A facile strategy for synthesis of α,α'-heterobifunctionalized poly (ε-caprolactones) and poly (methyl methacrylate)s containing "clickable" aldehyde and allyloxy functional groups using initiator approach

Article abstract of DOI:10.1002/pola.26598

Two new initiators, namely, 4-(4-(2-(4-(allyloxy) phenyl)-5-hydroxypentane 2-yl) phenoxy)benzaldehyde and 4-(4-(allyloxy) phenyl)-4-(4-(4-formylphenoxy) phenyl) pentyl 2-bromo-2-methyl propanoate containing "clickable" hetero-functionalities namely aldehy

Full text of DOI:10.1002/pola.26598

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A Facile Strategy for Synthesis of a,a0-Heterobifunctionalized
Poly(e-caprolactones) and Poly(methyl methacrylate)s Containing ‘‘Clickable’’
Aldehyde and Allyloxy Functional Groups Using Initiator Approach
Prakash S. Sane, Bhausaheb V. Tawade, Indravadan Parmar, Savita Kumari,
Samadhan Nagane, Prakash P. Wadgaonkar
Polymer Science and Engineering Division, Chemistry Department, CSIR-National Chemical Laboratory, Pune-411008, India
Correspondence to: P. P. Wadgaonkar (E-mail: pp.wadgaonkar@ncl.res.in)
Received 28 October 2012; accepted 23 January 2013; published online 14 February 2013
DOI: 10.1002/pola.26598
ABSTRACT: Two new initiators, namely, 4-(4-(2-(4-(allyloxy)
phenyl)-5-hydroxypentane 2-yl) phenoxy)benzaldehyde and 4-
(4-(allyloxy) phenyl)-4-(4-(4-formylphenoxy) phenyl) pentyl
2-bromo-2-methyl propanoate containing ‘‘clickable’’ hetero-
functionalities namely aldehyde and allyloxy were synthesized
starting from commercially available 4,40-bis(4-hydroxyphenyl)
pentanoic acid. These initiators were utilized, respectively, for
ring opening polymerization of e-caprolactone and atom transfer
radical polymerization of methyl methacrylate. Well-defined
a-aldehyde, a0-allyloxy heterobifunctionalized poly(e-caprolac-
tones) (Mn,GPC: 5900–29,000, PDI: 1.26–1.43) and poly(methyl
methacrylate)s (Mn,GPC: 5300–28800, PDI: 1.19–1.25) were synthe-
sized. The kinetic study of methyl methacrylate polymerization
demonstrated controlled polymerization behavior. The presence
of aldehyde and allyloxy functionality on polymers was confirmed
by 1H NMR spectroscopy. Aldehyde-aminooxy and thiol-ene
metal-free double click strategy was used to demonstrate reactiv-
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Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2091–2103
KEYWORDS: atom transfer radical polymerization; click chemis-
try; functional polymers; ring opening polymerization
INTRODUCTION The synthesis of well-defined functionally ter-
minated polymers is an area of great contemporary interest
due to the utility of such polymers as building blocks for the
construction of complex polymer architectures.1–5 As end-
groups are retained, ‘‘living’’ polymerization processes are, by
their nature, particularly suited for the synthesis of end-func-
tional polymers. Thus, various ‘‘living’’ polymerization meth-
ods, including nitroxide-mediated polymerization,6 atom
transfer radical polymerization (ATRP),7,8 reversible addition-
fragmentation chain transfer9,10 and ring opening polymeriza-
tion (ROP)11,12 have been successfully adapted for this pur-
pose. Of the various available techniques, ATRP process is the
most robust and promising. It tolerates traces of impurities,
and is compatible with a broader range of monomers and reac-
tion conditions.13 Generally, three approaches are applied to
prepare a-, x- or a, x-bifunctionalized polymers by ATRP. The
first approach is to use a designed functional initiator, wherein
functional group is tolerant to ATRP reaction conditions which
results in the formation of a-functionalized polymers. Poly-
mers with a- as well as a-,a0-homobifunctional groups such as
dihydroxyl,14,15 dicarboxylic acid,16 bis(4-fluorobenzoyl),17
bis(aromatic bromo),18 and tert-amino19 have been prepared
successfully using initiator approach. The second approach is
to transform the halogen end group of the polymer into differ-
ent useful functional groups by post-polymerization modifica-
tion. For example, dihydroxyl20,21 and diamino22 groups have
been introduced onto the x-chain end via nucleophilic substi-
tution of bromine end group with appropriate amino-diols and
triamine, respectively. The transformation of the terminal bro-
mine into azido group followed by the click reaction with trial-
kyne leads to x-dialkynyl polystyrene.23 The same strategy
was also used for the synthesis of x-hydroxyl-x0-alkynyl poly-
styrene via the click reaction of x-azido polystyrene with 3,5-
bis(propargyloxy)benzyl alcohol.24 The third approach is to
use functional initiator accompanied by post-polymerization
functionalization of the halogen end group of the polymer
into desired functional groups.25 A survey of literature
revealed that major attention has been paid to the synthesis of
a,a0- or x,x0-homobifunctionalized polymers via ATRP pro-
cess14–18,20–23 with scant attention to the preparation of, a,a0
or x,x0-heterobifunctinalized polymers.24,26 Notably, synthesis
of a,a0-heterobifunctionalized polystyrenes with well-defined
molecular weights via combination of ATRP and chemical mod-
ification of end-functional groups was reported by Bai and co
workers.27 To the best of our knowledge, synthesis of poly(-
alkyl methacrylate)s or poly caprolactones with a,a0-heterobi-
functional groups using initiator approach has not been
reported in the literature.
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Herein, we present a facile strategy for synthesis of a,a0-het-
erobifunctionalized poly(e-caprolactones) and poly(methyl
methacrylate)s using functional initiator approach. Two new
heterobifunctional initiators namely, 4-(4-(2-(4-(allyloxy)
phenyl)-5-hydroxypentane 2-yl) phenoxy) benzaldehyde and
4-(4-(allyloxy) phenyl)-4-(4-(4-formylphenoxy)phenyl) pentyl
2-bromo-2-methyl propanoate having ‘‘clickable’’ functional-
ities such as aldehyde and allyloxy were synthesized starting
from commercially available 4,40-bis(4-hydroxyphenyl) penta-
noic acid. These initiators were utilized for preparation of
well-defined a,a0-heterobifunctionalized poly(e-caprolactones)
and poly(methyl methacrylate)s containing aldehyde and
allyloxy groups. Furthermore, the possibilities provided by
these functionalities for the preparation of new functional-
ized polymers by carrying out aldehyde-aminooxy and thiol-
ene click reactions are explored.
min. The reaction mixture was refluxed for 8 h, cooled and
filtered. Acetone was evaporated under reduced pressure
and the reaction mixture was diluted with ethyl acetate (150
mL). The ethyl acetate solution was washed with saturated
brine solution (3 ꢂ 50 mL), sodium bicarbonate solution (3
ꢂ 50 mL), and water (2 ꢂ 50 mL). The ethyl acetate solu-
tion was dried over sodium sulfate, filtered and evaporated
under vacuum. The crude product was purified by column
chromatography using ethyl acetate:pet ether (20:80, v/v) to
yield 15.9 g (95%) of 4-(2-(4-(allyloxy) phenyl)5-hydroxy
pentan-2-yl)phenol as a thick yellow liquid.
IR (CHCl3, cmꢀ1): 3130, 1250
1H NMR (CDCl3, d/ppm): 7.08–6.96 (m, 4H, ArAH), 6.79–
6.66 (m, 4H, ArAH), 6.11–5.97 (m, 1H, CH¼¼CHA), 5.43–5.23
(q, 2H, C¼¼CH2), 4.49 (d, 2H, OACH2), 3.58 (t, 2H, OCH2),
2.12–2.05 (m, 2H, ACH2ACH2), 1.54 (s, 3H, ACH3), 1.40–
1.35 (m, 2H, ACH2ACH2).
EXPERIMENTAL
Materials
Synthesis of 4-(4-(2-(4-(Allyloxy) phenyl)-5-hydroxy)
pentan-2-yl)phenoxy)benzaldehyde
4,40-Bis(4-hydroxyphenyl) pentanoic acid, lithium aluminum
hydride, 4-fluoro benzaldehyde, N,N,N0,N0,N00-pentamethyldie-
thylenetriamine (PMDETA), 2-bromoisobutyryl bromide
(98%), and stannous octoate (Aldrich) were used as
received. Methyl ester of 4,40-bis(4-hydroxyphenyl) penta-
noate and 4,40-(5-hydroxypentane-2,2-diyl) diphenol were
prepared as reported by us earlier.28 Styrene, e-caprolactone
and N,N-dimethylformamide (DMF) were stirred over cal-
cium hydride for 4 h and distilled under reduced pressure.
Copper (I) bromide (Aldrich, 99.9%) was washed with gla-
cial acetic acid to remove any soluble oxidized species, fil-
tered, washed with ethanol, and dried. Azobisisobutyronitrile
was recrystalized from acetone. Triethyl amine was distilled
prior to the use. Dichloromethane, tetrahydrofuran, sodium
sulphate, potassium hydroxide, sodium hydrogen carbonate,
methanol and chloroform, all received from S.D. Fine-Chem,
India, were used as received.
Into a 500 mL two necked round-bottom flask equipped
with a reflux condenser were charged, 4-(2-(4-(allyloxy) phe-
nyl) 5-hydroxy pentan-2-yl) phenol (12.5 g, 40 mmol), K2CO3
(5.44 g, 40 mmol) and dry DMF (150 mL) and the reaction
mixture was stirred for 10 minutes. The solution of 4-fluoro
benzaldehyde (4.96 g, 40 mmol) in DMF (50 mL) was added
over a period of 30 minutes. The reaction mixture was
heated at 100 ꢁC for 8 h, cooled and filtered. DMF was
evaporated under vacuum and the reaction mixture was
diluted with ethyl acetate (150 mL). The ethyl acetate solu-
tion was washed with saturated brine solution (3 ꢂ 50 mL),
sodium bicarbonate solution (3 ꢂ 50 mL), and water (2 ꢂ
50 mL). The ethyl acetate solution was dried over sodium
sulfate, filtered and evaporated under vacuum. The crude
product was purified by column chromatography using ethyl
acetate:pet ether (20:80, v/v) to yield 14.80 g (92 %) of 4-
(4-(2-(4-(allyloxy)phenyl)-5-hydroxy)pentan-2-yl)phenoxy)
benzaldehyde as a thick yellow liquid.
Characterizations and Measurements
FT-IR spectra were recorded on a Perkin-Elmer Spectrum GX
spectrophotometer in chloroform. NMR spectra were
recorded on a Bruker 200 MHz spectrometer using CDCl3,
Acetone-d6 or DMSO-d6 as solvents. Molecular weight and
molecular weight distribution of polymers were determined
using GPC analysis at a flow rate of 1 mL minꢀ1 in chloro-
form at 30 ꢁC (Thermo separation products) equipped with
spectra series UV 100 and spectra system RI 150 detectors.
The sample concentration was 2 to 3 mg mLꢀ1. HPLC grade
chloroform was used as an eluent. Polystyrene was used as
the calibration standard.
IR (CHCl3, cm-1): 3200, 1715, 1250.
1H NMR (CDCl3, d/ppm): 9.91 (s, 1H, CHO), 7.84 (d, 2H, Ar-
H ortho to aldehyde), 7.25–6.82 (m, 10H, Ar-H), 6.13–5.97
(m, 1H, CH¼¼CHA), 5.45–5.26 (q, 2H, C¼¼CH2), 4.52 (d, 2H,
ꢀOCH2), 3.63 (t, 2H, OCH2), 2.16–2.08 (m, 2H, ACH2ACH2),
1.64 (s, 3H, ACH3), 1.46–1.39 (m, 2H, ACH2ACH2).
13C NMR (CDCl3, d/ppm): 191.4, 162.3, 156.0, 155.6, 154.7,
140.2, 139.8, 139.2, 133.4, 131.4, 130.4, 129.7, 128.1, 126.9,
122.5, 118.3, 114.9, 114.5, 71.7, 63.8, 42.5, 42.2, 30.7, and 23.8
Synthesis
Synthesis of 4-(2-(4-(Allyloxy) phenyl)5-hydroxy
pentan-2-yl) phenol
Synthesis of 4-(4-(Allyloxy)phenyl)-4-(4-(4-formylphenox-
y)phenyl)pentyl 2-bromo-2-methyl propanoate
Into a 500 mL two-necked round-bottom flask equipped
with a reflux condenser were charged, 4,40-(5-hydroxypen-
tane-2,2-diyl) diphenol (16.3 g, 60 mmol), K2CO3 (8.1 g, 60
mmol) and dry acetone (150 mL) and the reaction mixture
was stirred for 10 min. The solution of allyl bromide (7.20 g,
60 mmol) in acetone (50 mL) was added over a period of 30
Into a 250 mL two-necked round-bottom flask equipped with
a dropping funnel were charged, 4-(4-(2-(4-(allyloxy) phenyl)-
5-hydroxy) pentan-2-yl)phenoxy)benzaldehyde (10.40 g, 25
mmol), triethylamine (3 g, 30 mmol), and dry chloroform
(100 mL). The reaction mixture was cooled to 0 ꢁC and the
solution of 2-bromoisobutyryl bromide (6.80 g, 30 mmol) in
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dry chloroform (30 mL) was added dropwise into the reaction
mixture under stirring over a period of 30 min. The reaction
mixture was stirred at 0 ꢁC for 2 h, allowed to attain room
temperature and was further stirred for 12 h. The reaction
mixture was washed with 5% aqueous NaHCO3 solution (3 ꢂ
100 mL) and de-ionized water (3 ꢂ 100 mL). The chloroform
layer was dried over anhydrous sodium sulfate, filtered and
was evaporated under reduced pressure. The crude product
was purified by silica gel column chromatography using a
mixture of ethyl acetate/pet ether (25:75, v/v) as an eluent.
The removal of the solvent yielded 12.6 g (90%) of 4-4-(ally-
loxy) phenyl)-4-(4-(4-formylphenoxy) phenyl)pentyl 2-bromo-
2-methylpropanoate as a pale yellow liquid.
0.85 mmol), methyl methacrylate (4.25 g, 42.5 mmol) and ani-
sole (5 mL). The solution was degassed and was transferred
via argon-purged syringe into the Schlenk tube under argon
atmosphere. The reaction mixture was degassed three times
by freeze-pump-thaw cycles. Under argon atmosphere, the
reaction mixture was opened and PMDETA (173 lL, 0.85
mmol) was added. The Schlenk tube was sealed with a stopper
ꢁ
and was kept in an oil bath at 80 C. Kinetic study was per-
formed by taking aliquots at regular intervals. After appropri-
ate time; polymerization was quenched by cooling reaction
mixture in liquid nitrogen bath. The reaction mixture was
diluted with tetrahydrofuran (50 mL) and the solution was
passed through neutral alumina column to remove copper res-
idue. The solution was concentrated and poured into cold
methanol (500 mL) to precipitate the polymer. The polymer
was filtered, dried under vacuum for 24 h and weighed. The
monomer conversion was determined gravimetrically.
IR (CHCl3, cmꢀ1): 1735, 1710
1H NMR (CDCl3, d/ppm): 9.92 (s, 1H, aldehyde), 7.84 (d, 2H,
Ar-H ortho to aldehyde), 7.26–6.83 (m, 10H, ArAH),
6.13–6.05 (m, 1H, CH¼¼CA), 5.45–5.25 (q, 2H, C¼¼CH2), 4.52
(d, 2H, AOCH2), 4.15 (t, 2H, CH2OCO), 2.16–2.06 (m, 2H,
ACH2), 1.93 (s, 6H, OCOC(CH3)2) 1.64 (s, 3H, ACH3),
1.53–1.47 (m, 2H, ACH2)
13C NMR (CDCl3, d/ppm): 191.3, 170.5, 162.8, 155.2, 154.4,
141.6, 140.6, 134.0, 131.4, 128.1, 122.2, 118.5, 114.3, 70.4,
65.7, 51.3, 41.6, 39.8, 30.7, and 24.1.
1H NMR (CDCl3 d/ppm): 9.93 (s, aldehyde from initiator),
7.85 (d, ArAH ortho to aldehyde from initiator), 7.14–6.84
(m, ArAH from initiator), 6.14–6.00 (m, ACH¼¼CA), 5.46–5.26
(q, AC¼¼CH2), 4.53 (d, AOCH2), 3.60 (s, AOCH3 from poly
(methyl methacrylate), 1.90–0.84 (m, CH2, ACH from poly
(methyl methacrylate þ protons from initiator fragment))
Chemical Modification
Synthesis of a-Aldehyde, a0-allyloxy heterobifunctionalized
poly(e-caprolactones)
Aldehyde-Aminooxy Click Reaction
Into a 50 mL two-necked round-bottom flask equipped with
a dropping funnel were charged, a-aldehyde, a0-allyloxy het-
erobifunctionalized poly(e-caprolactone) (1.94 g, 0.20 mmol,
Mn,NMR ꢀ9700)/poly(methyl methacrylate) (740 mg, 0.20
mmol, Mn,NMR ꢀ3700), dichloromethane (20 mL) and a pinch
of sodium sulfate. Then, solution of O-(2-azidoethyl) hydrox-
ylamine (250 mg, 20 mmol) dissolved in dichloromethane (5
mL) was added and the reaction mixture was stirred at
room temperature for 24 h. The reaction mixture was pre-
cipitated into cold hexane (200 mL). The obtained polymer
In a typical experiment, Schlenk tube equipped with a mag-
netic stir bar was charged with, e-caprolactone (5.68 g, 50
mmol), stannous (II) octoate (1 mg, 0.002 mmol), 4-(4-(2-(4-
(allyloxy) phenyl)-5-hydroxy) pentan-2-yl)phenoxy)benzalde-
hyde (172 mg, 0.415 mmol) and toluene (30 mL) under
nitrogen atmosphere. The reaction mixture was degassed
three times by freeze-pump-thaw cycles. e-Caprolactone poly-
ꢁ
merization was carried out at 110 C. After a given time, the
polymerization was terminated by cooling the reaction mix-
ture to room temperature, diluted with dichloromethane (30
mL) and the solution was poured into cold methanol (300
mL). The polymer was collected by filtration and dried at
room temperature in vacuum for 24 h. The monomer conver-
sion was determined gravimetrically.
ꢁ
was filtered and dried under vacuum at 50 C for 8 h.
IR (CHCl3, cmꢀ1): 2110, 1730
1H NMR (CDCl3, d/ppm): 8.07 (s, ACH¼¼N), 7.52 (d, ArAH
ortho to oxime), 7.11–6.80 (m, ArAH), 6.12–6.00 (m,
ACH¼¼CH2), 5.45–5.23 (q, AHC¼¼CH2), 4.50 (d, AOCH2), 4.05
(t, ACH2OOC from poly(e-caprolactone)), 2.26 (t, ACH2CO,
from poly(e-caprolactone)), 1.61–1.53 (m, ACH2CH2 from
poly(e-caprolactone) þ protons from initiator fragment),
1.39–1.31 (m, ACH2CH2 from poly(e-caprolactone))
1H NMR (CDCl3, d/ppm): 8.11 (s, ACH¼¼N), 7.55 (d, ArAH
ortho to oxime), 7.14–6.82 (m, ArAH from initiator frag-
ment), 6.19–6.02 (m, ACH¼¼CA), 5.46–5.31 (q, AC¼¼CH2),
4.54 (d, AOCH2), 4.30 (t, OCH2), 3.60 (s,-OCH3 from poly
(methyl methacrylate)), 1.91–0.81 [m, ACH2, ACH from poly
(methyl methacrylate þ protons from initiator fragment)].
IR (CHCl3, cmꢀ1): 1730, 1710
1H NMR (CDCl3, d/ppm): 9.90 (s, ACHO), 7.84 (d, ArAH ortho to
aldehyde), 7.25–6.81 (m, ArAH), 6.12–5.99 (m, ACH2ACH¼¼CH2),
5.45–5.23 (q, AHC¼¼CH2), 4.50 (d, AOCH2),4.04 (t, ACH2OOC
from poly(e-caprolactone)), 2.29 (t, ACH2CO from poly(e-capro-
lactone)0, 1.58–1.53 (m, ACH2CH2 from poly(e-caprolactone) þ
protons from initiator fragment), 1.34–1.31 (m, ACH2CH2 from
poly(e-caprolactone) þ protons from initiator fragment)
Synthesis of a-Aldehyde, a0-Allyloxy heterobifunctionalized
Poly(methyl methacrylate)s
In a typical experiment, Schlenk tube equipped with a mag-
netic stir bar was charged with, CuBr (120 mg, 0.85 mmol) and
the tube was thoroughly flushed with argon. In a separate sam-
ple vial were taken, 4-4-(allyloxy) phenyl)-4-(4-(4-formylphe-
noxy) phenyl) pentyl 2-bromo-2-methylpropanoate (480 mg,
Thiol-Ene Thermal Click Reaction
Into a clean and dry Schlenk tube were charged, allyloxy func-
tionalized poly(e-caprolactone) (490 mg, 0.05 mmol)/poly
(methyl methacrylate) (185 mg, 0.05 mmol), 3-maracptopro-
pionic acid (53 mg, 0.5 mmol), AIBN (82 mg, 0.5 mmol), and
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SCHEME 1 Synthesis of 4-(4-(2-(4-(allyloxy)phenyl)-5-hydroxy)pentan-2-yl)phenoxy)benzaldehyde and 4-(4-(allyloxy)phenyl)-4-(4-(4-
formylphenoxy)phenyl)pentyl 2-bromo-2-methyl propanoate.
chlorobezene (20 mL). The mixture was degassed via three
freeze-pump-thaw cycles and subsequentlyꢁ sealed under
vacuum. The Schlenk tube was heated at 80 C for 6 h. The
reaction mixture was cooled at 0 ꢁC. Chlorobenzene was
removed under reduced pressure. The polymer was dis-
solved in dichloromethane (20 mL) and precipitated into
cold methanol (200 mL) and purified by carrying out
precipitation for another two times into cold methanol. The
tiators for ROP and ATRP was envisaged by considering the
fact that orthogonality of functional groups should remain
intact and functional groups could be further used for
chemical modifications. The selection of allyloxy and alde-
hyde functional groups was made by considering the fact
that both these functional groups are compatible with reac-
tion conditions of ROP as well as ATRP29–34 and both the
functional groups could further undergo metal-free click
reactions such as thiol-ene and aldehyde-aminooxy click
reaction, respectively. To the best of our knowledge, a,a0-
heterobifunctional initiators containing ‘‘clickable’’ groups
suitable for ROP and ATRP have not been reported in the
literature.
ꢁ
polymer was filtered and was dried under vacuum at 50 C
for 8 h.
1H NMR (CDCl3, d/ppm): 8.05 (s, ACH¼¼N), 7.51 (d, ArAH
ortho to oxime), 7.11–6.78 (m, ArAH), 4.01 (t, ACH2OOC
from poly(e-caprolactone)), 2.89 (t, SACH2), 2.75 (t, SACH2),
2.63 (t, CH2ACOOH), 2.31 (t, ACH2CO, from poly(e-caprolac-
tone)), 1.69–1.63 (m, ACH2CH2 from poly(e-caprolactone) þ
protons from initiator fragment), 1.41–1.35 (m, ACH2CH2
from poly(e-caprolactone))
Scheme 1 depicts route for synthesis of 4-(4-(2-(4-(allyloxy)
phenyl)-5-hydroxy)pentan-2-yl)phenoxy)benzaldehyde
and
4-(4-(allyloxy)phenyl)-4-(4-(4-formylphenoxy)phenyl)pentyl
2-bromo-2-methyl propanoate. The synthesis of new initia-
tors was carried out starting from commercially available
4,40-bis(4-hydroxyphenyl) pentanoic acid, which in turn is
derived from levulinic acid—a platform chemical obtained
from biomass. 4,40-Bis(4-hydroxyphenyl) pentanoic acid was
converted into 4,40-(5-hydroxypentane-2,2-diyl)-diphenol by
the reported procedure28,35 and was subjected to allylation
reaction using one equivalent of allyl bromide in the pres-
ence of K2CO3 in acetone. To minimize the formation of di-
allyloxy substituted compound, addition of allyl bromide was
performed slowly. However, the crude reaction product was
found to be a mixture of desired mono-allyloxy product and
minor amount of di-allyloxy compound. Column chromatog-
raphy was found to be essential for isolation of mono ally-
loxy product 4-(2-(4-(allyloxy) phenyl) (5-(hydroxypentan-2-
yl) phenol. The next step in the synthesis involved nucleo-
philic substitution reaction of 4-fluorobenzaldehyde with
4-(2-(4-(allyloxy) phenyl) (5-(hydroxypentan-2-yl) phenol in
dry DMF using K2CO3 as a base to obtain 4-(4-(2-(4-(ally-
loxy) phenyl)-5-hydroxy) pentan-2-yl)phenoxy)benzaldehyde.
1H NMR (CDCl3, d/ppm): 8.11 (s, ACH¼¼N), 7.55 (d, ArAH
ortho to oxime), 7.14–6.82 (m, ArAH from initiator), 4.54 (d,
AOCH2), 4.30 (t, OCH2), 3.60 (s, AOCH3 from poly(methyl
methacrylate), 2.89 (t, SACH2), 2.75 (t, SACH2), 2.63 (t,
CH2ACOOH), 1.87–0.81 (m, CH2, ACH from poly(methyl
methacrylate).
RESULTS AND DISCUSSION
Synthesis of 4-(4-(2-(4-(Allyloxy)phenyl)-5-
hydroxy)pentan-2-yl)phenoxy)benzaldehyde and 4-(4-
(allyloxy)phenyl)-4-(4-(4-formylphenoxy)phenyl)pentyl
2-bromo-2-methyl propanoate
a,a0-Bifunctionalized polymers possessing ‘‘clickable’’ func-
tional groups represent valuable precursors for synthesis of
different types of end functionalized polymers and Y-shaped
miktoarm star copolymers. In the present work, we report
synthesis of a,a0-hetero bifunctionalized polymers using ini-
tiator approach. The construction of heterobifunctional ini-
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FIGURE 1 1H NMR spectra of (A) 4-(2-(4-(allyloxy)phenyl)-5-hydroxypentan-2-yl)phenol, (B) 4-(4-(2-(4-allyloxy)phenyl)-5-hydroxy)pen-
tan-2-yl)phenoxy)benzaldehyde, and (C) 4-(4-(allyloxy) phenyl)-4-(4-(4-formylphenoxy) phenyl) pentyl 2-bromo-2-methyl propanoate.
The product was purified by column chromatography and
4-(4-(Allyloxy)phenyl)-4-(4-(4-formylphenoxy)phenyl)pentyl
2-bromo-2-methyl propanoate was synthesized by esterifica-
tion of 4-(4-(2-(4-(allyloxy)phenyl)-5-hydroxy)pentan-2-yl)
phenoxy)benzaldehyde with 2-bromoisobutyryl bromide in
dry chloroform Scheme 1. The product was purified by silica
gel column chromatography and was characterized using
1
was characterized by FT-IR, H NMR, and 13C NMR spectros-
copy. FT-IR spectrum of 4-(4-(2-(4-(allyloxy)phenyl)-5-
hydroxy)pentan-2-yl)phenoxy)benzaldehyde showed absorp-
tion band at 3200 cmꢀ1 which corresponds to asymmetric
stretching of alcohol functionality, and a characteristic peak
at 1715 cmꢀ1 that corresponds to asymmetric stretching to
1
FT-IR, H NMR and 13C NMR spectroscopy. FT-IR spectrum of
1
aldehyde functionality. Figure 1 represents H NMR spectrum
4-(4-(allyloxy)phenyl)-4-(4-(4-formylphenoxy) phenyl)pentyl
2-bromo-2-methyl propanoate showed absorption bands at
1735, and 1710 cmꢀ1 that correspond to ester carbonyl and
aldehyde asymmetric stretching, respectively. 1H NMR
spectrum of 4-(4-(allyloxy) phenyl)-4-(4-(4-formylphenoxy)
phenyl) pentyl 2-bromo-2-methyl propanoate along with
assignments is shown in Figure 1. The presence of aldehyde
functionality was confirmed by the appearance of a singlet at
9.92 ppm. The presence of allyloxy functionality was con-
firmed by the appearance of multiplets in the range
6.13–6.05 ppm, and 5.45–5.25 ppm corresponding to methine
and vinyl protons, respectively. The singlet appeared at 1.93
ppm could be attributed to protons corresponding to methyl
group of halo ester. The spectral data corresponding to other
protons was in good agreement with the proposed structure.
In 13C NMR spectrum, a peak corresponding to carbonyl
carbon of aldehyde group was observed at 191.3 ppm and
of 4-(4-(2-(4-(allyloxy) phenyl)-5-hydroxy) pentan-2-yl)phe-
noxy)benzaldehyde along with the assignments. The pres-
ence of aldehyde functionality was confirmed by the appear-
ance of a singlet at 9.91 ppm. The aromatic protons ortho to
aldehyde group were deshielded and appeared as a doublet
at 7.84 ppm. The presence of allyloxy functionality was con-
firmed by the appearance of multiplets in the range 6.13–
5.97 ppm, and 5.45–5.26 ppm corresponding to methine and
vinyl protons, respectively. The spectral data corresponding
to other protons was in good agreement with the proposed
structure. In 13C NMR spectrum, a peak due to aldehyde car-
bonyl carbon was observed at 191.4 ppm, the peaks corre-
sponding to carbon atoms in allyloxy functionality appeared
at 133.4 and at 118.3 ppm. The data corresponding to other
carbon atoms was in good agreement with the proposed
structure.
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SCHEME 2 Synthesis of (A) a-aldehyde, a0-allyloxy heterobifunctionalized poly(e-caprolactones) (B) a-aldehyde, a0-allyloxy heterobi-
functionalized poly(methyl methacrylate)s.
peak due to ester carbonyl carbon was observed at 170.5
ppm. The carbon atoms corresponding to allyloxy group
appeared at 134.0 and 118.5 ppm. The data corresponding to
other carbon atoms was in good agreement with the proposed
structure.
a0-allyloxy hetero functionalized poly(e-caprolactones) were
determined by 1H NMR spectroscopy by comparing integral in-
tensity of peak belonging to AOCH2 in poly(e-caprolactone) at
4.04 ppm to a singlet at 9.90 ppm corresponding to aldehyde
group. The degree of polymerization (Dpn) was calculated
from NMR analysis using the relation,
Synthesis of a-Aldehyde, a0-Allyloxy Hetero
Bifunctionalized Poly(e-caprolactones)
Dpn ¼ ½I4:04=2=ðI9:91=1Þ ðaldehyde protonÞꢃ
ROP of e-caprolactone was carried out using 4-(4-(2-(4-(ally-
loxy) phenyl)-5-hydroxy)pentan-2-yl)phenoxy)benzaldehyde
as initiator in the presence of stannous octoate as a catalyst
in toluene [Scheme 2(a)]. The conditions and results of syn-
thesis of a-aldehyde, a0-allyloxy heterobifunctionalized poly
(e-caprolactones) are summarized in Table 1. Poly(e-caprolac-
tones) with molecular weights ranging from Mn,GPC-5900 to
29,000 were synthesized by varying monomer to initiator
Molecular weights were calculated using the equation,
Mn;NMR ¼ ½Dpn ꢂ mol: wt: of monomer ð114Þꢃ
þ mol: wt: inititor ð416Þ
Mn,NMR values were in reasonably good agreement with theo-
retical molecular weights (Mn,theo) calculated from the mono-
mer to initiator ratio. In addition, GPC data revealed mono-
modal distribution with PDI values in the range 1.26–1.43,
for poly(e-caprolactone). Thus, 4-(4-(2-(4-(allyloxy) phenyl)-
5-hydroxy) pentan-2-yl)phenoxy)benzaldehyde was found to
be a useful ROP initiator for synthesis of a-aldehyde, a0-ally-
loxy heterobifunctionalized poly(e-caprolactones).
1
ratio. H NMR spectrum of a-aldehyde, a0-allyloxy heterobi-
functionalized poly(e-caprolactone) is represented in Figure 2.
The appearance of a singlet at 9.90 ppm and multiplets in the
range 6.12–5.99 ppm, and 5.45–5.23 ppm confirmed the pres-
ence of aldehyde and allyloxy functionality, respectively on
poly(e-caprolactone). Molecular weights for a-aldehyde,
TABLE 1 Reaction Conditions and Results for Synthesis of a-Aldehyde, a0-Allyloxy Heterobifunctionalized Poly(e-caprolactones)
a
c
d
e
Sr. No.
[M]0/[I]0
Time (h)
Conv. (%)b
Mn,theo
Mn,NMR
Mn,GPC
Mw/Mn
1
2
3
60:1
8
16
24
61
71
69
4,600
10,100
19,300
5,100
9,700
5,900
11,800
29,000
1.43
1.34
1.26
120:1
240:1
23,000
d
e
Temperature ¼ 110 ꢁC, Solvent: Toluene), [CL]/ [Sn (Oct)2] ¼ 200.
Mn,NMR ¼ determined from NMR.
Mn,GPC ¼ Polystyrene standard; CHCl3 eluent.
a
[M]O/[I]O ¼ [Monomer]O/[Initiator]O.
b
Gravimetry.
n
o
½MꢃO
½IꢃO
ð% Conv:Þ ꢂ mol: wt: of monomer ð114Þ
þ
c
Mn,theo
¼
ꢂ
100
mol. wt. initiator (416).
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1
FIGURE 2 H NMR spectra of (A) 4-(4-(2-(4-(allyloxy)phenyl)-5-hydroxy)pentan-2-yl)phenoxy)benzaldehyde and (B) a-aldehyde, a0-
allyloxy heterobifunctionalized poly(e-caprolactone) in CDCl3.
Synthesis of a-Aldehyde, a0-Allyloxy Hetero
Bifunctionalized Poly(methyl methacrylate)s
acrylate) and at 9.93 ppm corresponding to aldehyde proton,
respectively.
ATRP of methyl methacrylate was carried out using 4-4-(ally-
loxy) phenyl)-4-(4-(4-formylphenoxy) phenyl) pentyl 2-bromo-
2-methylpropanoate as the initiator Scheme 2(b). The condi-
tions and results of synthesis of a-aldehyde a0-allyloxy hetero
bifunctionalized poly(methyl methacrylate)s are summarized in
Table 2. Different ratios of [M0]/[I0] with different reaction
intervals were chosen so as to obtain polymers with a range of
molecular weights (Mn,GPC,: 5300–28,800). The conversions
were determined by gravimetric analysis. The reaction conver-
sions were kept in the range 55–67% to ensure avoidance of
potential side reactions related to allyloxy functional group, if
any.36 1H NMR spectrum of a-aldehyde, a0-allyloxy functional-
ized poly(methyl methacrylate) is reproduced in Figure 3. The
appearance of a singlet at 9.93 ppm confirmed the presence of
aldehyde functionality. The appearance of multiplets in the
range 6.14–6.0 ppm and 5.46–5.26 ppm confirmed the presence
of allyloxy functionality. Integration of signals for aldehyde func-
tionality and comparison with the integration values for
methoxy protons in poly(methyl methacrylate) allows the mo-
lecular weight to be determined. The Dpn was calculated from
NMR analysis using the relation,
Molecular weights were calculated by 1H NMR spectroscopy
using equation,
Mn;NMR ¼ ½Dpn ꢂ 100 ðmol: wt: of monomerÞꢃ
þ mol: wt: initiator ð565Þ
Molecular weights determined by 1H NMR spectroscopy
(Mn,NMR) were in reasonably good agreement with theoretical
molecular weights (Mn,theo) indicating good initiator effi-
ciency (Ieff ¼ 0.71–0.89). In addition, GPC trace revealed
monomodal distribution (Fig. 4) with PDI values in the range
1.19–1.25 for poly(methyl methacrylate)s. The PDIs were rel-
atively narrow, which is a characteristic behavior of a con-
trolled radical polymerization method. A kinetic study was
performed to verify the control over the polymerization of
methyl methacrylate. The linearity of plot of ln (M0/Mt) ver-
sus polymerization time (Fig. 5) where M0 and Mt are initial
and the actual monomer concentration indicated the pseudo
first order kinetics. The concentration of radicals remained
constant during the polymerization reaction and therefore it
can be concluded that no detectable side reactions occurred
during polymerization. The linear increase of molecular
weight with increasing conversion and PDI below 1.25
(Fig. 6) represents an additional indication for a controlled
Dpn ¼ fðI3:58=3Þ=I9:93=1ðaldehyde protonÞg
where I3.58 and I9.93 are integrals of the signals positioned at
3.58 corresponding to methoxy protons of poly(methyl meth-
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TABLE 2 Reaction Conditions and Results of Synthesis of a-Aldehyde a0-Allyloxy Hetero Bifunctionalized Poly(methyl
methacrylate)s
Conv. (%)b
Mn,theo
Mn,NMR
Mn,,GPC
Mw/Mn
Ieff
a
c
d
e
Sr. No.
[M]0/[I]0
Time (h)
1
2
3
50:1
3
5
8
55
60
67
3,300
6,600
14,000
d
3,700
7,800
5,300
12,100
28,800
1.21
1.25
1.19
0.89
0.84
0.71
100:1
200:1
19,700
Temperature ¼ 80 ꢁC, Solvent: Anisole (50%, w/v w.r.t monomer).
Mn,NMR ¼ determined from NMR.
a
e
[M]O/[I]O: [Monomer]O/[Initiator]O
Mn,GPC ¼ determined from GPC; Polystyrene standard; CHCl3 eluent
b
Gravimetry.
Ieff ꢀ Mn,theo/Mn,NMR
.
n
o
½MꢃO
½Iꢃ
c
Mn,theo
¼
ꢂ
ð% Conv:Þ ꢂ mol: wt: of monomer ð100Þ
þ
100
mol. wt. initiatoO r (565).
polymerization mechanism. Thus, 4-(4-(allyloxy) phenyl)-4-
(4-(4-formylphenoxy) phenyl)pentyl 2-bromo-2-methyl prop-
anoate was found to be an useful ATRP initiator for con-
trolled polymerization of methyl methacrylate.
by considering the fact that aldehyde group would undergo
addition reaction with thiol compounds and in the second
step, thiol-ene click reaction was performed.
Aldehyde-Aminooxy Click Reaction of a-Aldehyde,
a0-Allyloxy Hetero bifunctionalized Poly(e-caprolactone)/
Poly(methyl methacrylate) with O-(2-Azidoethyl)
hydroxylamine
Chemical Modification of a-Aldehyde, a0-allyloxy Hetero
Bifunctionalized Poly(e-caprolactone) /Poly(methyl
methacrylate)
The allyloxy and aldehyde functionalities introduced deliber-
ately on poly(e-caprolactone)/poly(methyl methacrylate) are
known to undergo different types of metal-free click reac-
tions. Aldehyde and allyloxy functional groups undergo alde-
hyde-aminooxy37,38 and thiol-ene click reactions,39–41 respec-
tively. Aldehyde-aminooxy click reaction was first performed
The reactivity of aldehyde functionality on poly(e-caprolac-
tone)/poly(methyl methacrylate) was illustrated by carrying
out click reaction on poly(e-caprolactone)/poly(methyl meth-
acrylate) with O-(2-azidoethyl) hydroxylamine at room tem-
perature (Scheme 3). The conversion was assessed by FT-IR
and 1H NMR spectroscopy. In FT-IR spectrum, in addition to
FIGURE 3 1H NMR spectra of (A) 4-(4-(allyloxy)phenyl)-4-(4-(4-formylphenoxy)phenyl)pentyl 2-bromo-2-methyl propanoate and (B)
a-aldehyde, a0-allyloxy heterobifunctionalized poly(methyl methacrylate) in CDCl3.
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FIGURE 6 Dependence of number-average molecular weight
and PDI on monomer conversion for ATRP of methyl methacry-
late at 80 ꢁC in anisole.
FIGURE 4 GPC trace of a-aldehyde, a0-allyloxy heterobifunc-
tionalized poly(methyl methacrylate) (Table 2, Run 2).
(2-azidoethyl) hydroxylamine introduces azido moiety on
palycaprolactone/poly(methylmethacrylate) chain which fur-
ther opens up plethora of opportunities to introduce differ-
ent types of functional groups on poly(e-caprolactone)/poly(-
methylmethacrylate) by well-known azide-alkyne click
reaction.42 Furthermore, polymers possessing clickable end
functional groups represent valuable precursors for synthesis
of Y-shaped miktoarm star copolymers.
the peaks corresponding to ester carbonyl of poly(e-caprolac-
tone)/poly(methyl methacrylate) around 1730 cmꢀ1, charac-
teristic peak corresponding to azido functionality appeared
at 2110 cmꢀ1 confirming introduction of azido groups via
1
oxime formation. Figure 7 represents H NMR spectrum of a-
aldehyde, a0-allyloxy heterobifunctionalized poly(e-caprolac-
tone) (Mn: 9700) and its click reaction product with O-(2-
azidoethyl) hydroxylamine while Figure 8 represents 1H
NMR spectra of a-aldehyde, a0-allyloxy heterobifunctionalized
poly(methyl methacrylate) (Mn: 3700) and its click reaction
product with O-(2-azidoethyl) hydroxylamine. 1H NMR spec-
tra showed complete disappearance of the peak correspond-
ing to aldehyde functionality and appearance of a new peak
around 8.11 ppm (ACH¼¼NAO) which elucidates oxime for-
mation without affecting peaks related to poly(e-caprolac-
tone)/poly(methyl methacrylate) attesting completion of the
reaction without any side reaction such as degradation of
poly(e-caprolactone)/poly(methyl methacrylate) backbone.
The model aldehyde-aminooxy click reaction study with O-
Thiol-Ene Click Reaction on a-Allyloxy a0-Azido Hetero
Bifunctionalized Poly(e-caprolactone) and Poly(methyl
methacrylate)
The reactivity of allyloxy functionality on poly(e-caprolac-
tone) and poly(methyl methacrylate) was illustrated by car-
rying out click reaction with 3-mercaptopropionic acid in the
presence of AIBN at 80 ꢁC in chlorobenzene as a solvent
1
[Schemes 3(b) and 4(b)]. The conversion was assessed by H
1
NMR spectroscopy. H NMR spectra of a-allyloxy a0-azido
functionalized poly(e-caprolactone) (Mn: 9700) and its click
reaction product with 3-mercaptopropionic acid is depicted
in Figure 9. Although Figure 10 represents 1H NMR spectra
of a-allyloxy a0-azido functionalized poly(methyl methacry-
late) (Mn: 3700) and its click reaction product with 3-mer-
captopropionic acid. 1H NMR spectra showed complete dis-
appearance of the peak corresponding to allyloxy
functionality and appearance of new signals at 2.89, 2.75,
and 2.63 ppm (CHAS) which evidenced the successful thiol
addition reaction. The peaks corresponding to polycaprolac-
tone/poly(methylmethacrylate) backbones remained intact in
the product polymer attesting completion of the reaction
without any side reaction. The model thiol-ene click reaction
study with 3-mercaptopropionic acid opens up additional
opportunities to introduce different functionalities on poly(e-
caprolactone)/poly(methyl methacrylate) via utilization of
functionalized thiols.43 For instanace, thiols containing func-
tional groups such as furan, pyrene, trimethoxysilyl, perfluor-
oalkyl, and so forth, could be utilized to introduce the corre-
sponding functional groups on poly(e-caprolactone)/
poly(methyl methacrylate).
FIGURE 5 Relationship between ln ([M0]/[Mt]) and the polymer-
ization time for ATRP of methyl methacrylate at 80 ꢁC in
anisole.
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SCHEME 3 Post-functionalization of a-aldehyde, a0-allyloxy heterobifunctionalized poly(e-caprolactone)/poly(methyl methacrylate)
by (A) aldehyde-aminooxy and (B) thiol-ene click rection.
1
FIGURE 7 H NMR spectra of (A) a-aldehyde, a0-allyloxy heterobifunctionalized poly(e-caprolactone) and (B) the product of alde-
hyde-aminooxy click reaction in CDCl3.
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1
FIGURE 8 H NMR spectra of (A) a-aldehyde, a0-allyloxy heterobifunctionalized poly(methyl methacrylate) and (B) the product of
aldehyde-aminooxy click reaction in CDCl3.
1
FIGURE 9 H NMR spectra of (A) a-allyloxy, a0-azido heterobifunctionalized poly(e-caprolactone) and (B) the product of thiol-ene
click reaction in CDCl3.
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1
FIGURE 10 H NMR spectra of (A) a-allyloxy, a0-azido heterobifunctionalized poly(methylmethacrylate) and (B) the product of thiol-
ene click reaction in CDCl3.
CONCLUSIONS
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