Synthesis of a sun-shaped amphiphilic copolymer consisting of a cyclic perfluorocyclobutyl aryl ether-based backbone and lateral PMAA side chains

Article abstract of DOI:10.1039/c4ra11630e

A novel amphiphilic sun-shaped copolymer, c-PMBTFVB-g-PMAA (MBTFVB: 2-methyl-1,4-bistrifluorovinyloxybenzene, MAA: methacrylic acid) containing a cyclic perfluorocyclobutyl (PFCB) aryl ether-based backbone and PMAA lateral side chains with narrow molecular weight distribution (M_w/M_n ≤ 1.38), was synthesized via the site transformation strategy. First, a PMBTFVB linear precursor was prepared by thermal step-growth cycloaddition polymerization of MBTFVB trifluorovinyl monomer. After the end functionalization of the linear precursor with alkyne, Glaser coupling reaction was performed to produce c-PMBTFVB cyclic homopolymer. The pendant methyls on PFCB aryl ether-based backbone were then converted to Br-containing ATRP initiating groups by a mono-bromination reaction with N-bromosuccinimide (NBS) and benzoyl peroxide (BPO) to give c-PMBTFVB-Br cyclic macroinitiator without affecting the main chain, where the density of ATRP initiation groups could be tuned from 33% to 58% by varying the feeding ratio of NBS to the pendant methyl. Subsequently, c-PMBTFVB-g-PMAA sun-shaped amphiphilic copolymers with hydrophilic PMAA side chains were synthesized by ATRP of tert-butyl methacrylate (tBMA) initiated by c-PMBTFVB-Br, followed by the selective hydrolysis of hydrophobic PtBMA segment into hydrophilic PMAA segment using CF₃COOH. The obtained c-PMBTFVB cyclic homopolymer and its precursor were well characterized by GPC, NMR, and DSC and all the observations indicated a high efficiency of the intra-macromolecular cyclization via Glaser coupling reaction. The critical micelle concentrations of the obtained amphiphilic copolymers were determined by fluorescence probe technique and the morphologies of the formed micelles were investigated by TEM. This journal is

Full text of DOI:10.1039/c4ra11630e

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Page 1 of 38
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DOI: 10.1039/C4RA11630E
Synthesis of Sun-Shaped Amphiphilic Copolymer
Consisting of Cyclic Perfluorocyclobutyl Aryl
Ether-Based Backbone and Lateral PMAA Side Chains
Wenqiang Yao, Yongjun Li, Chun Feng,* Guolin Lu, Xiaoyu Huang*
Key Laboratory of Synthetic and SelfꢀAssembly Chemistry for Organic Functional
Molecules, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Road, Shanghai 200032, P. R. China
* To whom correspondence should be addressed, Eꢀmail: xyhuang@mail.sioc.ac.cn (Tel:
+86ꢀ21ꢀ54925310, Fax: +86ꢀ21ꢀ64166128), cfeng@mail.sioc.ac.cn (Tel: +86ꢀ21ꢀ
54925520, Fax: +86ꢀ21ꢀ64166128).

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Abstract
A novel amphiphilic sunꢀshaped copolymer, cꢀPMBTFVBꢀgꢀPMAA (MBTFVB:
2ꢀmethylꢀ1,4ꢀbistrifluorovinyloxybenzene, MAA: methacrylic acid) containing cyclic
perfluorocyclobutyl (PFCB) aryl etherꢀbased backbone and PMAA lateral side chains
with narrow molecular weight distribution (Mw/Mn≤1.38), was synthesized via the site
transformation strategy. First, PMBTFVB linear precursor was prepared by stepꢀgrowth
thermal cycloaddtion polymerization of MBTFVB trifluorovinyl monomer. After the end
functionalization of linear precursor with alkyne, Glaser coupling reaction was performed
to produce cꢀPMBTFVB cyclic homopolymer. The pendant methyls on PFCB aryl
etherꢀbased backbone were then converted to Brꢀcontaining ATRP initiating groups by a
monoꢀbromination reaction with Nꢀbromosuccinimide (NBS) and benzoyl peroxide (BPO)
to give cꢀPMBTFVBꢀBr cyclic macroinitiator without affecting the main chain, where the
density of ATRP initiation groups could be tuned from 33% to 58% by the feeding ratio
of NBS to the pendant methyl. Subsequently, cꢀPMBTFVBꢀgꢀPMAA sunꢀshaped
amphiphilic copolymers with hydrophilic PMAA side chains were synthesized by ATRP
of tertꢀbutyl methacrylate (tBMA) initiated by cꢀPMBTFVBꢀBr, followed by the
selective hydrolysis of hydrophobic PtBMA segment into hydrophilic PMAA segment
using CF3COOH. The obtained cꢀPMBTFVB cyclic homopolymer and its precursor were
well characterized by GPC, NMR, and DSC and all the observations indicated a high
efficiency of intraꢀmacromolecular cyclization via Glaser coupling reaction. The critical
micelle concentrations of the obtained amphiphilic copolymers were determined by
fluorescence probe technique and the morphologies of the formed micelles were
investigated by TEM.
2

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Introduction
Perfluorocyclobutyl (PFCB) aryl etherꢀbased polymers discovered by Babb et al. of
Dow Chemical in early 1990s,1,2 represented a class of partially fluorinated polymers.3ꢀ5
They have attracted much interests in many fields such as photonics,6ꢀ8 polymer
lightꢀemitting diodes,9,10 and proton exchange membranes for fuel cells.4 Since PFCB
aryl etherꢀbased polymers were synthesized via a relatively unusual thermal [2π + 2π]
stepꢀgrowth thermal cycloaddition polymerization of trifluorovinyl monomer at very high
polymerization temperature (>150oC), the majority of researches on PFCB aryl etherꢀ
based polymers were focused on linear PFCB aryl etherꢀbased homopolymers and
copolymers with different functionalities.11 It is wellꢀknown that physical properties of a
certain polymer are inherently related to its architecture. Therefore, a range of nonꢀlinear
PFCB aryl etherꢀbased (co)polymer architectures, such as star, graft, crossꢀlinked, and
hyperbranched, have been prepared and investigated to address their potentially diverse
applications.11 Recently, cyclic polymers with “endless” and closed ring macromolecular
topology have attracted considerable interests in fundamental polymer science because
cyclic polymers possessed a particularly unique set of physical properties compared to
their linear analogues, including higher glass transition temperature, smaller
hydrodynamic volume, lower intrinsic viscosity and so on.12ꢀ18 Up to now, a variety of
cyclic polymers, such as polystyrene (PS),19ꢀ22 poly(Nꢀisopropylacrylamide),23,24
poly(αꢀpeptiod),25 oligonucleotide,26 poly(2ꢀvinylnaphthalene),27 poly(ethylene oxide)
(PEO),28 poly(octene),29 polyester,16 and their related copolymers,30ꢀ32 have been prepared
either by the combination of “living”/controlled polymerization with “click” chemistry
through an endꢀtoꢀend linking process, or ringꢀexpansion metathesis polymerization.
3

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However, to our best knowledge, cyclic PFCB aryl etherꢀbased polymer has not been
explored yet possibly due to its unusual preparation approach of thermal [2π + 2π]
stepꢀgrowth thermal cycloaddition polymerization of trifluorovinyl monomer.
With the development of polymer chemistry and the desire for better understanding
the fundamental structureꢀproperty relationships exhibited by novel and complicated
polymer architectures, cyclic polymers were also employed as building blocks for the
fabrication of copolymers with much more complex topological architectures, including
8ꢀshaped,33,34 Pꢀshaped,35 jellyfishꢀshaped,36 tadpoleꢀshaped,37 flowerꢀshaped,38
sunꢀshaped18,39ꢀ49 and so on.50ꢀ57 Among them, sunꢀshaped copolymer, consisting of a
cyclic backbone and lateral side chains connected to the backbone, was a kind of special
graft copolymer. Therefore, sunꢀshaped copolymer is also endowed with fascinating
properties such as wormlike conformation, compact molecular dimension, and notable
chain end effects resulting from their confined and compact structure.58 For examples,
Deffieux et al. grafted polystyryllithium or both polystyryllithium and
polyisoprenyllithium into cyclic poly(chloroethyl vinyl ether) to afford sunꢀshaped
macrocyclic graft copolymers40 and these cyclic graft copolymers could be visualized by
atomic force microscopy (AFM), which was similar with previous reports on “linear”
graft copolymers.59,60 Interestingly, the obtained sunꢀshaped copolymer with both PS and
polyisoprene side chains could selfꢀassemble into rigid tubular objects up to 700 nm in
heptane. Similarly, Frechet and Grubbs et al. utilized a direct route of ringꢀexpansion
metathesis polymerization of dendronized macromonomer to prepare sunꢀshaped
copolymers and the individual copolymer ring, thus, was also able to be visualized by
AFM.41 Tew et al. also employed ringꢀexpansion metathesis polymerization to synthesize
4

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cyclic backbone and lateral side chains were connected to the backbone via metalloꢀ
supramolecular interactions instead of covalent bones.42 The obtained sunꢀshaped
copolymers could be visualized by transmission electron microscopy (TEM). Besides
these, some other groups also reported sunꢀshaped copolymers with different
compositions. Kricheldorf et al. reported sunꢀshaped copolymer with cyclic poly(ether
ketone)s backbone and hyperbranched side chains derived from a polycondensation of
3,5ꢀbis(4ꢀfluorobenzoyl)phenol.43 Huang el al. demonstrated the synthesis of a series of
amphiphilic sunꢀshaped copolymers of cꢀPEOꢀgꢀPS, cꢀPEOꢀgꢀpoly(εꢀcaprolactone), and
cꢀpoly(2ꢀhydroxylethyl methacrylate)ꢀgꢀ(PSꢀbꢀPEO) on the basis of anionic ring opening
polymerization, atom transfer radical polymerization, nitroxide radical coupling reaction,
and azideꢀalkyne “click” chemistry.44ꢀ47 Experiments on the extraction for dyes of Cresol
red, Thymol blue, Bromophenol blue, and Thymolphthalein by sunꢀshaped cꢀPEOꢀgꢀPS
and combꢀlike PEOꢀgꢀPS showed that cꢀPEOꢀgꢀPS had a stronger conjugation ability
with the dyes than that of combꢀlike PEOꢀgꢀPS.47 Jerome and his coꢀworkers reported the
preparation of amphiphilic sunꢀshaped copolymers of cꢀPCLꢀgꢀPEO by the esterification
of carboxylic acid terminated PEO with hydroxyls of cyclic PCL.48 Grayson et al.
prepared sunꢀshaped copolymers with cyclic PS backbone and dendritic polyester side
chains up to 4th generation by azideꢀalkyne “click” reaction of alkyneꢀfunctionalized PS
cyclic backbone with azideꢀfunctionalized polyester dendrons.49 Although significant
advances have occurred in the synthesis of sunꢀshaped copolymers, a straightꢀforward
and efficient synthetic method is still challenging for the preparation of sunꢀshaped
copolymer with PFCB aryl etherꢀbased segment considering the unusual approach for its
preparation.
5

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Here for the first time, we reported the synthesis of PFCB aryl etherꢀbased cyclic
homopolymer and sunꢀshaped amphiphilic copolymers with PFCB aryl etherꢀbased
cyclic backbone and PMAA side chains. PFCB aryl etherꢀbased cyclic homopolymer was
first prepared via [2π + 2π] stepꢀgrowth thermal cycloaddition polymerization of
4ꢀmethoxytrifluorovinyloxybenzene and Glaser coupling reaction. Subsequently, pendant
methyls on the backbone were transformed into ATRP initiating groups using NBS and
BPO. Amphiphilic sunꢀshaped copolymers were synthesized by ATRP graft
copolymerization of tBMA using brominated PFCB aryl etherꢀbased cyclic polymer as
macroinitiator, followed by the acidic selective hydrolysis of hydrophobic PtBMA side
chains into hydrophilic PMAA side chains. In addition, critical micelle concentrations
(cmc) of resulting amphiphilic sunꢀshaped copolymers were measured by fluorescence
spectroscopy and their micellar morphologies were visualized by TEM. It should be
noticed that with the view of high tolerance of Glaser coupling reaction, the versatility of
thermal [2π + 2π] stepꢀgrowth thermal cycloaddition polymerization on the synthesis of
PFCB aryl etherꢀbased polymers with diverse functional moieties, and ATRP on the
preparation of (co)polymers with different functionalities, composition, and architectures,
it is logical to assume that the approach addressed in this work can lead to a variety of
PFCB aryl etherꢀbased sunꢀshaped copolymers with desired features.
Experimental Section
Materials
4ꢀMethoxytrifluorovinyloxybenzene was prepared according to previous literatures.1ꢀ5
1,2ꢀDibromotetrafluoroethane (BrCF2CF2Br) was prepared by condensing equimolar
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amounts of Br2 and tetrafluoroethylene at ꢀ195°C followed by warming up to 22°C
according the previous reports.1ꢀ5 Granular zinc was activated by washing in 0.1 M HCl
followed by drying at 100°C in vacuo for 16 h. NꢀBromosuccinimide (NBS, Aldrich,
99%) was recrystallized from water and dried in vacuo at 35°C for 1 day. Benzoyl
peroxide (BPO, Alfa Aesar, 97%) was purified by dissolving in acetone and precipitating
in water followed by drying in vacuo at room temperature for 1 day. tertꢀButyl
methacrylate (tBMA, Aldrich, 99%) was distilled with CaH2 under reduced pressure prior
to use. Copper (I) bromide (CuBr, Aldrich, 99%) was purified by stirring overnight over
CH3COOH at room temperature followed by washing the solid with ethanol, acetone, and
diethyl ether before drying at 40°C in vacuo for 1 day. NꢀPhenylꢀ1ꢀnaphthylamine (PNA,
Alfa Aesar, 97%) was purified by recrystallization in ethanol for three times. Diphenyl
ether (Aldrich, ≥99%), acetonitrile (Aldrich, ≥99.5%), and 1,4ꢀdioxane (Aldrich, ≥99%)
were dried with CaH2 and distilled under vacuum. 4ꢀMethoxyphenol (Aldrich, 99%),
2ꢀmethylhydroquinone (Aldrich, 99%), N,N,N’,N’,N’’ꢀpentamethyldiethylenetriamine
(PMDETA, Aldrich, 99%), borontribromide (BBr3, Aldrich, 99%), sodium hydride (NaH,
Aldrich, 99%), and 3ꢀbromoꢀ1ꢀpropyne (Aldrich, 99%) were used as received. Other
solvents were obtained from commercial sources and used as received.
Measurements
All NMR analyses were performed on a Bruker Avance 500 spectrometer (500 MHz).
Tetramethylsilane (1H NMR) and CDCl3 (13C NMR) were used as internal standards and
CF3CO2H was used as an external standard for 19F NMR. EIꢀMS was measured by an
Agilent 5937N system. FTꢀIR spectra were recorded on a Nicolet AVATARꢀ360 FTꢀIR
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spectrophotometer with a resolution of 4 cmꢀ1. Relative molecular weights and molecular
weight distributions were measured by gel permeation chromatography (GPC) system
equipped with a Waters 1515 Isocratic HPLC pump, a Waters 2414 refractive index
detector and a set of Waters Styragel columns (HR3,HR4 and HR5, 7.8×300 mm). GPC
measurements were carried out at 35ºC using THF as eluent with a flow rate of 1.0
mL/min. The system was calibrated with linear polystyrene standards. Absolute
molecular weights of the sunꢀshaped copolymers were determined by GPC equipped with
a multiangle light scattering detector (GPC/MALS), THF was used as the eluent with a
flow rate of 1.0 mL/min, detectors: Wyatt Optilab rEX refractive index detector and
Wyatt DAWN HELEOS 18ꢀangle light scattering detector with a 50 mW solidꢀstate laser
operating at 658 nm. Differential scanning calorimetry (DSC) measurements were run on
a TA Q200 system under N2 purge with a heating rate of 10oC/min. The glass transition
temperature (Tg) was record from the second heating process after a quick cooling from
200oC, and the value was determined from the midpoint of Cp curve. Steadyꢀstate
fluorescence spectra of PNA were measured on a Hitachi Fꢀ2700 spectrofluorometer with
the bandwidth of 5 nm for excitation and emission; the emission intensity at 418 nm was
recorded to determine the cmc with an excitation wavelength (λex) of 340 nm.
Hydrodynamic diameter (Dh) was measured by dynamic light scattering (DLS) with a
Malvern NanoꢀZS90 Zetasizer. Transmission Electron Microscope (TEM) images were
obtained by a JEOL JEMꢀ1230 instrument operated at 80 kV.
Synthesis of MBTFVB 1
MBTVFB 1 trifluorovinyl monomer was first prepared from 2ꢀmethylhydroquinone
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via standard fluoroalkylation followed by Znꢀmediated dehalogenation1 as shown in
Scheme 1. The crude product was purified by silica column chromatography to afford
MBTFVB 1 as clear oil. 1H NMR (500 MHz, CDCl3): δ (ppm): 2.32 (s, 3H, CH3), 6.93
(m, 1H, phenyl), 6.97 (m, 1H, phenyl), 7.03 (m, 1H, phenyl). 19F NMR (470 MHz,
CDCl3): δ (ppm): ꢀ120.1, ꢀ127.2. ꢀ134.2. 13C NMR (125 MHz, CDCl3): δ (ppm): 15.6
(CH3), 114.3, 115.5, 119.0, 129.9, 132.7, 135.2, 143.9, 144.6, 146.5, 147.1, 149.3, 149.9
(OCF=CF2), 150.5, 151.8 (phenyl). EIꢀMS: m/z: 284. FTꢀIR (KBr): ν (cmꢀ1): 3045, 2968,
1832, 1603, 1496, 1422, 1311, 1275, 1178, 1157, 1009, 945, 874, 808, 766.
Scheme 1. Synthesis of cꢀPMBTFVB via Glaser coupling reaction.
Thermal Cycloaddition Polymerization of MBTFVB 1
PMBTFVB 2 homopolymer was prepared according to previous report.1 To a 30 mL
Schlenk flask (flameꢀdried under vacuum prior to use) sealed with a rubber septum,
MBTFVB 1 (1.42 g, 5.00 mmol) and diphenyl ether (5.0 mL) were first added under N2.
The flask was degassed by three cycles of freezingꢀpumpingꢀthawing followed by
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immersing the flask into an oil bath set at 200°C. 4ꢀMethoxytrifluorovinyloxybenzene
(0.612 g, 3.00 mmol) was introduced via a gastight syringe after 6 h for endꢀcapping. The
polymerization was terminated by putting the flask into liquid N2 after another 6 h. THF
(5.0 mL) was added to dilute the solution followed by precipitating into methanol. After
repeated purification by dissolving in THF and precipitating in methanol, 0.91 g of
MeOꢀPMBTFVBꢀOMe 2 as a white powder was obtained after drying in vacuo overnight.
1
GPC: Mn = 6,100 g/mol, Mw /Mn = 1.27. H NMR (500 MHz, CDCl3): δ (ppm): 2.07,
2.28 (3H, CH3), 3.77 (3H, OCH3), 6.81ꢀ7.10 (3H, phenyl). 19F NMR (470 MHz, CDCl3):
13
δ (ppm): ꢀ128.2 ~ ꢀ132.8 (cyclobutylꢀF6). C NMR (125 MHz, CDCl3): δ (ppm): 16.0,
55.6 (OCH3), 105.8, 109.2, 112.7, 116.6, 121.5, 131.0, 148.2. FTꢀIR (KBr): ν (cmꢀ1):
3052, 2933, 1599, 1498, 1314, 1267, 1201, 1128, 1006, 962, 925, 813, 741.
Demethylation of MeOꢀPMBTFVB-OMe 2
To a 100 mL threeꢀneck flask (flameꢀdried under vacuum prior to use),
MeOꢀPMBTFVBꢀOMe 2 (Mn,GPC = 6,100 g/mol, Mw /Mn = 1.27, 0.90 g, 0.25 mmol
ꢀOCH3) was first added followed by three cycles of evacuating and backfilling with N2.
CH2Cl2 (30 mL) was introduced via a gastight syringe and the mixture was stirred at 0°C.
BBr3 (0.20 mL, 2.12 mmol) in 5 mL of CH2Cl2 was added dropwise within 30 min, and
the mixture was stirred for another 1 h. The reaction was terminated by adding 10 mL of
methanol and the organic phase was washed twice with brine. The content was
concentrated and precipitated into 20 mL of methanol twice. The obtained product,
HOꢀPMBTFVBꢀOH 3, was dried in vacuo until a constant weight (0.823 g, 91.4%). GPC:
Mn = 6,000 g/mol, Mw/Mn = 1.25. 1H NMR (500 MHz, CDCl3): δ (ppm): 2.07, 2.28 (3H,
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CH3), 6.80ꢀ7.11 (3H, phenyl). 19F NMR (470 MHz, CDCl3): δ (ppm): ꢀ128.1~ ꢀ132.8
(cyclobutylꢀF6). 13C NMR (125 MHz, CDCl3): δ (ppm): 16.0, 105.8, 109.2, 112.7, 116.6,
121.5, 131.0, 148.0. FTꢀIR (KBr): ν (cmꢀ1): 3051, 2933, 1596, 1314, 1267, 1125, 1007,
962, 925, 813, 742.
Propargylation of HO-PMBTFVB-OH 3
To a 100 mL threeꢀneck flask, NaH (0.20 g, 8.33 mmol) was first added followed by
three cycles of evacuating and backfilling with N2. THF (20 mL) was introduced via a
gastight syringe and the mixture was stirred for 30 min. HOꢀPMBTFVBꢀOH 3 (Mn,GPC
=
6,000 g/mol, Mw/Mn = 1.25, 0.50 g, 0.139 mmol ꢀOH) in 10 mL of THF was added
dropwise slowly, and the mixture was stirred for another 1 h. 3ꢀbromoꢀ1ꢀpropyne (0.991
g, 8.33 mmol) was introduced via a gastight syringe and the mixture was stirred overnight
at room temperature. The organic phase was washed twice with brine and the content was
concentrated and precipitated into 10 mL of methanol twice. The obtained product,
≡
ꢀPMBTFVBꢀ
≡
4, was dried in vacuo until a constant weight (0.468 g, 93.6%). GPC: Mn
1
= 6,300 g/mol, Mw/Mn = 1.27. H NMR (500 MHz, CDCl3): δ (ppm): 2.07, 2.28 (3H,
ꢀCH3), 2.49 (1H, CH), 4.65 (2H, OCH2), 6.80ꢀ7.12 (3H, phenyl). 19F NMR (470 MHz,
13
CDCl3): δ (ppm): ꢀ128.0 ~ ꢀ132.8 (cyclobutylꢀF6). C NMR (125 MHz, CD3COCD3): δ
(ppm): 16.0, 56.1, 76.3, 78.1, 105.8, 109.2, 112.7, 116.7, 121.3, 131.3, 148.2. FTꢀIR
(KBr): ν (cmꢀ1): 2930, 1732, 1497, 1317, 1266, 1203, 1110, 1015, 962, 812, 743.
Glaser Coupling of
≡
-PMBTFVB- 4
≡
CuBr (0.318 g, 2.22 mmol), PMDETA (0.463 mL, 2.22 mmol), and pyridine (400 mL)
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were first added to a 1000 mL threeꢀneck flask (flameꢀdried under vacuum prior to use)
and the mixture was stirred for 1 h. ꢀPMBTFVBꢀ 4 (Mn,GPC = 6,300 g/mol, Mw/Mn =
≡
≡
1.27, 0.4433 g, 0.123 mmol alkynyl) in 100 mL of pyridine was added dropwise slowly
using a syringe pump (5.0 mL/h) within 20 h and the mixture was stirred for another 24 h.
The content was concentrated and filtered through a short neutral Al2O3 column to
remove the residual copper catalyst. The crude product was precipitated into methanol
twice and dried in vacuo at room temperature to afford a yellow powder with a constant
weight (0.425 g, 95.9%), cꢀPMBTFVB 5 cyclic homopolymer. GPC: Mn = 5,600 g/mol,
1
Mw/Mn = 1.26. H NMR (500 MHz, CDCl3): δ (ppm): 2.07, 2.28 (3H, CH3), 4.65 (1H,
OCH2), 6.81ꢀ7.10 (3H, phenyl). 19F NMR (470 MHz, CDCl3): δ (ppm): ꢀ127.8 ~ ꢀ132.5
(cyclobutylꢀF6). 13C NMR (125 MHz, CD3COCD3): δ (ppm): 16.1, 56.1, 70.2, 75.5,
105.8, 109.2, 112.5, 116.6, 121.5, 131.2, 148.1. FTꢀIR (KBr): ν (cmꢀ1): 3042, 2933, 2227,
1595, 1313, 1267, 1125, 1009, 961, 929, 813, 741.
Bromination of c-PMBTFVB 5
NBS and BPO were used to monoꢀbrominate the pendant methyls of cꢀPMBTFVB 5
homopolymer according to our previous reports61,62 as shown in Scheme 2. In a typical
procedure, cꢀPMBTFVB 5 (Mn,GPC = 5,600 g/mol, Mw/Mn = 1.26, 0.1260 g, 0.444 mmol
ꢀCH3), NBS (37.5 mg, 0.21 mmol), and BPO (15.3 mg, 0.063 mmol) were first added to a
100 mL threeꢀneck flask (flameꢀdried under vacuum prior to use) fitted with a reflux
condenser and the solution was deoxygenated under N2. Next, CCl4 (30 mL) was
introduced via a gastight syringe and the solution was refluxed at 80°C for 12 h. After
filtration, CCl4 was removed from the filtrate by rotary evaporation. The obtained solid
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was dissolved in 20 mL of ethyl acetate and the resulting solution was washed with brine
twice followed by drying over MgSO4. The solution was concentrated and precipitated
into methanol. After repeated purification by dissolving in THF and precipitating in
methanol, 0.108 g of cꢀPMBTFVBꢀBr 6a macroinitiator as a yellow powder was obtained
after drying in vacuo overnight. GPC: Mn = 5,700 g/mol, Mw/Mn = 1.27. EA: Br% : 8.1%.
1H NMR (500 MHz, CDCl3): δ (ppm): 2.03, 2.26 (3H, CH3), 4.27, 4.45 (2H, CH2Br),
6.75ꢀ7.30 (3H, phenyl). 19F NMR (470 MHz, CDCl3): δ (ppm): ꢀ127.6 ~ ꢀ132.5
13
(cyclobutylꢀF6). C NMR (125 MHz, CD3COCD3): δ (ppm): 15.5 (CH3), 24.6 (CH2Br),
105.0, 107.8, 111.7 (4C, PFCB), 116.5, 118.7, 120.9, 129.1, 148.6. FTꢀIR (KBr): ν (cmꢀ1):
3050, 2933, 1599, 1498, 1315, 1203, 1125, 1009, 962, 813, 741.
ATRP Graft Copolymerization of tBMA
ATRP bulk graft copolymerization of tBMA was performed using cꢀPMBTFVBꢀBr 6
as macroinitiator and CuBr/PMDETA as catalytic system. ATRP macroinitiator 6a
(Mn,GPC = 5,700 g/mol, Mw/Mn = 1.27, 0.02 g, 0.02 mmol –CH2Br) and CuBr (2.9 mg,
0.02 mmol) were first added to a 10 mL Schlenk flask (flameꢀdried under vacuum prior
to use) sealed with a rubber septum under N2. After three cycles of evacuating and
purging with N2, tBMA (0.65 mL, 4 mmol), PMDETA (0.004 mL, 0.02 mmol), and
1,4ꢀdioxane (0.5 mL) were charged via a gastight syringe. The flask was degassed by
three cycles of freezingꢀ pumpingꢀthawing followed by immersing the flask into an oil
bath set at 70°C. The polymerization was terminated by immersing the flask into liquid
N2 after 12 h. The reaction mixture was diluted with THF and passed through a neutral
Al2O3 column to remove the residual copper catalyst. The solution was concentrated and
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precipitated into methanol. After repeated purification by dissolving in THF and
precipitating in methanol, cꢀPMBTFVBꢀgꢀPtBMA 7a cyclic copolymer as a white
powder was obtained after drying in vacuo overnight. GPC: Mn = 11,300 g/mol，Mw/Mn =
1.36. 1H NMR (500 MHz, CDCl3): δ (ppm): 1.01, 1.21 (3H, CCH3 of polymethacrylate),
1.36 (9H, C(CH3)3), 1.81, 2.03 (2H, CH2 of polymethacrylate), 6.75ꢀ7.30 (3H, phenyl).
19F NMR (470 MHz, CDCl3): δ (ppm): ꢀ127.2 ~ ꢀ132.3 (m, cyclobutylꢀF6). 13C NMR
(125 MHz, CDCl3): δ (ppm): 18.8, 19.0, 27.9, 46.3, 80.7, 119.5, 128.1, 176.2. FTꢀIR
(KBr): ν (cmꢀ1): 3051, 2963, 1733, 1558, 1540, 1497, 1456, 1435, 1398, 1317, 1263,
1205, 1123, 1017, 963, 898, 801, 741.
Acidolysis of c-PMBTFVB-g-PtBMA 7
In a typical procedure, cꢀPMBTFVBꢀgꢀPtBMA 7a (Mn,GPC = 11,300 g/mol，Mw/Mn =
1.36, 30 mg) was first dissolved in 2 mL of dichloromethane at 0oC and 2 mL of
CF3COOH was added via a syringe. The mixture was stirred at room temperature for 24
h followed by concentration. The solution was precipitated into cold nꢀhexane followed
by drying in vacuo at room temperature for 1 day to provide 15.6 mg of white powder,
cꢀPMBTFVBꢀgꢀPMAA 8a. GPC: Mn = 11,500 g/mol, Mw/Mn = 1.35. 1H NMR (500 MHz,
DMSOꢀd6): δ (ppm): 0.95 (3H, CCH3), 1.60ꢀ1.83, 2.23 (2H, CH2), 6.81ꢀ7.23 (3H,
phenyl), 12.30 (1H, COOH). 19F NMR (470 MHz, CD3COCD3): δ (ppm): ꢀ127.5 ~ ꢀ133.2
(cyclobutylꢀF6). 13C NMR (125 MHz, CDCl3): δ (ppm): 16.2, 18.0, 27.9, 46.3, 108.2,
111.5, 129.3, 176.5. FTꢀIR (KBr): ν (cmꢀ1): 3322, 2931, 1736, 1483, 1447, 1407, 1372,
1255, 1151, 1009, 966, 842, 813, 745.
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Scheme 2. Synthesis of cꢀPMBTFVBꢀgꢀPMAA.
Determination of Critical Micelle Concentration
PNA was used as fluorescence probe to measure the cmc of cꢀPMBTFVBꢀgꢀPMAA 8.
Acetone solution of PNA (1 mg/mL) was added to a large amount of water until the
concentration of PNA reached 0.001 mg/mL. Next, different amounts of THF solutions
of cꢀPMBTFVBꢀgꢀPMAA 8 (1, 0.1, or 0.01 mg/mL) were added to water containing
PNA ([PNA] = 0.001 mg/mL). All fluorescence spectra were recorded at 20°C.
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Micellar Morphology
THF solution of cꢀPMBTFVBꢀgꢀPMAA 8 ([copolymer] = 0.2 mg/mL) was first
filtered through a membrane with a pore size of 0.45 ꢀm. Next, a certain amount of
deionized water was added slowly (0.36 mL/h) to 1.00 g of THF solution of copolymer
by a microꢀsyringe, and the solution was dialyzed against water with stirring for 5 days.
For TEM measurement, a drop of micelle solution (10 ꢀL) was deposited on an electron
microscopy copper grid coated with carbon film, and the water was evaporated at room
temperature for 1 day prior to measurement.
Results and Discussion
End Capping of PMBTFVB
MBTVFB 1 trifluorovinyl monomer was first synthesized according to standard
procedures of fluoroalkylation with BrCF2CF2Br and Znꢀmediated elimination in two
steps using commercially available 2ꢀmethylhydroquinone as starting material.1
PMBTFVB 2 homopolymer was then obtained via thermal [2π + 2π] stepꢀgrowth thermal
cycloaddition polymerization of bifunctional monomer 1 in diphenyl ether at 200°C and
both ends of the homopolymer were capped by monoꢀfunctional 4ꢀmethoxytrifluoroꢀ
vinyloxybenzene according to our previous reports.61ꢀ64 It should be point out that
1
because during the cyclization reaction, only chain ends took part in the reaction and H
and 13C NMR analyses were employed to determine the transformation of end groups,
thus PFCB aryl etherꢀbased backbone with a relative low molecular weight was prepared
1
13
on purpose for getting better H and C NMR spectra with high resolution. Figure 1A
shows 1H NMR spectrum of the homopolymer endꢀcapped with methoxyl. The signals at
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2.07 and 2.28 ppm (peak “a”) corresponded to 3 protons of the pendant methyls on the
benzene ring and the broad peaks between 6.81 ppm and 7.10 ppm were attributed to 3
protons of benzene ring. The signal originating from 3 protons of terminal methoxyl was
found to clearly appear at 3.77 ppm (peak “b”), which confirmed the effective
endꢀcapping with 4ꢀmethoxytrifluorovinyloxybenzene.
1
Figure 1. H NMR spectra of linear MeOꢀPMBTFVBꢀOMe 2 (A) and HOꢀPMBTFVBꢀ
OH 3 (B) in CDCl3.
The relative molecular weight of MeOꢀPMBTFVBꢀOMe 2 (Mn = 6,100 g/mol) was
obtained by conventional GPC which utilized linear polystyrene for calibration, therefore,
1H NMR was employed to determine the “absolute” molecular weight of the polymer
according to eq 1 (Sa and Sb are the integration area of peaks “a” at 2.07 and 2.28 ppm,
and peak “b” at 3.77 ppm in Figure 1A, respectively; 284 and 204 are the molecular
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weights of MBTFVB 1 monomer and 4ꢀmethoxytrifluorovinyloxybenzene) assuming the
complete endꢀcapping with 4ꢀmethoxytrifluorovinyloxybenzene. The molecular weight
was calculated to be 7,200 g/mol and this value indicated that every PMBTFVB chain
possessed 24.0 MBTFVB repeated units.
Mn,NMR = 284×(2Sa/Sb) + 204×2
(1)
Preparation of Linear Precursor for Glaser Coupling
Glaser coupling reaction between two terminal alkynyls has shown to be a powerful
tool for the formation of 1,3ꢀdiyne with a high yield and excellent functionality tolerance,
which has been employed in a variety of fields.65 In 2010, Huang et al. reported the
utilization of this reaction for the preparation of cyclic polymer under pseudoꢀultradilute
conditions with a nearly quantitative yield.28 Compared to widely used azideꢀalkyne and
thiolꢀene “click” reactions between two different and complementary groups, Glaser
coupling reaction is a homoꢀcoupling reaction taking place between two same terminal
alkynyls while it is more convenient and easier to introduce two same end alkynyls into a
polymeric chain. Since the mechanism of stepꢀgrowth thermal cycloaddition
polymerization made the obtained PMBTFVB homopolymer possess two same end
groups, Glaser coupling reaction was naturally chosen for synthesizing cyclic polymer
considering the ease in the functionalization of two same chain ends of PMBTFVB.
Firstly, methoxylꢀterminated PMBTFVB was converted to hydroxylꢀꢀterminated
PMBTFVB via demethylation of MeOꢀPMBTFVBꢀOMe 2 using BBr3. As shown in
Figure 1B, the signal of 3 protons of terminal methoxyl at 3.77 ppm (peak “b” in Figure
1A) completely disappeared after the demethylation while keeping the signals of pendant
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methyls and benzene ring. Moreover, the signal of carbon of terminal methoxyl at 55.6
ppm (peak “a” in Figure S1A) also could not be detected after the demethylation (Figure
S1B). We need to point out that GPC curves of polymers 2 and 3 (Figure S2) could be
overlapped very well and no peak attributed to the product with larger and smaller
molecular weights were observed. These results verified the efficient demethylation of
MeOꢀPMBTFVBꢀOMe 2 without crosslinking or degradation of PMBTFVB backbone.
Next, two terminal hydroxyls PMBTFVB were transformed into alkynyls using
3ꢀbromoꢀ1ꢀpropyne for providing Glaser coupling reaction precursor,
≡
ꢀPMBTFVBꢀ
≡
4.
1
The chemical structure of
≡
ꢀPMBTFVBꢀ
≡
4 was characterized by H and 13C NMR.
Typical proton resonance signal of alkynyl was located at 2.49 ppm (peak “a” in Figure
Figure 2. 1H NMR spectra of
≡
ꢀPMBTFVBꢀ 4 (A) and cꢀPMBTFVB 5 (B) in CDCl3.
≡
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2A) and the peak at 4.65 ppm in Figure 2A was attributed to 2 protons of methylene
adjacent to alkynyl. The resonance signals of 2 carbons of alkynyl were found to be
located at 76.3 (peak “a”) and 78.1 (peak “a”) ppm in Figure 3 and the peak at 56.1 ppm
(peak “c”) was ascribed to the carbon of methylene. These results clearly evidenced the
effective propargylation of HOꢀPMBTFVBꢀOH 3 by 3ꢀbromoꢀ1ꢀpropyne.
Figure 3. 13C NMR spectra of
≡
ꢀPMBTFVBꢀ
≡
4 (A) and cꢀPMBTFVB 5 (B) in
CD3COCD3.
Preparation of c-PMBTFVB via Glaser Coupling
The intramolecular cyclization of
≡
ꢀPMBTFVBꢀ 4 was achieved under high dilution
≡
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conditions by the endꢀtoꢀend Glaser coupling reaction between terminal alkynyls. The
coupling reaction was carried out using CuBr/PMDETA as catalytic system in pyridine
according to previous report.28 Utilizing a syringe pump, 100 mL pyridine solution of
linear precursor 4 with a concentration of 7.04×10ꢀ4 mol/L was added dropwise into 400
1
mL pyridine solution containing catalysts at the speed of 5.0 mL/h. H and 13C NMR
were employed to examine the obtained product. The peak at 4.65 ppm in Figure 2B
belonged to 2 protons of methylene adjacent to alkynyl while the signal of alkynyl at 2.49
ppm in Figure 2A was found to vanish completely in Figure 2B. Furthermore, the
resonance signals of 2 carbons of alkynyl were shifted to 70.2 and 75.5 ppm after the
cyclization via Glaser coupling reaction as shown in Figure 3B, which was consistent
with previous report about chemical shift of 1,3ꢀdiyne.66 The complete disappearance of
1H NMR signal of one proton of terminal alkynyl and the obvious shift of 13C NMR
signals originating from two carbons of alkynyl after Glaser reaction distinctly suggested
the efficient occurrence of cyclization of PMBTFVB polymeric chain.
GPC profiles of linear
≡
ꢀPMBTFVBꢀ 4 and cyclic cꢀPMBTFVB 5 were presented in
≡
Figure 4A. One can see a unimodal GPC profile of cꢀPMBTFVB 5 with a relatively
narrow molecular weight distribution (Mw/Mn = 1.26) and a clear shift of the elution peak
of cꢀPMBTFVB 5 to the lower molecular weight side as compared to its linear precursor
4. This observation indicated that cyclic cꢀPMBTFVB 5 had a smaller hydrodynamic
volume than that of linear precursor 4, which was consistent with previous studies on
cyclic polymer.23,24 It is worthy of noting that there is no peak or shoulder at the higher
molecular weight side, which meant that possible intermolecular coupling reaction was
effectively suppressed under current high dilution conditions. In addition, glass transition
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temperatures (Tg) of the obtained cyclic cꢀPMBTFVB 5 and linear precursor 4 were
measured by DSC. As shown in Figure 4B, Tg of cꢀPMBTFVB 5 was 62.9oC, which was
2.2oC higher than that of linear precursor 4. Previous studies showed that Tgs of cyclic
polymers with relative low molecular weights were higher than those of corresponding
linear counterparts because fewer degrees of freedom needed to be frozen out.16,67
Therefore, the shift of elution peak and higher Tg of cyclic polymers compared to those of
linear analogues were regarded as invaluable tools for differentiating cyclic polymers
from linear analogs, and in the current case, all aforementioned results proved the
efficient intramolecular cyclization of
≡
ꢀPMBTFVBꢀ 4 to afford cyclic cꢀPMBTFVB 5.
≡
Figure 4. GPC (A) and DSC (B) curves of
≡
ꢀPMBTFVBꢀ 4 (A) and cꢀPMBTFVB 5.
≡
Preparation of PMBTFVB-Br Macroinitiator
Monoꢀbromination of pendant methyls of cꢀPMBTFVB 5 for introducing
Brꢀcontaining ATRP initiating sites into cyclic backbone was conducted in CCl4 by
reacting with NBS and BPO according to our previous reports61,62 so that cꢀPMBTFVB 5
was transformed into cꢀPMBTFVBꢀBr 6 macroinitiator. The degree of bromination was
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tuned by the feeding ratio of NBS to methyl as listed in Table 1. It can be seen from
Table 1 that 0.5 equivalent of NBS to methyls of cꢀPMBTFVB 5 gave a bromination
content of 8.1% for cꢀPMBTFVBꢀBr 6a. With increasing the amount of NBS to 0.8 and
1.0 equivalent, the bromination extent was raised to 10.8 % and 13.5%, respectively.
Table 1. Preparation of cꢀPMBTFVBꢀBr 6 Macroinitiator
b
Mn,GPC
b
Br%a
c
d
Mw/Mn
DBr
NBr
NBS (eq)
(g/mol)
5,700
5,800
6,100
0.5
0.8
1.0
8.1%
10.8%
13.5%
1.27
1.28
1.28
33%
46%
58%
8
6a
6b
6c
11
14
a
b
Determined by the titration with Hg(NO3)2. Measured by GPC in THF at 35ºC. c
Grafting density of ꢀCH2Br ATRP initiating group obtained from element analysis. d The
number of ꢀCH2Br ATRP initiating group per chain obtained from element analysis.
1
Figure 5 shows H NMR spectrum of cꢀPMBTFVBꢀBr 6. It is clear that two new
peaks at 4.27 and 4.45 ppm (peak “a”) appeared in Figure 5, which corresponded to 2
protons of newly formed ꢀCH2Br group. The signals at 2.03 and 2.26 ppm (peak “b”)
belonged to 3 protons of the methyls which were not brominated. In addition, Figure S3
shows a series of peaks between ꢀ127.6 and ꢀ132.5 ppm attributed to PFCB linkage in 19F
NMR spectrum of cꢀPMBTFVBꢀBr 6 (Figure S3A) and a unimodal and symmetric
elution peak without tail or shoulder at the lower or higher molecular weight sides in
GPC trace (Figure S3B), which illustrated that the architecture of cyclic polymeric chain
was not destroyed during the bromination. Overall, it can be concluded from the
aboveꢀmentioned results that Brꢀcontaining ATRP initiating sites were successfully
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introduced by the monoꢀbromination of methyls of cꢀPMBTFVB 5 without affecting the
polymeric backbone.
Figure 5. 1H NMR spectrum of cꢀPMBTFVBꢀBr 6 macroinitiator in CDCl3.
The number of ꢀCH2Br ATRP initiating group per cyclic chain (NBr) and grafting
density of ꢀCH2Br ATRP initiating group (D could be estimated via the data of Br%
)
Br
(Table 1) according to eq 2 and 3 (24.0 is the total number of repeated unit with pendant
ꢀCH2Br or ꢀCH3 group in cꢀPMBTFVBꢀBr cyclic chain, 454 and 80 are the molecular
weights of connection group and bromine atom, 363 and 284 are the molecular weights
of repeated units with ꢀCH2Br or ꢀCH3 group). The values of NBr for cꢀPMBTFVBꢀBr 6a,
6b, and 6c were calculated to be 8, 11, and 14, respectively. Thus, the grafting densities
of ꢀCH2Br ATRP initiating group were 33%, 46%, and 58% for macroinitiators 6a, 6b,
and 6c (Table 1), respectively.
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80NBr/(363NBr +284(24-NBr) + 454) = Br%
DBr = NBr /24.0
(2)
(3)
Construction of Sun-Shaped c-PMBTFVB-g-PMAAAmphiphilic Copolymer
ATRP of tBMA was initiated by cꢀPMBTFVBꢀBr 6 macroinitiator at 70°C using
CuBr/PMDETA as catalytic system to yield two sunꢀshaped cꢀPMBTFVBꢀgꢀPtBMA 7
copolymers through the graftingꢀfrom strategy and the results are summarized in Table 2.
One can see that both copolymers have relatively narrow molecular weight distributions
(Mw/Mn≤1.38) and the molecular weights were much higher than that of macroinitiator.
These observations demonstrated that ATRP of tBMA was successfully performed and
intermolecular coupling reactions were suppressed using a high feeding ratio of tBMA to
ꢀCH2Br initiating group (200:1).68,69
Table 2. Synthesis of cꢀPMBTFVBꢀgꢀPtBMA 7 Copolymera
d
e
Mn
[tBMA]:[6
Mn,GPC/MALS
d
f
g
Mw/Mn
NtBMA
ntBMA
]
(g/mol)
(g/mol)
11,300
12,100
7ab
7bc
200:1
200:1
1.36
1.38
21,600
96.5
12.1
9.6
23,200
106.1
a
b
Polymerization temperature: 70°C, time: 12 h. Initiated by 6a (Mn = 5,700 g/mol,
Mw/Mn= 1.27). c Initiated by 6b (Mn = 5,800 g/mol, Mw/Mn= 1.28). d Measured by GPC in
e
THF at 35°C. Obtained by GPC equipped with a multiangle light scattering detector
f
(GPC/MALS) in THF with a flow rate of 1.0 mL/min. Total number of tBMA repeated
g
unit obtained from GPC/MALS. The number of tBMA repeated unit per PtBMA side
chain obtained from GPC/MALS.
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1H NMR spectrum of cꢀPMBTFVBꢀgꢀPtBMA 7 copolymer is shown in Figure 6A. It
was found that the signal of 2 protons of ꢀCH2Br initiating group connected to the
benzene ring located at 4.27 and 4.45 ppm (peak “a” in Figure 5) disappeared, indicative
of the complete involvement of ATRP initiating groups in graft copolymerization of
tBMA. Moreover, one also can see the signal of 9 protons of tertꢀbutyl of PtBMA side
chains located at 1.36 ppm (peak “c”) and the broad peak between 6.75 ppm and 7.30
ppm attributed to phenyl of cyclic PMBTFVB backbone. These evidences confirmed the
structure of cꢀPMBTFVBꢀgꢀPtBMA 7 sunꢀshaped copolymer.
1
Figure 6. H NMR spectra of (A) cꢀPMBTFVBꢀgꢀPtBMA 7 in CDCl3 and (B)
cꢀPMBTFVBꢀgꢀPMAA 8 in DMSOꢀd6.
Since the molecular weight of graft copolymer measured by GPC is much lower than
the “real” value,44,68 absolute molecular weights of cꢀPMBTFVBꢀgꢀPtBMA 7 sunꢀshaped
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copolymers were measured by GPC/MALS in THF. As listed in Table 2, the molecular
weights obtained from GPC/MALS were much higher than those measured by GPC as
expected. From the data of absolute molecular weights of copolymers (Mn,GPC/MALS in
Table 2), the total number of tBMA repeated unit (NtBMA) and the number of tBMA
repeated unit per PtBMA side chain (ntBMA) were calculated according to eq 4 and 5 (NBr
is the number of ꢀCH2Br ATRP initiating group per chain as listed in Table 1, 24 is the
total number of repeated unit with pendant ꢀCH2Br or ꢀCH3 group in the cyclic backbone,
454, 363, 284, and 142 are the molecular weights of connection group, repeated units
with ꢀCH2Br or ꢀCH3 group, and tBMA monomer, respectively) and the results are
summarized in Table 2. The molecular weights of copolymers 7a and 7b were also
1
calculated on the basis of H NMR results, which were 20,200 and 21,600 g/mol,
respectively. These values were very close to those obtained by GPC/MALS.
NtBMA = [Mn,GPC/MALS-363NBr-284(24-NBr)-454]/142
ntBMA = NtBMA/NBr (5)
(4)
Thus, we can conclude that cꢀPMBTFVBꢀgꢀPtBMA 7 sunꢀshaped copolymers
consisting of a PFCB aryl etherꢀbased backbone with 24 repeated units and PtBMA side
chains (7a: 8 PtBMA side chains with 12.1 tBMA repeated units per side chain; 7b: 11
PtBMA side chains with 9.6 tBMA repeated units per side chain) were synthesized by
ATRP of tBMA initiated by cꢀPMBTFVBꢀBr 6 macroinitiator.
Selective acidic hydrolysis of tertꢀbutoxycarbonyls of PtBMA side chain was achieved
by treating cꢀPMBTFVBꢀgꢀPtBMA 7 copolymer with anhydrous CF3COOH in CH2Cl2 at
room temperature for 1 day according to previous reports,70ꢀ72 to give cꢀPMBTFVBꢀgꢀ
PMAA 8 copolymer. Figure 6B shows 1H NMR spectrum of the hydrolyzed product. It is
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clear that the signal at 1.36 ppm originating from 9 protons of tertꢀbutyl of PtBMA side
chain (peak “c” in Figure 6A) completely disappeared with the appearance of a new peak
at 12.30 ppm attributed to 1 proton of newly formed carboxyl (peak “c” in Figure 6B).
Form FTꢀIR spectra before and after the hydrolysis, we can also notice that a much
stronger peak at 3322 cm−1 attributed to the newly formed carboxyl appeared in FTꢀIR
spectrum after the hydrolysis in comparison with that before the hydrolysis. Furthermore,
it was found from 13C NMR spectra of cꢀPMBTFVBꢀgꢀPtBMA 7 and cꢀPMBTFVBꢀgꢀ
PMAA 8 (Figure 7) that typical peaks at 80.7 ppm (peak “a” in Figure 7A) and 27.9 ppm
(peak “b” in Figure 7A) corresponding to tertꢀbutyl of PtBMA side chain completely
disappeared after the hydrolysis (Figure 7B), indicative of quantitative hydrolysis of tertꢀ
butoxycarbonyl into carboxyl. It should be noted that typical signals of other groups in
Figure 7. 13C NMR spectra of cꢀPMBTFVBꢀgꢀPtBMA 7 (A) and cꢀPMBTFVBꢀgꢀ
PMAA 8 (B) in CDCl3.
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1
the sunꢀshape copolymer were found to remain in H (Figure 6B) and 13C (Figure 7B)
NMR spectra after the hydrolysis. This observation suggested that other groups in the
sunꢀshaped copolymer kept intact during the hydrolysis, consistent with previous results
demonstrating the selectively hydrolysis of tertꢀbutoxycarbonyl in CF3COOH.70ꢀ72 All
these results evidenced the efficient and selective hydrolysis of tertꢀbutoxycarbonyl in
cꢀPMBTFVBꢀgꢀPtBMA 7 so that hydrophobic cꢀPMBTFVBꢀgꢀPtBMA 7 was converted
to amphiphilic cꢀPMBTFVBꢀgꢀPMAA 8 with hydrophilic PMAA side chains.
Self-Assembly of c-PMBTFVB-g-PMAA in Aqueous Media
Similar to small molecule surfactant, amphiphilic copolymer also has critical
micellization concentration (cmc), which is an important indicator reflecting the
solubility (dispersibility) or amphiphilicity of an amphiphilic copolymer. The cmc value
of cꢀPMBTFVBꢀgꢀPMAA 8 sunꢀshaped copolymer was analyzed by fluorescence
spectroscopy using PNA as probe. As it is wellꢀknown, fluorescence spectrum of PNA is
sensitively to the environment with a high fluorescence activity in nonpolar surroundings,
and a rather low fluorescence in polar solvents such as water.73 Figure 8A shows
fluorescent spectra of aqueous solutions containing PNA with a constant content of
2.0×10ꢀ6 g/mL and cꢀPMBTFVBꢀgꢀPMAA 8a with different concentrations. We can see
that the fluorescent intensity rose with the increasing of the concentration of copolymer.
We plotted the fluorescence intensity ratios of I/I0 in Figure 8B (I0 and I are the
fluorescent intensities at 418 nm for the aqueous solutions without and with the
copolymer, respectively) against the concentration of copolymer. It can be seen from the
figure that I/I0 increased sharply when the concentration of cꢀPMBTFVBꢀgꢀPMAA 8a
29