copolymer.
d
e
1
f
g
benzophenone, and N,N-dimethylformamide (DMF) was dried over
MgSO4 and distilled under reduced pressure. 4-Vinylbenzyl chloride
(90%) and tin(II) 2-ethylhexanoate (Sn(Oct)2, 95%) were used as
received. N,N′-Dicyclohexylcarbodiimide (DCC), 4-dimethylamino-
pyridine (DMAP), and other chemicals with analytical grade were
purchased from Sinopharm Chemical Reagent and used as received. 4-
Vinylbenzyl dithiobenzoate (VBDB),75 4-cyanopentanoic acid-4-
dithiobenzoate (4-CPDB),87 and S-(4-trimethoxysilyl)benzyl S′-
propyltrithiocarbonate (TBPT)88 were synthesized and purified
according to literature methods. Silica gel (Qingdao Haiyang Chemical
Co., Ltd.) had an average particle size of 10 μm, BET specific surface
area of 297.1 m2/g, and average pore size of 11.5 nm.
Synthesis of 3-Bromopropyl 4-(Benzodithioyl)-4-cyanopen-
tanoate (BBCP). To a round flask were added 3-bromo-1-propanol
(1.53 g, 11.0 mmol), 4-CPDB (2.80 g, 10.0 mmol), DMAP (0.122 g,
1.0 mmol), and dry DCM (50 mL), and then 20 mL of DCM solution
containing 2.50 g (12.1 mmol) of DCC was slowly added to the above
solution cooled with ice−water bath under nitrogen. The contents
were further stirred at ambient temperature for 18 h. After filtration,
the filtrate was partitioned between water and DCM, and the aqueous
layer was extracted twice with DCM. The combined extracts were
dried over MgSO4 overnight. The crude product was purified by flash
column chromatography eluting with hexane/DCM (1:1, v/v), and
3.76 g (93.9% yield) of BBCP was obtained. 1H NMR (CDCl3): δ 7.90
(d, J 8.4, PhH, 2H), 7.57 (t, J 6.8, PhH, 1H), 7.40 (t, J 9.6, PhH, 2H),
4.27 (t, J 7.6, CH2O, 2H), 3.47 (t, J 8.4, CH2Br, 2H), 2.67 (m, CH2,
2H), 2.60 and 2.44 (m, CH2, each 1H), 2.20 (m, CH2, 2H), 1.94 (s,
CH3, 3H). 13C NMR (CDCl3): δ 222.1 (CS), 171.2 (CO), 144.2,
132.9, 128.4, 126.5 (PhC), 118.3 (CN), 62.6 (CH2O), 45.5
(CH3CCN), 33.1, 31.3, 29.5, 29.3 (CH2), 23.9 (CH3). FT-IR (KBr):
3455, 3078, 3058, 3028, 2962, 2932, 2856, 2230, 1739, 1590, 1513,
1483, 1445, 1421, 1393, 1293, 1233, 1183, 1108, 1081, 1046, 1026,
999, 926, 868, 762, 688, 650, 617, 576 cm−1. Elem. Anal. Calcd for
C16H18BrNO2S2: C, 48.00%; H, 4.53%; N, 3.50%; S, 16.02%. Found:
C, 48.12%; H, 4.55%; N, 3.48%; S, 15.96%.
unit), 44.5 (CH2S, CHS and CH3CS), 44.2 (CH3CCOO of DMA
unit), 25.9−37.4 (CH2), 18.2, 16.4 (CH3 of DMA unit). FT-IR (KBr):
3431, 2947, 2860, 2821, 2770, 1730, 1655, 1638, 1560, 1542, 1508,
1459, 1400, 1364, 1272, 1150, 1100, 1058, 1042, 1017, 965, 853, 779,
748, 689 cm−1.
Synthesis of PDMA Grafted Silica Particles. To a round flask
were added S-(4-trimethoxysilyl)benzyl S′-propyltrithiocarbonate
(TBPT, 10.9 g, 30 mmol), silica particles (15.0 g), and dry toluene
(150 mL), and the mixture was stirred at 85 °C overnight. After
filtration, the crude product was thoroughly washed with THF (100
mL × 3), toluene (100 mL × 3), and acetone (100 mL × 2). After
vacuum drying, TBPT tethered silica (Si-TBPT, 17.5 g) was obtained
as a yellow powder. The CTA loading was determined to be 0.541
mmol/g by elemental analysis (based on sulfur content of 5.20%).
Si-TBPT (10.0 g, 5.41 mmol), DMA (51.2 g, 326 mmol), AIBN
(0.178 g, 1.09 mmol), and dioxane (100 mL) were added to a 250 mL
of round flask under nitrogen. The contents were cooled with ice−
water bath and degassed with bubbled nitrogen for 1 h. The RAFT
graft polymerization was performed at 60 °C for 20 h. After
concentration and precipitation into a large amount of cold hexane,
monomer conversion was determined to be 83.2% by gravimetry. The
mixture was dispersed into 150 mL of THF and filtered, and crude
product was collected and thoroughly washed with THF. After vacuum
drying, 14.1 g of SiO2-g-PDMA was obtained. The weight ratio of
grafted PDMA to silica particles determined by TGA was 42.5%,
corresponding to 1.68 mmol of DMA unit per g of SiO2-g-PDMA.
Synthesis of Bromide-Terminated Polymers. In a typical
polymerization (run 1 of Table S1), St (10.4 g, 100 mmol), BBCP
(0.40 g, 1.0 mmol), and AIBN (33 mg, 0.2 mmol) were added to a
Schlenk tube with a magnetic stirring bar, and the contents were
degassed with bubbled nitrogen for 20 min. The polymerization was
performed at 60 °C for 16 h, and monomer conversion was
1
determined to be 21.6% by H NMR analysis. The polymerization
solution was concentrated and precipitated into cold hexane thrice,
and 2.55 g of PSt-Br was obtained. Number-average molecular weight
and polydispersity estimated by GPC were Mn,GPC = 2370 and PDI =
1.10. Other bromide-functionalized polymers were synthesized and
isolated according to a similar approach, in which PCL-Br was
synthesized by CL polymerization initiated with 3-bromo-1-propanol
using Sn(Oct)2 catalyst.
Synthesis of Hyperbranched Copolymers by RAFT Copoly-
merization of VBDB with DMA. In a typical experiment (run 4 of
Table 1), DMA (15.7 g, 100 mmol), VBDB (1.35 g, 5.0 mmol), AIBN
(82 mg, 0.5 mmol), and 150 mL of dioxane were added to a round
flask with a magnetic stirring bar, and the contents were degassed with
bubbled nitrogen for 1 h. The polymerization was performed at 80 °C
1
1
PSt-Br. H NMR (CDCl3): δ 6.2−8.0 (PhH), 4.6−5.0 (broad, CH
for 20 h, and DMA conversion was determined to be 46.2% by H
of terminal St unit), 4.17 (CH2O), 3.42 (CH2Br), 2.61
(CH2CH2CH2Br), 0.6−2.4 (CH2CO, CH and CH2 of PSt). 13C
NMR (DMSO-d6): δ 222.7 (CS), 171.8 (CO), 145.1, 128.6,
128.0, 127.3, 126.5, 125.7 (PhC), 118.2 (CN), 62.1
(BrCH2CH2CH2O), 43.8 (CH2 of PSt and CHS), 36.5, 35.6, 35.0,
34.3, 33.3, 32.8 (CH of PSt and CH2CH2COO), 31.3 (CH2Br), 31.0
(CH2CH2Br), 29.3 (CH2COO), 23.4 (CH3), 22.6 (CH3CCN). FT-IR
(KBr): 3082, 3059, 3025, 3003, 2922, 2849, 1943, 1870, 1802, 1740,
1601, 1583, 1493, 1452, 1371, 1180, 1155, 1069, 1045, 1028, 907, 876,
841, 758, 698 cm−1.
NMR analysis. The polymerization solution was concentrated and
precipitated into cold hexane thrice, and 8.35 g of branched PVD was
obtained. Number-average molecular weight and polydispersity
determined by GPC-MALLS were Mn,MALLS = 8630 and PDI = 1.32.
Other branched copolymers were synthesized and isolated according
to a similar approach. 1H NMR (CDCl3): δ 7.98 (PhH), 6.7−7.7
(PhH and ArH), 5.0−5.5 (CHS), 4.53 (CH2S), 4.24 (CH2O, terminal
DMA unit), 4.06 (CH2O, DMA unit), 3.30 (CH2, terminal DMA
unit), 2.57 (CH2N, DMA unit), 2.28 ((CH3)2N, DMA unit), 0.5−2.2
(CH, CH2 and CH3). 13C NMR (DMSO-d6): δ 222.7 (CS), 176.8,
176.5, 175.8 (CO), 145.0, 144.4, 136.2, 135.0, 133.9, 130.7, 129.1,
128.9, 128.2, 126.7 (ArC and PhC), 62.4 (CH2O of DMA unit), 56.6
(CH2N of DMA unit), 53.6 (CH2 of DMA unit), 45.1 (CH3N of DMA
1
PMMA-Br. H NMR (CDCl3): δ 7.90, 7.53, and 7.37 (m, PhH),
4.25 (CH2O), 3.60 (CH3O of PMMA), 3.40 (CH2Br), 2.51
(CH2CH2CH2Br), 2.20 (CH2CO), 0.5−2.1 (CH2 and CH3 of
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Table 2. Results for Synthesis of Multiarm Am-Type Stars with a Branched Core by Menschutkin Reaction between PVD and
a
PM-Br
b
c
d
e
f
f
g
h
i
run
arm
PSt20.0
r0
t (h)
Y (%)
Mn,th
Mn,MALLS
PDI
Mn,NMR
m
CE (%)
1
0.80
0.80
0.80
0.80
0.80
0.80
0.80
0.20
0.50
1.0
3.5
8
23.6
40.2
50.5
59.8
33.5
22.4
19.2
76.8
45.5
28.3
52.4
36.6
90.5
95.6
92.8
30 200
45 300
54 700
63 200
64 400
60 200
76 500
40 600
55 900
67 500
64 500
76 600
71 700
23 500
31 100
46 600
56 900
66 900
63 800
66 300
76 900
40 100
56 200
66 400
66 800
81 100
76 500
23 800
67 900
1.21
1.23
1.19
1.18
1.20
1.16
1.11
1.24
1.16
1.22
1.10
1.13
1.24
1.11
1.20
30 800
46 900
56 600
65 300
64 900
64 400
75 200
40 900
53 300
67 200
66 200
79 500
75 600
23 900
69 300
9.1
15.3
19.5
23.5
12.2
9.2
24.6
41.6
52.9
63.8
33.2
25.1
19.3
75.7
45.8
27.8
54.6
39.0
97.4
97.6
95.3
2
PSt20.0
PSt20.0
PSt20.0
PSt39.6
PSt56.2
PSt88.3
PSt39.6
PSt39.6
PSt39.6
3
20
40
40
40
40
40
40
40
40
40
40
40
40
4
5
6
7
7.1
8
7.0
9
10.5
12.8
15.1
14.4
22.4
9.0
10
11
12
13
14
15
PMMA34.6
PtBA36.3
0.60
0.80
0.50
0.20
0.80
PNIPAM23.3
PCL13.6
PCL13.6
66 300
35.1
a
b
1
Reaction conditions: Wpolymer:VDMF = 0.20 g/mL, in DMF at 60 °C. The subscript denoted polymerization degree of PM-Br determined by H
c
d
e
NMR. Molar ratio of PM-Br to DMA unit of PVD. Yield of isolated star polymer. Theoretical molecular weight, Mn,th = Mn,PVD + 46Mn,arm × yield,
in which 46 was average number of DMA unit per branched PVD, and Mn,PVD and Mn,arm were molecular weights of PVD and PM-Br. Number-
average molecular weight and polydispersity of star polymer determined by GPC-MALLS. Number-average molecular weight determined by H
NMR. Average arm number of PM segments grafted onto the surface of PVD. Coupling efficiency of Menschutkin reaction.
f
g
1
h
i
PMMA). FT-IR (KBr): 2996, 2951, 2844, 1732, 1485, 1449, 1387,
1272, 1242, 1193, 1149, 1063, 988, 966, 912, 842, 810, 750, 695 cm−1.
PtBA-Br. 1H NMR (CDCl3): δ 7.97, 7.53, and 7.38 (m, PhH), 4.64
(CH of terminal tBA unit), 4.24 (CH2O), 3.47 (CH2Br), 2.51
(CH2CH2CH2Br), 2.23 (CH of PtBA and CH2CO), 0.5−2.0 (CH2
and CH3 of PtBA). FT-IR (KBr): 2979, 2934, 2876, 1728, 1481, 1458,
1394, 1368, 1258, 1147, 1033, 909, 846, 753, 689 cm−1.
1663, 1601, 1582, 1493, 1452, 1385, 1177, 1153, 1028, 908, 842, 758,
699 cm−1.
1
PMMA Star. H NMR (CDCl3): δ 7.85 (PhH), 6.2−7.8 (PhH and
ArH), 5.35 (CHS), 4.58 (CH2O of reacted DMA unit), 4.53 (CH2S),
4.21 (CH2O of DMA unit and CH2OCO of PMMA segment), 3.60
(OCH2CH2N+(CH3)2CH2 and CH3O of PMMA), 3.15
(OCH2CH2N+(CH3)2CH2, and CH2 of terminal DMA unit), 0.5−
2.9 (CH2N and CH3 of DMA unit, and CH, CH2 and CH3 of PMMA
and branched core). FT-IR (KBr): 3448, 2998, 2951, 2851, 1730,
1484, 1450, 1389, 1274, 1243, 1196, 1148, 1061, 987, 911, 841, 750,
690 cm−1.
1
PNIPAM-Br. H NMR (CDCl3): δ 7.97, 7.58, and 7.41 (m, PhH),
6.0−7.2 (broad, NH of PNIPAM), 4.24 (CH2O), 4.01 (CH of
PNIPAM), 3.47 (CH2Br), 2.80 (CHCO of PNIPAM), 2.50
(CH2CH2CH2Br), 0.5−2.4 (CH2CO, CH2 and CH3 of PNIPAM).
FT-IR (KBr): 3432, 3300, 3074, 2973, 2934, 2876, 1725, 1652, 1545,
1459, 1387, 1368, 1326, 1275, 1236, 1173, 1131, 1047, 984, 927, 880,
840, 764, 688 cm−1.
1
PtBA Star. H NMR (CDCl3): δ 7.96, 7.53, and 7.37 (PhH), 4.63
(CH2O of reacted DMA unit, CHS of PtBA and CH2S), 4.26 and 4.19
(CH2O of DMA unit and CH2OCO of PtBA segment), 3.62 and 3.33
(OCH2CH2N+(CH3)2CH2, and CH2 of terminal DMA unit), 0.5−3.0
(CH2N and CH3 of DMA unit, and CH, CH2 and CH3 of PtBA and
branched core). FT-IR (KBr): 3436, 3003, 2979, 2934, 2873, 1729,
1481, 1458, 1394, 1368, 1258, 1149, 1034, 909, 846, 752, 690 cm−1.
PNIPAM Star. 1H NMR (CDCl3): δ 7.96, 7.56, and 7.40 (m, PhH),
6.0−7.2 (broad, NH of PNIPAM), 4.58 (CH2O of reacted DMA unit
and CH2S), 4.24 (CH2O of DMA unit and PNIPAM segment), 4.01
(CH of PNIPAM), 3.62 and 3.18 (OCH2CH2N+(CH3)2CH2, and CH2
of terminal DMA unit), 0.5−3.0 (CH2N and CH3 of DMA unit, and
CH, CH2 and CH3 of PNIPAM and branched core). FT-IR (KBr):
3436, 3073, 2973, 2933, 2876, 1735, 1655, 1545, 1459, 1388, 1368,
1264, 1173, 1131, 1036, 913, 840, 691 cm−1.
PCL-Br. 1H NMR (CDCl3): δ 4.24 (t, J 5.1, BrCH2CH2CH2O), 4.06
(s, CH2O of PCL), 3.65 (t, J 6.3, CH2OH), 3.47 (t, J 6.3, CH2Br), 2.31
(s, CH2CO of PCL), 2.18 (t, J 6.0, BrCH2CH2CH2O), 1.65 (m,
CH2CH2CH2CH2CH2O of PCL), 1.38 (m, CH2CH2CH2CH2CH2O
of PCL). 13C NMR (DMSO-d6): δ 172.7 (CO), 63.5 (CH2O of
PCL), 61.8 (BrCH2CH2CH2O), 60.6 (CH2OH), 33.4 (CH2COO of
PCL), 32.3 (CH2CH2OH), 31.3 (CH2Br), 30.9 (CH2CH2Br), 27.9
(CH2CH2CH2CH2CO of PCL), 25.0 (CH2CH2CH2CH2CO of PCL),
24.5 (CH2CH2CH2OH), 24.2 (CH2CH2CH2CH2CO of PCL). FT-IR
(KBr): 3437, 2945, 2865, 1724, 1471, 1437, 1420, 1398, 1368, 1296,
1246, 1191, 1108, 1066, 1047, 962, 933, 841, 772, 732, 710 cm−1.
Synthesis of Am-Type Multiarm Stars with a Branched Core
by Menschutkin Reaction. In a typical experiment (run 4 of Table
1
PCL Star. H NMR (CDCl3): δ 7.95 (PhH), 6.2−7.8 (PhH and
2), branched PVD (50 mg, 0.267 mmol DMA unit), PSt-Br (Mn,NMR
=
ArH), 5.35 (CHS), 4.58 (CH2O of reacted DMA unit and CH2S), 4.21
(CH2O of DMA unit, and CH2OCO of PCL segment), 4.06 (CH2O
of PCL), 3.64 and 3.42 (OCH2CH2N+(CH3)2CH2, and CH2 of
terminal DMA unit), 0.5−2.7 (CH2N and CH3 of DMA unit, and CH,
CH2 and CH3 of PCL and branched core). 13C NMR (DMSO-d6): δ
222.5 (CS), 177.2, 176.7, 176.2, 172.9, 172.8 (CO), 132.2, 124.6
(ArC and PhC), 63.5 (CH2O of PCL and DMA unit), 60.8
(CH2N+(CH3)2CH2CH2CH2OCO), 60.6 (CH2OH), 60.3
(CH2N+(CH3)2CH2CH2CH2OCO), 56.5 (CH2N of DMA unit),
53.2 (CH2 of DMA unit), 50.6 (CH2N+(CH3)2CH2CH2CH2OCO),
45.3 (CH3N of DMA unit), 44.3 (CH3CCOO of DMA unit, CH2S,
CHS and CH3CS), 33.4 (CH2COO of PCL), 32.2 (CH2CH2OH),
27.9 (CH2CH2CH2CH2CO of PCL), 25.0 (CH2CH2CH2CH2CO of
PCL), 24.5 (CH2CH2CH2OH), 24.2 (CH2CH2CH2CH2CO of PCL),
22.4, 22.1, 21.8 (CH2N+(CH3)2CH2CH2CH2OCO), 18.6, 16.7 (CH3
2480, 0.53 g, 0.214 mmol), and DMF (2.9 mL) were added to a glass
tube under nitrogen and stirred at 60 °C for 40 h. After that, to the
mixture was added 0.50 g of SiO2-g-PDMA, and the contents were
further reacted at 60 °C for 20 h. PSt star (0.367 g, 59.8% yield) was
recovered by filtration, concentration, and precipitation into cold
methanol. GPC-MALLS: Mn,MALLS = 66 900, PDI = 1.18. Other
multiarm stars were synthesized and isolated according to a similar
approach.
1
PSt Star. H NMR (CDCl3): δ 7.85 (PhH), 6.2−7.6 (PhH and
ArH), 5.35 (CHS), 4.67 (CH2O of reacted DMA unit), 4.53 (CH2S),
4.06 (CH2O of DMA unit and CH2OCO of PSt segment), 3.64
(OCH2CH2N+(CH3)2CH2), 3.30 (OCH2CH2N+(CH3)2CH2, and
CH2 of terminal DMA unit), 0.5−2.7 (CH2N and CH3 of DMA
unit, and CH, CH2 and CH3 of PSt and branched core). FT-IR (KBr):
3423, 3082, 3059, 3025, 3001, 2923, 2850, 1944, 1871, 1806, 1732,
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of DMA unit). FT-IR (KBr): 3437, 2945, 2866, 1727, 1471, 1420,
1396, 1367, 1296, 1245, 1190, 1164, 1106, 1046, 962, 732 cm−1.
Synthesis of AmBn Miktoarm Stars by Successive Menschut-
kin Reaction. To a glass tube were added branched PVD (0.50 g,
2.67 mmol DMA unit), PCL-Br (Mn,NMR = 1690, 0.90 g, 0.53 mmol),
and DMF (7.0 mL), and the contents were reacted at 60 °C for 40 h
under nitrogen. PCL star (1.36 g, 95.6% yield) was recovered by
concentration and precipitation into cold hexane. GPC-MALLS:
Mn,MALLS = 23 800, PDI = 1.11.
The mixture of PCL star (100 mg, 0.155 mmol of DMA unit), PSt-
Br (Mn,NMR = 2480, 193 mg, 0.078 mmol), and DMF (1.5 mL) was
stirred at 60 °C for 40 h under nitrogen and followed by addition of
0.35 g of SiO2-g-PDMA and further stirring at 60 °C for 20 h.
(PCL)m(PSt)n star (183 mg, 43.0% yield) was recovered by filtration,
concentration, and precipitation into cold hexane. GPC-MALLS:
Mn,MALLS = 45 700, PDI = 1.13. Other miktoarm stars were synthesized
and purified according to a similar approach.
magnetic stirring bar was sealed with a rubber septum, and the
contents were degassed with bubbled nitrogen for 20 min and
polymerized at 60 °C for 24 h. Monomer conversion was determined
to be 22.4% by 1H NMR analysis of polymer solution. After
concentration and precipitation into cold hexane, 0.52 g of
(PCL)m(PSt)n star was obtained. GPC-MALLS: Mn,MALLS = 48 100,
PDI = 1.11. Other star copolymers were synthesized and purified
according to a similar approach.
Characterization. The apparent number-average molecular weight
(Mn,GPC) and polydispersity index (PDI) of homopolymers were
measured on a Waters 150-C gel permeation chromatography using
three Ultrastyragel columns (pore size 50, 100, and 1000 nm, with
molecular weight ranges of 100−10 000, 500−30 000, and 5000−600
000, respectively) with 10 μm bead size at 35 °C. THF was used as an
eluent at a flow rate of 1.0 mL/min, polystyrene samples were
calibrated with PSt standard samples, and other samples were
calibrated using PMMA standard samples. Gel permeation chromatog-
raphy with multiple angle laser scattering detection (GPC-MALLS)
systems was used to determine the absolute number-average molecular
weight (Mn,MALLS) of branched and star polymers, in which GPC was
conducted in DMF at 40 °C with a flow rate of 1.0 mL/min, and three
MZ-Gel SDplus columns (pore size 103, 104 and 105 Å, with molecular
weight ranges of 1000−40 000, 4000−500 000, and 10 000−2 000 000,
respectively) with 10 μm bead size were used. Detection systems
consisted of a RI detector (Optilab rEX) and a multiangle (14°−145°)
laser light scattering (MALLS) detector (DAWN HELLOS) with the
He−Ne light wavelength at 658.0 nm. The refractive index increment
dn/dc for samples were measured off-line by Optilab rEX refractive
index detector (λ = 658 nm) at 25 °C using a series of different
concentration solutions, and the data were collected and processed by
(PCL)m(PSt)n Star. 1H NMR (CDCl3): δ 7.96 and 7.85 (PhH), 6.2−
7.7 (PhH and ArH), 5.35 (CHS), 4.64 (CH2O of reacted DMA unit
and CH2S), 4.23 and 4.18 (CH2O of DMA unit, and CH2OCO of
PCL and PSt segments), 4.06 (CH2O of PCL), 3.63 and 3.37
(OCH2CH2N+(CH3)2CH2, and CH2 of terminal DMA unit), 0.5−3.0
(CH2N and CH3 of DMA unit, and CH, CH2 and CH3 of PCL, PSt
and branched core). 13C NMR (DMSO-d6): δ 222.6 (CS), 177.3,
176.9, 176.4, 176.1, 175.6, 174.4, 172.8, 171.8 (CO), 145.2, 128.6,
128.0, 127.4, 126.6, 125.7 (ArC and PhC), 118.1 (CN), 63.5 (CH2O of
PCL and DMA unit), 61.5, 61.1, 60.8 (CH2N+(CH3)2CH2CH2CH2-
OCO), 60.6 (CH2OH), 60.2 (CH2N+(CH3)2CH2CH2CH2OCO),
55.8 (CH2N of DMA unit), 53.5 (CH2 of DMA unit), 50.6
(CH2N+(CH3)2CH2CH2CH2OCO), 45.2 (CH3N of DMA unit),
44.5 (CH3CCOO of DMA unit, CH2 of PSt, CH2S, CHS and
CH3CS), 35.5, 35.2, 35.0, 34.4 (CH of PSt), 33.4 (CH2COO of PCL),
32.3 (CH2CH2OH), 29.2 (CH2COO), 27.9 (CH2CH2CH2CH2CO of
PCL), 25.0 (CH2CH2CH2CH2CO of PCL), 24.5 (CH2CH2CH2OH),
24.2 (CH2CH2CH2CH2CO of PCL), 22.4, 22.1, 21.8
(CH2N+(CH3)2CH2CH2CH2OCO), 18.1, 16.4 (CH3 of DMA unit).
FT-IR (KBr): 3437, 3060, 3027, 2962, 2929, 2855, 1733, 1659, 1630,
1601, 1492, 1453, 1396, 1261, 1162, 1097, 1021, 909, 864, 803, 760,
699 cm−1.
1
use of ASTRA software from Wyatt Technology. H (400 MHz) and
13C (100 MHz) NMR spectra were recorded on a Varian spectrometer
at 25 °C using CDCl3 or DMSO-d6 as a solvent. Fourier transform
infrared (FT-IR) spectra were recorded on a PerkinElmer 2000
spectrometer using KBr discs. C, H, N, and S were determined by
combustion followed by chromatographic separation and thermal
conductivity detection using a Carlo-Erba EA 1110CHNO-S elemental
analyzer. Differential scanning calorimetry (DSC) analysis was
performed under a nitrogen atmosphere using a SDT 2960
simultaneous DSC−TGA of TA Instruments, and the heating rate
was 10 °C/min.
(PCL)m(PMMA)n Star. 1H NMR (CDCl3): δ 6.5−8.1 (PhH and
ArH), 5.35 (CHS), 4.22 (broad, CH2O of DMA unit, CH2S, and
CH2OCO of PCL and PMMA segments), 4.06 (CH2O of PCL), 3.68
and 3.41 (CH3O of PMMA, OCH2CH2N+(CH3)2CH2, and CH2 of
terminal DMA unit), 0.5−3.0 (CH2N and CH3 of DMA unit, and CH,
CH2 and CH3 of PCL, PMMA and branched core). FT-IR (KBr):
3438, 2995, 2951, 2868, 1732, 1658, 1628, 1484, 1452, 1390, 1365,
1276, 1245, 1190, 1148, 1062, 986, 966, 911, 843, 809, 749, 692 cm−1.
(PCL)m(PtBA)n Star. 1H NMR (CDCl3): δ 6.5−8.1 (PhH and ArH),
4.24 (broad, CH2O of DMA unit, CH2S, and CH2OCO of PCL and
PtBA segments), 4.06 (CH2O of PCL), 3.64 and 3.48
(OCH2CH2N+(CH3)2CH2, and CH2 of terminal DMA unit), 0.5−
3.0 (CH2N and CH3 of DMA unit, and CH, CH2 and CH3 of PCL,
PtBA and branched core). FT-IR (KBr): 3438, 2978, 2936, 2865,
1730, 1629, 1478, 1461, 1393, 1369, 1258, 1151, 1037, 963, 846, 751,
690 cm−1.
RESULTS AND DISCUSSION
■
This study aimed at versatile synthesis of multiarm and
miktoarm stars by combination of Menschutkin reaction and
RAFT process. Three types of ion-containing star polymers,
namely, Am-type multiarm stars, and AmBn- and AmCo-type
miktoarm stars, were synthesized to confirm the great potential
of these approaches, in which the branched core was generated
by RAFT copolymerization of VBDB and DMA via SCVP, A
and B segments were prefabricated by ring-opening polymer-
ization (ROP) or RAFT polymerization, and C segment was
directly grown from the surface of PVD via RAFT polymer-
ization. The detailed syntheses are described below.
1
(PCL)m(PNIPAM)n Star. H NMR (CDCl3): δ 5.8−8.1 (PhH, ArH,
and NH of PNIPAM), 4.1−5.0 (broad, CH2O of DMA unit, CH of
PNIPAM, CH2S, and CH2OCO of PCL and PNIPAM segments), 4.06
(CH2O of PCL), 3.61 and 3.50 (OCH2CH2N+(CH3)2CH2, and CH2
of terminal DMA unit), 0.5−3.0 (CH2N and CH3 of DMA unit, and
CH, CH2 and CH3 of PCL, PNIPAM and branched core). FT-IR
(KBr): 3436, 3069, 2970, 2936, 2872, 2826, 2775, 1733, 1652, 1548,
1460, 1387, 1367, 1270, 1236, 1171, 1101, 1043, 966, 881, 841, 691
cm−1.
Synthesis of Branched PVD. Our previous study has
confirmed that the target branched copolymers with variable
degree of branching (DB), polydispersity index (PDI), and
CTA functionality (F) could be achieved by changing feed
ratios and reaction time during RAFT copolymerization of S-
(4-vinyl)benzyl S′-propyltrithiocarbonate (VBPT) and vinyl
monomers.78 In this study, RAFT copolymerization of VBDB
and DMA was performed to generate branched PVD with
controlled number of dithiobenzoate moieties and tertiary
amino groups, in which a polymerizable dithiobenzoate
VBDB75 was used instead of VBPT so as to better control
RAFT process of methacrylate-type monomer.
Synthesis of AmCo Miktoarm Stars by Tandem Menschutkin
Reaction and RAFT Polymerization. In a typical reaction (run 1 of
Table 4), branched PVD (100 mg, 0.058 mmol CTA, 0.533 mmol
DMA unit), PCL-Br (180 mg, 0.107 mmol), St (1.21 g, 11.6 mmol),
and AIBN (1.9 mg, 0.012 mmol) were added to a glass tube, and dry
DMF was added until the total volume was 7.7 mL. The tube with a
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The utilization of branched copolymers with narrow
molecular weight distribution was crucial to the subsequent
synthesis of well-defined star polymers. To this end, RAFT
copolymerization of VBDB and DMA was performed under
different conditions to generate branched PVD with relatively
low polydispersity (Table 1). In H NMR spectra of PVD
(Figure 1), the signals of aromatic protons originating from
between 0.20 and 0.30, and their Fn (Fw) values were within the
range of 5.0−31.7 (6.6−154).
As expected, RAFT SCVP allowed for synthesis of branched
PVD with tunable molecular parameters. The copolymeriza-
tions ([DMA]0:[VBDB]0:[AIBN]0 = DP0:1:0.2, [DMA]0 = 3.0
mol/L, DP0 = 5−30, runs 1−3 of Table 1) using high
concentrations of comonomers and thermal initiator were liable
to generate branched copolymers with broad molecular weight
distribution (PDI = 3.24−4.96) in which the copolymer
fractions had Mn values in the range of 2000−2 000 000 (Figure
2a−c). As the concentrations of comonomers and AIBN were
1
Figure 2. GPC traces (normalized weight distribution) of PVD
branched copolymers.
significantly reduced, the copolymerization could afford
branched PVD with polydispersity as low as 1.32 (Figure
2d), similar to those obtained by RAFT copolymerization of
VBPT and methyl acrylate in our previous study.78 The
branched PVD (Mn,MALLS = 8630, Fn = 5.0, with average DMA
units of 46) synthesized by run 4 of Table 1 was chosen as an
ideal multifunctional core for the subsequent synthesis of
multiarm and miktoarm stars.
Synthesis of Bromide-Terminated Polymers. RAFT-
synthesized polymers involving polystyrene (PSt-Br, Mn,NMR
=
Figure 1. 1H NMR spectra of PVD branched copolymers obtained by
RAFT copolymerization of VBDB and DMA. The samples were
synthesized by runs 1 (a), 2 (b), 3 (c), and 4 (d) of Table 1.
2480, 4520, 6250, and 9600), poly(methyl methacrylate)
(PMMA-Br, Mn,NMR = 3860), poly(tert-butyl acrylate) (PtBA-
Br, Mn,NMR = 5050), and poly(N-isopropylacrylamide)
(PNIPAM-Br, Mn,NMR = 3030) were synthesized by RAFT
polymerization mediated by BBCP, and bromide-terminated
poly(ε-caprolactone) (PCL-Br, Mn,NMR = 1690) was generated
by ROP initiated with 3-bromo-1-propanol (Table S1).
VBDB were noted at δ 7.98 (2H of PhH, terminal PhC(S)S)
and 6.7−7.7 (ArH and other PhH), and characteristic signals of
CH2C(CH3)(COOCH2CH2N(CH3)2)S (terminal DMA unit),
CH2S (VBDB unit with unreacted dithiobenzoate moiety) and
CHS (styryl unit originated from reacted VBDB) appeared at δ
3.30 (k), 4.53 (q), and 5.0−5.5 (b); thus, proportions of
dithiobenzoate functionality connecting with benzyl group (P1),
terminal DMA unit (P2), and styryl unit (P3) in branched PVD
were obtained. The real repeat units per branch (RB) were
deduced by equation RB = RB(th)/(1 − P1) since the
proportion of remaining VBDB unit only acted as conventional
vinyl monomer during copolymerization. Meanwhile, number-
average (Fn) and weight-average (Fw) CTA functionalities per
In 1H NMR spectra of PM-Br (Figure 3 and Figure S3), the
signals of characteristic protons were observed at around δ 6.2−
7.2 (PhH of PSt), 3.60 (CH3O of PMMA), 2.23 (CH of PtBA),
1.44 (CH3 of PtBA), 4.01 (CH of PNIPAM), 4.06 (CH2O of
PCL), 4.24 (terminal CH2O), and 3.47 (terminal CH2Br), and
thus number-average molecular weights determined by 1H
NMR (Mn,NMR) were obtained. The apparent number-average
molecular weights (Mn,GPC) of PM-Br were estimated to be
2790−9450 by GPC, and their polydispersity indices ranged
from 1.06 to 1.15 (Figure 4 and Figure S4). The Mn,NMR values
were very close to those expected (Mn,th), as expected from the
“living”/controlled polymerization systems.
branched copolymer were determined by equations Fn
=
Mn,MALLS × I7.98/(MWVBDB × I7.98 + MWDMA × I4.06), and Fw =
Fn × PDI, in which Mn,MALLS was number-average molecular
weight of PVD determined by GPC-MALLS, and MWVBDB and
MWDMA were molecular weights of VBDB and DMA. In Table
1, the P1 values of various branched copolymers ranged
Synthesis of Am-Type Multiarm Stars by Menschutkin
Reaction. Menschutkin reaction was normally applied for
postmodification of functional polymers to generate ion-
bearing segments.79,81−86 In this study, bromide-terminated
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of PSt stars were taken as typical examples to investigate the
influence of reaction conditions. After coupling reaction,
PDMA grafted silica particles (SiO2-g-PDMA) were added to
the reaction system to perform a subsequent Menschutkin
reaction, and the remaining unreacted PM-Br was grafted onto
the surface of solid substrates to form comblike copolymer
grafted silica via quaternization process. The resultant star
polymers with a branched core could be efficiently isolated via
filtration and precipitation owing to the poor solubility of
silica−copolymer hybrids in organic solvents. This method
could completely remove unreacted PM-Br, evident from GPC-
MALLS and NMR spectra as described later, in which GPC
traces of the isolated stars only exhibited monomodal
distribution, and characteristic signals corresponding to
1
CH2CH2CH2Br of PM-Br were absent in H and 13C NMR
spectra.
1
In H NMR spectrum of PSt star (Figure 5a), the signals of
characteristic protons were noted at around δ 6.2−7.6 (PhH of
1
Figure 3. H NMR spectra of bromide-functionalized polymers.
Figure 4. GPC traces of PM-Br (PM = PSt (a), PMMA (b), PtBA (c),
PNIPAM (d), and PCL (e)).
Figure 5. 1H NMR spectra of Am-type multiarm stars with a branched
core.
polymers were directly coupled onto the surface of branched
PVD via quaternization process, allowing for the straightfor-
ward synthesis of multiarm and miktoarm stars with lowered
reaction steps.
Many factors such as reaction temperature, concentration,
time, feed ratio, chain rigidity, and molecular weight can affect
Menschutkin reaction between branched copolymer bearing
DMA units and bromide-terminated polymers. The reactions
between PVD and PM-Br using a fixed polymer concentration
of 0.20 g/mL were conducted in DMF at 60 °C, and syntheses
PSt arm, PhH and ArH of PVD core), 4.06 (CH2O of DMA
unit), 3.64 (OCH2CH2N+(CH3)2CH2), and 3.30
(OCH2CH2N+(CH3)2CH2, and CH2 of terminal DMA unit).
By comparing the integrated peak areas of protons at δ 6.2−7.6
and 4.06, the Mn,NMR values of PSt stars were obtained.
Meanwhile, their absolute Mn values and polydispersity indices
were determined by GPC-MALLS. On this basis, average arm
number (m) of PSt star and coupling efficiency (CE) of
Menschutkin reaction were calculated by equations m = (Mn,star
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− Mn,PVD)/Mn,arm, and CE = m/46r0, in which Mn,star and Mn,PVD
were number-average molecular weights of PSt star and PVD
determined by GPC-MALLS, Mn,arm was Mn,NMR value of PSt-
Br, 46 meant average number of DMA units per PVD, and r0
was feed ratio of PSt-Br to DMA unit of PVD.
Various stars were liable to exhibit symmetric distribution in
GPC traces, with polydispersity indices in the range of 1.10−
1.24 (Figure 6), suggesting the coupling reaction was efficiently
performed.
First, the reactions (Mn,NMR(PSt-Br) = 2480, r0 = 0.80) were
performed in DMF at 60 °C for different time to reveal the
effects of reaction time on coupling reaction (runs 1−4 of
Table 2 and Figure S6). No significant enhancement in
Mn,MALLS was noted as the time was beyond 40 h, originating
from the enhanced shielding effect with increasing grafting
density. When the arm number of the resultant stars was high
enough, the surface of branched PVD was almost surrounded
by the grafted PSt chains, and thus PSt-Br segments were quite
difficult to approach DMA units to further perform the
quaternization process. As the reaction time increased from 3.5
to 40 h, the m value of multiarm stars was significantly
increased from 9.1 to 23.5, and the coupling efficiency was
enhanced from 24.6% to 63.8%. Their PDI values were around
1.2, and no obvious shoulder and tailing were noted in GPC
traces, revealing the absence of notable side reactions during
star formation. Consequently, 40 h was chosen as the suitable
time for quaternization reaction between PVD and PM-Br.
Second, the reactions of branched PVD and PSt-Br with
different molecular weights were conducted to investigate the
effects of arm length on coupling reaction (runs 4−7 of Table
2). The resultant PSt stars were of Mn,MALLS values in the range
of 63 800−76 900, with polydispersity indices lower than 1.22.
As Mn,NMR values of PSt segments increased from 2480 to 9600,
the arm number of star polymers obtained significantly
decreased from 23.5 to 7.1, and the CE values were
correspondingly reduced from 63.8% to 19.3% (Figure S7),
which could be ascribed to lowered quaternization reactivity
and enhanced steric hindrance with increasing molecular weight
of grafted chains. These results indicated that Menschutkin
reaction between PVD and PSt-Br could be efficiently
performed as the grafted PSt chains were relatively short.
Last, quaternization reactions in a wide range of r0
(Wpolymer:VDMF = 0.20 g/mL, Mn,NMR(PSt-Br) = 4520) were
carried out to reveal the influence of feed ratio on coupling
reaction (runs 5 and 8−10 of Table 2). As r0 increased from
0.20 to 1.0, the Mn,MALLS values of PSt stars increased from 40
100 to 66 400, and their polydispersity indices were in the
range of 1.16 and 1.24. With increasing amount of PM-Br, the
coupling efficiency was significantly decreased from 75.7% to
27.8% although the arm number gradually increased from 7.0 to
12.8 (Figure S8). Consequently, multiarm PSt stars with higher
arm number could be achieved at the cost of using sacrificial
PSt-Br.
Figure 6. GPC traces (normalized weight distribution) of multiarm
stars with a branched core. Mn,MALLS and PDI of various stars were 76
900, 1.11 (PSt star), 66 800, 1.10 (PMMA star), 81 100, 1.13 (PtBA
star), and 76 500, 1.24 (PNIPAM star).
The above results revealed that the Menschutkin reaction
between polymers was strongly dependent on reaction
conditions, and Am-type multiarm stars with arm number up
to 35.1 could be achieved under optimized conditions, in which
the coupling efficiency was normally in the range of 19.3−
97.6%. The versatile and general reaction allowed for efficient
synthesis of a variety of well-defined multiarm stars by control
over reaction time and feed ratio of PM-Br to PVD.
Synthesis of AmBn-Type Miktoarm Stars by Successive
Menschutkin Reactions. Multistep Menschutkin reactions
are potentially applicable for the synthesis of many types of
miktoarm stars such as AmBn, AmBnCo, and even AmBnCoDp
stars. As the chain lengths of various arms are relatively short,
the approach to synthesis of miktoarm stars via successive
coupling reactions is very promising due to its high coupling
efficiency.
To confirm the great potential of such methodology in
construction of miktoarm stars, Am-type PCL star (Mn,MALLS
=
23 800, m ≈ 9.0, with average DMA units of 37) synthesized by
run 14 of Table 2 was used to synthesize AmBn stars by a second
Menschutkin reaction. The reactions using feed ratio of PM-Br
to DMA unit of PCL star (r0) in the range of 0.30−0.80
afforded AmBn (A = PCL; B = PSt, PMMA, PtBA, and
1
PNIPAM) miktoarm stars (Table 3). In H NMR spectra of
isolated stars (Figure 7), the signals of characteristic protons of
each arm and branched core appeared at about δ 6.2−7.2 (PhH
of PSt), 3.60 (CH3O of PMMA), 2.23 (CH of PtBA), 1.44
(CH3 of PtBA), 6.0−7.2 (NH of PNIPAM), 4.01 (CH of
PNIPAM), 4.06 (CH2O of PCL and DMA unit of branched
core), 7.8−8.1 (terminal PhH of PhC(S)S moieties), and
3.2−3.8 (OCH2CH2N+(CH3)2CH2). The Mn,MALLS and Mn,NMR
values of various stars were similar, and both were close to Mn,th
values expected from Mn,MALLS(PCL star), Mn,NMR(PM-Br), and
yield of isolated stars. GPC-MALLS results revealed Men-
schutkin reaction between PCL star and PM-Br could afford
AmBn stars with Mn,MALLS values ranging from 42 200 to 67 500,
and the arm number of B segment was calculated to be in the
range of 6.1−11.3. These results indicated that the target AmBn
stars with tunable arm number of B segment could be efficiently
On this basis, other PMMA, PtBA, PNIPAM, and PCL stars
were synthesized (runs 11−15 of Table 2). In 1H NMR spectra
of isolated stars (Figure 5b−e), the signals of characteristic
protons of each arm and branched core were noted at about δ
3.60 (CH3O of PMMA), 2.23 (CH of PtBA), 4.01 (CH of
PNIPAM), 4.06 (CH2O of PCL and DMA unit), 7.8−8.1
(terminal PhH of PhC(=S)S moieties), and 3.2−3.8
(OCH2CH2N+(CH3)2CH2). By comparing the integrated
peak areas of these protons, Mn,NMR values of various stars
were obtained. The Mn,MALLS and Mn,NMR values were usually
similar, and both were in good accordance with Mn,th results
expected from Mn,MALLS(PVD), Mn,NMR(PM-Br), and yield of
isolated stars. The number of arms was in the range of 9.0−
35.1, with coupling efficiency ranging from 39.0% to 97.6%.
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Table 3. Results for Synthesis of AmBn (A = PCL, B = PM, m ≈ 9.0) Miktoarm Stars by Menschutkin Reaction between PCL Star
a
and PM-Br
b
c
d
e
e
f
g
h
run
PM
PSt
r0
Y (%)
Mn,th
Mn,MALLS
PDI
Mn,NMR
n
CE (%)
1
2
3
4
0.50
0.80
0.50
0.30
43.0
39.5
34.8
50.6
43 500
68 900
56 300
40 800
45 700
67 500
58 100
42 200
1.13
1.16
1.20
1.09
44 100
66 900
55 800
41 600
8.8
11.3
6.8
47.6
38.2
36.8
55.0
PMMA
PtBA
PNIPAM
6.1
a
Reaction conditions: Mn,NMR = 2480 (PSt-Br), 3860 (PMMA-Br), 5050 (PtBA-Br), 3030 (PNIPAM-Br); Wpolymer:VDMF = 0.20 g/mL, in DMF at 60
b
c
d
e
°C for 40 h. Molar ratio of PM-Br to DMA unit of PCL star. Yield of isolated star copolymer. Theoretical molecular weight. Number-average
molecular weight and polydispersity determined by GPC-MALLS. Number-average molecular weight determined by H NMR. Average arm
f
g
1
number of PM segment in star copolymer, n = (Mn,AmBn − Mn,Am)/Mn,PM, in which Mn,AmBn and Mn,Am were molecular weights of AmBn and PCL stars
h
determined by GPC-MALLS, and Mn,PM was Mn,NMR value of PM-Br. Coupling efficiency of Menschutkin reaction, CE = n/37r0, in which 37 was
average number of DMA unit per PCL star.
Figure 8. GPC traces (normalized weight distribution) of branched
PVD (a), PCL star (b), and (PCL)m(PM)n miktoarm stars (PM = PSt
(c), PMMA (d), PtBA (e), and PNIPAM (f)).
within the range 36.8−55.0%, suggesting the above-mentioned
limitations were not obvious.
In addition to 1H NMR and GPC-MALLS, 13C NMR
spectroscopy was also used to characterize representative
polymer samples. In 13C NMR spectrum of branched PVD
(Figure 9a), characteristic signals of aromatic carbons and
DMA units appeared at δ 175.8−176.8 (CO), 126.7−145.0
(ArC and PhC), 62.4 (CH2O), 56.6 (CH2N), 45.1 (CH3N),
and 44.2 (CH3CCOO). For PCL star with a branched core
(Figure 9b), characteristic signals corresponding to PCL arms
were noted at δ 172.8 (CO), 63.5 (CH2O), 60.6 (CH2OH),
33.4 (CH2COO), 32.2 (CH2CH2OH), 27.9 (CH2CH2-
CH2 CH2 CO), 25.0 (CH2 CH2 CH2 CH2 CO), 24.5
1
Figure 7. H NMR spectra of AmBn stars with a branched core (A =
(CH2CH2CH2OH), and 24.2 (CH2CH2CH2CH2CO), and
PCL; B = PSt (a), PMMA (b), PtBA (c), and PNIPAM (d)).
typical signals beside the ionic bonds appeared at δ 60.8
(CH2N+(CH3)2CH2CH2CH2OCO), 60.3 (CH2N+(CH3)2CH2-
synthesized via two-step Menschutkin reactions, in which A and
B arms could be any polymer segments soluble in DMF.
With increasing grafting density of arm segments, the
successive syntheses of miktoarm stars may suffer from some
disadvantages involving enhanced steric hindrance and
heterogeneity, resulting in lowered coupling efficiency,
broadened molecular weight distribution, and different
chemical composition among miktoarm stars. As compared
with their precursors PVD and PCL star, the GPC traces of the
resultant AmBn stars exhibited monomodal distribution and
completely shifted to higher molecular weight sides (Figure 8),
corresponding to the efficient quaternization process. The
polydispersity indices of AmBn stars determined by GPC-
MALLS were lower than 1.20, and the coupling efficiency was
CH2CH2OCO), 50.6 (CH2N+(CH3)2CH2CH2CH2OCO), and
22.4 (CH2N+(CH3)2CH2CH2CH2OCO). In the 13C NMR
spectrum of (PCL)m(PSt)n star (Figure 9c), characteristic
signals of PSt and PCL segments appeared at δ 145.2, 125.7−
128.6 (PhC), 44.5 (CH2 of PSt), 63.5, 33.4, 27.9, 25.0, and 24.2
(CH2 of PCL), and typical signals corresponding to
CH2N+(CH3)2CH2CH2CH2OCO were observed at δ 61.5,
61.1, 60.8, 60.2, 50.6, and 22.4. The signals of unbonded DMA
units at δ 56.6 (CH2N) and 45.1 (CH3N) were significantly
weakened, and new signals corresponding to
CH2N+(CH3)2CH2CH2CH2OCO were noted in 13C NMR
spectra, confirming the success of Menschutkin reaction
between branched PVD and PM-Br. The isolated multiarm
stars lacked signals at 62.1, 31.3, and 31.0 (BrCH2CH2CH2O of
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cessive coupling reactions are very efficient to synthesize
miktoarm stars with relatively short arms; however, the
coupling efficiency is gradually decreased with increasing
grafting density, and thus it is difficult to synthesize AmBn
miktoarm stars with same arm number and different arm length
of B segment. The tandem reaction is promising to partly
address this disadvantage. The alternative method allows for
facile construction of AmCo stars via simultaneous Menschutkin
reaction and RAFT polymerization, in which A segment can be
efficiently grafted onto the surface of PVD via quaternization
process, the arm length of RAFT generated C segment can be
adjusted by control over reaction time and feed ratio, and the
arm number of C segment is equal to the CTA functionality of
branched core as the dithiobenzoate moieties are quantitatively
activated.78
The simultaneous coupling reaction between PCL-Br
(Mn,NMR = 1690) and PVD (Mn,MALLS = 8630) and RAFT
polymerization of St, tBA, and NIPAM mediated by PVD were
1
performed in DMF at 60 °C for 24 h (Table 4). In H NMR
spectrum of branched PVD (Figure 1), the signals of
characteristic protons connecting with terminal CTA function-
alities were noted at δ 5.0−5.5 (CHS), 4.53 (CH2S), and 3.30
(CH2 of terminal DMA unit). These signals completely
1
disappeared in H NMR spectra of isolated AmCo stars (Figure
10), and a new signal corresponding to CHS of terminal
monomer unit was quantitatively observed at δ 4.6−4.9,
confirming the RAFT polymerization to grow C segment was
efficiently conducted. Other characteristic signals appeared at δ
4.06 (CH2O of PCL), 6.2−7.2 (PhH of PSt), 2.23 (CH of
PtBA), 1.44 (CH3 of PtBA), 6.0−7.0 (NH of PNIPAM), 4.01
(CH of PNIPAM), 1.13 (CH3 of PNIPAM), and about 7.96
(2H of PhH, terminal PhC(S)S). By comparing the
integrated peak areas of dithiobenzoate moiety, CH2O of
PCL and characteristic protons of other C segment, the total
number of monomer units per star could be deduced. The
average arm number of PCL in AmCo stars was estimated to be
in the range of 8.8−9.0 by 1H NMR analysis, corresponding to
coupling efficiency of 96−98%. By assuming the arm number of
C segment was equal to the CTA functionality of PVD (Fn =
5.0), the polymerization degree of each arm was determined to
be 43.2 (PSt), 49.5 (PtBA), and 43.6 (PNIPAM) by 1H NMR.
The Mn,NMR and Mn,MALLS values of AmCo stars were
comparable, and both were close to the Mn,th values expected
from Mn,MALLS(PVD), monomer conversion (C%), and the
assumed quantitative coupling efficiency. The GPC traces of
various stars exhibited monomodal distribution and wholly
shifted to higher molecular weight side than PVD, with PDI
values lower than 1.12 (Figure 11), revealing the successful
synthesis of the target miktoarm stars via the tandem approach.
The above results revealed the tandem reaction could afford
AmCo stars (m ≈ 8.8−9.0, o ≈ 5.0) with well-controlled
molecular weight and low polydispersity. It should be
mentioned that a subsequent Menschutkin reaction or
fractional precipitation may be necessary to remove the
unreacted bromide-functionalized A segment if its chain length
is too long to efficiently perform the quaternization reaction.
This approach is promising for the synthesis of miktoarm stars
with tunable arm number of short A segment and variable chain
length of C segment since it combines the advantages of
“grafting onto” and “core first” approaches.21−27 Owing to the
nature of “living”/controlled polymerization of RAFT process,
AmCo star copolymers with molecular weight of C segment up
Figure 9. 13C NMR spectra of branched PVD (a), PCL star (b), and
(PCL)m(PSt)n star (c) in DMSO-d6.
PSt-Br) and 61.8, 31.3, and 30.9 (BrCH2CH2CH2O of PCL-Br)
in 13C NMR spectra and only exhibited monomodal
distribution in GPC traces, revealing the absence of PM-Br.
These results further demonstrated Menschutkin reaction
between polymers could be efficiently applied for the synthesis
of multiarm and miktoarm stars with low polydispersity.
Synthesis of AmCo-Type Multiarm Stars by Tandem
Menschutkin Reaction and RAFT Polymerization. Suc-
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Table 4. Results for Synthesis of AmCo (A = PCL, C = PM, o ≈ 5.0) Miktoarm Stars by Tandem Menschutkin Reaction and
a
RAFT Polymerization
b
c
d
d
e
f
run
M
[M]0 (mol/L)
C (%)
Mn,th
Mn,MALLS
PDI
Mn,NMR
m
1
2
3
St
1.5
1.0
1.0
22.4
48.8
44.5
47 500
55 400
49 300
48 100
53 600
50 000
1.11
1.06
1.12
46 000
55 200
48 500
8.8
8.8
9.0
tBA
NIPAM
a
Reaction conditions: [PCL-Br]0:[DMA unit of PVD]0 = 0.20, [M]0:[CTA]0:[AIBN]0 = 200:1:0.2 (run 1) or 100:1:0.2 (runs 2 and 3), in DMF at
b
c
60 °C for 24 h. Monomer conversion determined by 1H NMR. Theoretical molecular weight by assuming Menschutkin reaction was quantitatively
d
e
performed. Number-average molecular weight and polydispersity determined by GPC-MALLS. Number-average molecular weight determined by
f
1H NMR. Average arm number of PCL segment grafted onto the surface of PVD estimated by H NMR.
1
Figure 11. GPC traces (normalized weight distribution) of branched
PVD (a) and AmCo-type miktoarm stars (A = PCL; C = PSt (b), PtBA
(c), and PNIPAM (d)).
increased value in Tg was 11.6 (PM = PSt), 15.7 (PM =
PMMA), 5.2 (PM = PtBA), and 17.7 °C (PM = PNIPAM).
Although the Tg values of multiarm stars were higher than their
arm precursors, they were usually lower than linear PM
segments with similar molecular weight. For instance, the Tg
value of PSt (Mn,NMR = 65 300, Tg = 77.6 °C) and PMMA
(Mn,NMR = 66 200, Tg = 93.1 °C) stars was lower than that of
their linear chains with comparable Mn values (Tg ≈ 100 °C).
This phenomenon could be primarily attributed to the
branching effect although all the factors involving arm number,
segment rigidity and polarity, and chain length played
important roles in chain relaxation.
On the contrary, the chain relaxation and melting behaviors
of miktoarm stars were more complex. For all the miktoarm
stars bearing PCL arms, the presence of noncrystalline
segments strongly prevented the PCL chains from folding
and rearrangement, evident from the absence of melting peak in
DSC curves of AmBn stars (Figure 12c). Meanwhile, PCL chains
were liable to act as “plasticizers” during glass transition of star
copolymers, resulting in remarkably decreased Tg values. No
notable Tg appeared in DSC curve of (PCL)m(PSt)n star, and
the glass transition temperature of (PCL)m(PM)n stars as noted
at 71.2 (PM = PMMA), 24.9 (PM = PtBA), and 87.0 °C (PM =
PNIPAM) was distinctly lower than that of PM-Br. These
results indicated the presence of PCL arms tended to weaken
the interactions among polymer segments, and thus the chain
relaxation of PM segments in miktoarm stars could smoothly
perform at significantly reduced temperatures.
1
Figure 10. H NMR spectra of AmCo stars with a branched core.
to tens of thousands are potentially achieved by the tandem
approach under optimized conditions.
DSC Analysis and Solubility of Multiarm and
Miktoarm Stars. To investigate the effects of macromolecular
architecture on chain relaxation and melting behaviors, glass
transition (Tg) and melting (Tm) temperatures of various stars
and their precursors were determined by DSC (Table S2).
Branched PVD (Mn,MALLS = 8630) had a glass transition at 28.6
°C, and bromide-terminated polymers possessed a glass
transition at 66.0 (PSt-Br, Mn,NMR(PSt-Br) = 2480), 77.4
(PMMA-Br), 31.3 (PtBA-Br), and 107.6 °C (PNIPAM-Br).
PCL-Br was of two melting peaks at 43.3 and 46.6 °C (Figure
12a), while PCL star had a decreased Tm at 35.4 °C (Figure
12b). As compared with PM-Br, the resultant (PM)m-type
multiarm stars except PCL star exhibited one enhanced Tg in
the range of 36.5−125.3 °C (Figure 12b), in which the
In addition, multiarm and miktoarm stars with quaternary
ammonium bromides usually had solubility different from those
without quaternization interaction, and nine solvents compris-
ing deionized water and hexane were used to check their
solubility (Table 5). Branched PVD was insoluble in hexane
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and diethyl ether, partly soluble in water and DCM, and soluble
in other solvents. All the multiarm and miktoarm stars were
soluble in DMF, soluble or partly soluble in DCM and THF,
and insoluble in hexane and diethyl ether (except PtBA star).
Interestingly, all the stars except (PCL)m(PtBA)n star were
insoluble in water, and star polymers except PSt, PNIPAM, and
(PSt)m(PNIPAM)n stars were at least partly soluble in
methanol. The special solubility in different solvents revealed
the presence of quaternary ammonium bromides was liable to
remarkably change the solubility parameter of various stars,
which was mainly dependent on chain length, chemical
composition, segment rigidity, and degree of ionization of
DMA units. Consequently, these ion-bearing star polymers
potentially exhibit unique self-assembly behaviors in water and
cosolvents, which are in progress in our laboratory.
CONCLUSION
■
Menschutkin reaction and controlled polymerization were
efficiently combined to generate Am-, AmBn-, and AmCo-type
star polymers with a branched core and PCL, PSt, PMMA,
PtBA, and PNIPAM arms. The Menschutkin reaction between
branched PVD and PM-Br was strongly dependent on reaction
conditions, and the coupling efficiency was in the range 19.3−
97.6%. A series of multiarm and miktoarm stars with roughly
controlled arm number and tunable chain length were achieved
by control over reaction conditions and utilization of different
methods, confirming the generality and versatility of these
synthetic approaches. DSC results indicated the Tg values
decreased in the order PM star > PM-Br > (PCL)m(PM)n star
(PM = PMMA, PtBA, and PNIPAM), revealing the strong
dependence of chain relaxation on macromolecular architec-
ture. In addition, the resultant star polymers exhibited solubility
different from conventional stars without ionic bonds. The ion-
bearing stars may have a potential utility in supramolecular
chemistry and smart biomaterials due to their special thermal
and solution properties.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR spectra of BBCP and PSt-Br, 13C NMR spectra of
PCL-Br, PSt-Br, and branched PVD, GPC traces of PSt-Br, IR
spectra of BBCP, PVD, PM-Br, and star polymers, and TGA
curves of Si-TBPT and SiO2-g-PDMA. This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 12. DSC curves of PVD and PM-Br (a), PM stars (b), and
AmBn stars (c).
a
Table 5. Solubility of PVD and Star Polymers in Various Solvents
sample
methanol
acetone
water
toluene
+
hexane
diethyl ether
DCM
THF
DMF
PVD
+
+
−
−
−
−
−
−
−
−
+
+
+
+
+
+
+
+
+
+
+
+
+
+
PSt star
−
−
−
−
−
−
−
−
−
PMMA star
+
+
PCL star
PtBA star
+
+
−
+
+
+
−
+
−
+
+
+
−
+
PNIPAM star
(PCL)m(PSt)n
(PCL)m(PMMA)n
(PCL)m(PtBA)n
(PCL)m(PNIPAM)n
−
−
−
−
−
−
−
−
−
−
−
+
+
+
+
+
+
+
+
−
−
a
Solubility: + (soluble), (partly soluble, with solubility lower than 0.2 mg/mL), and − (insoluble).
9323
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(29) Fox, M. E.; Szoka, F. C.; Frec
42, 1141−1151.
(30) Rodionov, V.; Gao, H. F.; Scroggins, S.; Unruh, D. A.; Avestro,
A. J.; Frec
́
het, J. M. J. Acc. Chem. Res. 2009,
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ylzhao@suda.edu.cn.
́
het, J. M. J. J. Am. Chem. Soc. 2010, 132, 2570−2572.
(31) Bapat, A. P.; Roy, D.; Ray, J. G.; Savin, D. A.; Sumerlin, B. S. J.
Notes
Am. Chem. Soc. 2011, 133, 19832−19838.
The authors declare no competing financial interest.
(32) Syrett, J. A.; Haddleton, D. M.; Whittaker, M. R.; Davis, T. P.;
Boyer, C. Chem. Commun. 2011, 47, 1449−1451.
ACKNOWLEDGMENTS
(33) Zhang, Q.; Li, G. Z.; Becer, C. R.; Haddleton, D. M. Chem.
Commun. 2012, 48, 8063−8065.
■
This work was financially supported by the National Natural
Science Foundation of China (Grants 20874067, 21074081,
and 21274096), the Key Project of Chinese Ministry of
Education (No. 209049), and the Project Funded by the
Priority Academic Program Development of Jiangsu Higher
Education Institutions.
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