Hydrogen phosphates: Self initiated organocatalysts for the controlled ring-opening polymerization of cyclic esters

Article abstract of DOI:10.1016/j.ica.2013.02.006

A series of arylhydrogenphosphates and aryldihydrogenphosphates was synthesized and characterized using spectroscopic methods and single crystal X-ray diffraction. These compounds were assessed as catalysts towards the ring-opening polymerization and proved to be potent organocatalysts for the ring-opening polymerization of cyclic esters. The bulk polymerizations were performed in the absence of external initiator. The polymerization proceeds in a controlled fashion which leads to well defined polyesters with narrow molecular weight distributions. In the post polymerization experiments, kinetics, mechanism and monomer concentration effects were investigated. The kinetic results have confirmed the pseudo-living character of the polymerizations and mechanistic studies suggest that the polymerization operates through a cationic mechanism.

Full text of DOI:10.1016/j.ica.2013.02.006

Inorganica Chimica Acta 400 (2013) 32–41
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Inorganica Chimica Acta
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Hydrogen phosphates: Self initiated organocatalysts for the controlled
ring-opening polymerization of cyclic esters
⇑
Payal Malik, Debashis Chakraborty
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of arylhydrogenphosphates and aryldihydrogenphosphates was synthesized and characterized
using spectroscopic methods and single crystal X-ray diffraction. These compounds were assessed as cat-
alysts towards the ring-opening polymerization and proved to be potent organocatalysts for the ring-
opening polymerization of cyclic esters. The bulk polymerizations were performed in the absence of
external initiator. The polymerization proceeds in a controlled fashion which leads to well defined polye-
sters with narrow molecular weight distributions. In the post polymerization experiments, kinetics,
mechanism and monomer concentration effects were investigated. The kinetic results have confirmed
the pseudo-living character of the polymerizations and mechanistic studies suggest that the polymeriza-
tion operates through a cationic mechanism.
Received 26 November 2012
Received in revised form 6 February 2013
Accepted 7 February 2013
Available online 17 February 2013
Keywords:
Arylhydrogenphosphates
Ring-opening polymerization
Cyclic esters
Pseudo-living character
Cationic mechanism
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
and their subsequent use in biomedical and micro-electronics
[24,25].
Polymer synthesis has been revolutionized by modern synthetic
methods through the extraordinary development of new strategies
for the controlled synthesis of complex macro-molecule architec-
tures [1–6]. In the last three decades, biodegradable polymers have
received attention as resorbable biomaterials as well as commodity
polymers [7,8]. The interest in design and synthesis of biodegrad-
able aliphatic polyesters and their copolymers is constantly
increasing due to their potential applications in biological and
biomedical areas [9,10]. Conventionally, these polyesters were
synthesized through ring-opening polymerization of the cyclic
esters and lactides [11–13]. In this context, numerous metallic
and organic derivatives have been used as initiators/catalysts for
the ring-opening polymerization of cyclic esters and lactides and
spectacular improvements have been achieved in terms of activity,
productivity and stereocontrol [14–19]. A wide range of metal
alkoxide complexes have developed using metals such as Al, Mg,
Zn, Ca, Sn, Fe, Ti, Zr, Y, Sm and Lu [20,21]. However, the metal
contaminant derived from the chain end of the polymer products
is the main limitation in the application of these polymers as
biomaterials and micro-electronics [22]. Since last few years, the
development of biocompatible metal catalysts [19,23] or metal-free
organic catalysts (organocatalysts) are the paramount challenges in
polymer science, to synthesize polymer without metal contaminant
Organocatalysis provides great opportunities in polymer synth-
esis. Among the earliest reports, in 1940, Flory has used organoca-
talyst for the synthesis of polyesters [26]. In the recent years,
organocatalysts have been developed as novel catalytic systems
for various types of controlled/living polymerizations leading to
well-defined polymers, such as end-functionalized polymers and
block copolymers [27–29]. The diversity of mechanistic pathways
of organocatalysis has provided new opportunities for control over
polymerization and enhancing the rate and selectivity of ring-
opening polymerization reactions [30,31]. In the realm of organo-
catalysis Hedrick, Waymouth and co-workers have developed a
broad class of highly active, environmentally benign organocata-
lysts for the ring-opening polymerization of cyclic esters and
lactide [32–37].
Within the last decade, organocatalysis has tremendously
accelerated the development of polymer synthesis [7]. The organic
acids catalyzed ring-opening polymerizations of cyclic esters and
lactides [38–42] evoked us to investigate the catalytic activity of
arylhydrogenphosphate derivatives. In fact, Kakuchi and co-
workers in 2011 have reported diphenyl phosphate catalyzed
ring-opening polymerization of e-caprolactone (CL) and d-valero-
lactone (VL) in the presence of alcohols as initiator [43]. Here in,
we have synthesized and characterized a series of arylhydrogen-
phosphates and aryldihydrogenphosphates and investigated their
catalytic activity towards the ring-opening polymerization of CL
and VL in the absence of any external initiator. The bulk polymeri-
zations were carried out at 80 °C under a nitrogen atmosphere.
⇑
Corresponding author. Tel.: +91 44 2257 4223; fax: +91 44 2257 4202.
E-mail addresses: dchakraborty@iitm.ac.in, debashis.iit@gmail.com (D. Chakra-
borty).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.02.006

P. Malik, D. Chakraborty / Inorganica Chimica Acta 400 (2013) 32–41
33
was removed under reduced pressure. The crude products were
crystallized from toluene/CH2Cl2.
2. Experimental
2.1. General methods
2.4.1. (2-MeC6H4O)2P(O)Cl (1)
Compound (1) was synthesized from 2-MeC6H4OH (1.0 g,
8.9 mmol), POCl3 (2.7 g, 17.8 mmol) and LiCl (8.4 mg, 0.2 mmol);
Reaction Time: 26 h; Yield = 1.5 g (62%); Physical state: Colourless
viscous liquid; 1H NMR (400 MHz, CDCl3, ppm): d 2.23 (s, 6H, Ar–
Me), 7.15–7.10 (m, 4H, Ar), 7.30–7.21 (m, 4H, Ar); 13C{1H} NMR
(100 MHz, CDCl3, ppm): d 16.16 (Ar–Me), 120.07 (Ar–C), 125.14
(Ar–C), 127.10 (Ar–C), 129.92 (Ar–C), 131.32 (Ar–C), 149.16 (Ar–
O); 31P{1H} NMR (161 MHz, CDCl3, ppm): d –1.91 (s); HRMS
(M+H)+: 228.15; Anal. Calc. for C14H14ClO3P: C, 56.68; H, 4.76.
Found: C, 57.11; H, 4.54%.
The reactions for the synthesis of arylphosphodichloridates and
arylphosphochlo-ridates were performed under a nitrogen atmo-
sphere using standard Schlenk techniques with rigorous exclusion
of moisture and air. Toluene was dried by heating under reflux over
sodium and benzophenone and distilled fresh prior to use. All the
phenols used in this study along with phosphorus oxychloride and
lithium chloride were purchased from Aldrich and used without
subsequent purification. The monomers CL and VL were purchased
from Aldrich and dried over CaH2 and distilled fresh prior to use.
Toluene, hexane, ethyl acetate and methylene chloride were pur-
chased from Rankem India. CDCl3 used for NMR spectral measure-
ments was purchased from Aldrich.
2.4.2. (4-MeOC6H4O)2P(O)Cl (2)
Compound (2) was synthesized from 4-MeOC6H4OH (1.0 g,
8.0 mmol), POCl3 (2.4 g, 16.0 mmol) and LiCl (7.6 mg, 0.17 mmol);
Reaction Time: 24 h; Yield = 1.6 g (60%); Physical state: Colourless
viscous liquid; 1H NMR (400 MHz, CDCl3, ppm): d 3.80 (s, 6H, Ar–
OMe), 6.91–6.89 (m, 4H, Ar), 7.21–7.19 (m, 4H, Ar); 13C{1H} NMR
(100 MHz, CDCl3, ppm): d 55.77 (Ar–OMe), 115.19 (Ar–O), 121.54
(Ar–C), 143.36 (Ar–O), 158.32 (Ar–OMe); 31P{1H} NMR (161 MHz,
CDCl3, ppm): d À1.34 (s); HRMS (M+H)+: 228.15; Anal. Calc. C14H14-
ClO5P: C, 51.16; H, 4. 29. Found: C, 51.78; H, 4.10%.
2.2. Instrumentation
1H and 13C NMR spectra were recorded with a Bruker Avance
400 instrument. Chemical shifts for 1H and 13C NMR spectra were
referenced to residual solvent resonances and are reported as parts
per million relative to SiMe4. 31P NMR spectra were recorded
relative to 85% H3PO4 as an external standard. Mass spectra of the
samples were recorded on a Micro mass QToF instrument, low
resolution Electro-Spray Ionization (ESI) mass spectrometer using
methanol solvent. GC–MS were recorded using Jeol JMS GC-Mate
II instrument. Infrared spectra were recorded using a Nicolet 6700
FTIR instrument and MALDI-TOF measurements were performed
on a Bruker Daltonics instrument in dihydroxy benzoic acid matrix.
Elemental analyses were done with a Perkin Elmer Series 11
analyzer.
2.4.3. (2-ClC6H4O)2P(O)Cl (3)
Compound (3) was synthesized from 2-ClC6H4OH (1.0 g,
7.8 mmol), POCl3 (2.3 g, 15.6 mmol) and LiCl (7.4 mg, 0.17 mmol);
Reaction Time: 48 h; Yield = 1.6 g (59%); Physical state: Colourless
viscous liquid; 1H NMR (400 MHz, CDCl3, ppm): d 7.34–7.27 (m,
4H, Ar), 7.51–7.46 (m, 4H, Ar); 13C{1H} NMR (100 MHz, CDCl3,
ppm): d 121.68 (Ar–C), 126.15 (Ar–C), 126.48 (Ar–C), 127.96 (Ar–
C), 130.83 (Ar–Cl), 146.54 (Ar–O); 31P{1H} NMR (161 MHz, CDCl3,
ppm): d À2.13 (s); HRMS (M+H)+: 228.15; Anal. Calc. C12H8Cl3O3P:
C, 42.70; H, 2.39. Found: C, 43.01; H, 2.74%.
2.3. X-ray crystallography
Single crystals of 13 and 14 suitable were for structural studies.
These crystals were obtained by crystallization from toluene at 0 °C
by slow evaporation. X-ray data collection was performed with
Bruker AXS (Kappa Apex 2) CCD diffractometer equipped with gra-
2.4.4. (2-BrC6H4O)2P(O)Cl (4)
Compound (4) was synthesized from 2-BrC6H4OH (1.0 g,
5.7 mmol), POCl3 (1.7 g, 11.5 mmol) and LiCl (5 mg, 0.12 mmol);
Reaction Time: 46 h; Yield = 1.5 g (66%); Physical state: Colourless
viscous liquid; 1H NMR (400 MHz, CDCl3, ppm): d 7.24–7.20 (m,
2H, Ar), 7.43–7.39 (m, 2H, Ar), 7.61–7.59 (m, 2H, Ar), 7.74–7.72
(m, 2H, Ar); 13C{1H} NMR (100 MHz, CDCl3, ppm): d 114.95 (Ar–
C), 121.58 (Ar–C), 126.66 (Ar–C), 128.75 (Ar–C), 133.84 (Ar–Br),
148.32 (Ar–O); 31P{1H} NMR (161 MHz, CDCl3, ppm): d À2.10 (s);
HRMS (M+H)+: 228.15; Anal. Calc. C12H8Br2ClO3P: C, 33.80; H,
1.89, Found: C, 33.17; H, 1.52%.
phite monochromated Mo K
data were collected with 100% completeness for h up to 25°.
And / scans were employed to collect the data. The frame width
for was set to 0.5° for data collection. The frames were integrated
a (k = 0.7107 Å) radiation source. The
x
x
and data were reduced for Lorentz and polarization corrections
using SAINT-NT. The multi-scan absorption correction was applied
to the data set. All structures were solved using SIR-92 and refined
using SHELXL-97 [44]. The non-hydrogen atoms were refined with
anisotropic displacement parameter. All the hydrogen atoms could
be located in the difference Fourier map. The hydrogen atoms
bonded to carbon atoms were fixed at chemically meaningful posi-
tions and were allowed to ride with the parent atom during
refinement.
2.4.5. (3-FC6H4O)2P(O)Cl (5)
Compound (5) was synthesized from 3-FC6H4OH (1.0 g,
8.9 mmol), POCl3 (2.7 g, 17.8 mmol) and LiCl (8.4 mg, 0.2 mmol);
Reaction Time: 42 h; Yield = 1.4 g (61%); Physical state: Colourless
viscous liquid; 1H NMR (400 MHz, CDCl3, ppm): d 6.96–6.82 (m,
6H, Ar), 7.27–7.15 (m, 2H, Ar); 13C{1H} NMR (100 MHz, CDCl3,
ppm): d 103.26 (Ar–C), 108.01 (Ar–C), 111.31 (Ar–C), 130.64 (Ar–
C), 156.68 (Ar–O), 162.51 (Ar–F); 31P{1H} NMR (161 MHz, CDCl3,
ppm): d À1.44 (s); HRMS (M+H)+: 228.15; Anal. Calc. C12H8Cl
F2O3P: C, 47.32; H, 2.65, Found: C, 47.94; H, 2.41%.
2.4. Synthesis of arylhydrogenphosphates (1–20)
Phenol (2 mmol) was stirred under reflux with POCl3 (1 mmol)
and LiCl (0.02 mmol) under a nitrogen atmosphere to obtain the
corresponding arylphosphochloridates. The reaction mixture was
purified by distillation under reduced pressure. The reactions were
monitored using TLC (80% EtOAc–hexane solution) against the re-
spective phenols. For the synthesis of arylhydrogenphosphates, the
arylphosphochloridates and water (1 mL) were stirred at 90 °C. The
reactions were monitored using TLC (80% EtOAc–hexane solution)
against arylphosphochloridates. The products were extracted using
CH2Cl2 (3 Â 5 mL). The organics were combined and the solvent
2.4.6. (4-FC6H4O)2P(O)Cl (6)
Compound (6) was synthesized from 4-FC6H4OH (1.0 g,
8.9 mmol), POCl3 (2.7 g, 17.8 mmol) and LiCl (8.4 mg, 0.2 mmol);
Reaction Time: 48 h; Yield = 1.5 g (60%); Physical state: Colourless
viscous liquid; 1H NMR (400 MHz, CDCl3, ppm): d 7.15–7.09 (m,
4H, Ar), 7.31–7.26 (m, 4H, Ar); 13C{1H} NMR (100 MHz, CDCl3,

34
P. Malik, D. Chakraborty / Inorganica Chimica Acta 400 (2013) 32–41
ppm): d 116.26 (Ar–C), 116.31 (Ar–C), 151.56 (Ar–O), 158.59 (Ar–
F); 31P{1H} NMR (161 MHz, CDCl3, ppm): d À1.29 (s); HRMS
(M+H)+: 228.15; Anal. Calc. C12H8Cl F2O3P: C, 47.32; H, 2.65. Found:
C, 47.73; H, 2.15%.
1207 (P@O), 3049 (C–H), 3321 (O–H); 1H NMR (400 MHz, CDCl3,
ppm): d 7.38–7.25 (m, 4H, Ar), 7.52–7.45 (m, 4H, Ar), 8.17(b, 1H,
OH); 13C{1H} NMR (100 MHz, CDCl3, ppm): d 121.69 (Ar–C),
126.07 (Ar–C), 126.50 (Ar–C), 127.98 (Ar–C), 130.82 (Ar–Cl),
146.55 (Ar–O); 31P{1H} NMR (161 MHz, CDCl3, ppm): d À15.98
(s); HRMS (M+H)+: Calc.: 318.9612; Found: 318.9690.
2.4.7. (3-IC6H4O)2P(O)Cl (7)
Compound (7) was synthesized from 3-IC6H4OH (1.0 g,
4.5 mmol), POCl3 (1.3 g, 9.0 mmol) and LiCl (8.4 mg, 0.2 mmol); Re-
action Time: 47 h; Yield = 1.3 g (60%); Physical state: Colourless
viscous liquid; 1H NMR (400 MHz, CDCl3, ppm): d 7.22–7.11 (m,
2H, Ar), 7.33–7.28 (m, 2H, Ar), 7.68–7.65 (m, 4H, Ar); 13C{1H}
NMR (100 MHz, CDCl3, ppm): d 94.43 (Ar–I), 115.05 (Ar–C),
124.71 (Ar–C), 130.19 (Ar–C), 131.15 (Ar–C), 156.14 (Ar–O);
31P{1H} NMR (161 MHz, CDCl3, ppm): d À1.74 (s); HRMS (M+H)+:
228.15; Anal. Calc. C12H8Cl I2O3P: C, 27.69; H, 1.55. Found: C,
28.24; H, 1.88%.
2.4.14. (2-BrC6H4O)2P(O)OH (14)
Compound 14 was synthesized by the hydrolysis of 4 (0.85 g,
2 mmol); Reaction Time: 12 h; Yield = 0.50 g (62%); Physical state:
Brown crystalline solid; m.p. = 179 °C; IR (neat, cmÀ1): 756 (P–O),
1189 (P@O), 3083 (C–H), 3512 (O–H); 1H NMR (400 MHz, CDCl3,
ppm): d 7.07–7.03 (m, 2H, Ar), 7.26–7.22 (m, 2H, Ar), 7.57–7.55
(m, 2H, Ar), 8.31 (b, 1H, OH); 13C{1H} NMR (100 MHz, CDCl3,
ppm): d 115.06 (Ar–C), 121.57 (Ar–C), 126.79 (Ar–C), 128.73 (Ar–
C), 133.91 (Ar–Br), 147.77 (Ar–O); 31P{1H} NMR (161 MHz, CDCl3,
ppm): d –16.05 (s); HRMS (M+H)+: Calc.: 406.1204; Found:
406.8680.
2.4.8. (4-IC6H4O)2P(O)Cl (8)
Compound (8) was synthesized from 4-IC6H4OH (1.0 g,
4.5 mmol), POCl3 (1.3 g, 9.0 mmol) and LiCl (8.4 mg, 0.2 mmol); Re-
action Time: 48 h; Yield = 1.4 g (62%); Physical state: Colourless
viscous liquid; 1H NMR (400 MHz, CDCl3, ppm): d 7.05 (d, 4H,
Ar), 7.34 (d, 4H, Ar); 13C{1H} NMR (100 MHz, CDCl3, ppm): d
82.97 (Ar–I), 117.93 (Ar–C), 138.52 (Ar–C), 155.15 (Ar–O); 31P{1H}
NMR (161 MHz, CDCl3, ppm): d À1.84 (s); HRMS (M+H)+: 228.15;
Anal. Calc. C12H8Cl I2O3P: C, 27.69; H, 1.55. Found: C, 28.07; H,
1.94%.
2.4.15. (3-FC6H4O)2P(O)OH (15)
Compound 15 was synthesized by the hydrolysis of 5 (0.57 g,
2 mmol); Reaction Time: 16 h; Yield = 0.36 g (64%); Physical state:
Reddish brown oily liquid; IR (neat, cmÀ1): 709 (P–O), 1226 (P@O),
2921 (C–H), 3557 (O–H); 1H NMR (400 MHz, CDCl3, ppm): d 6.96–
6.81 (m, 6H, Ar), 7.28–7.15 (m, 2H, Ar), 7.78 (b, 1H, OH); 13C{1H}
NMR (100 MHz, CDCl3, ppm): d 103.50 (Ar–C), 107.91 (Ar–C),
111.31 (Ar–C), 130.73 (Ar–C), 156.57 (Ar–O), 164.94 (Ar–F);
31P{1H} NMR (161 MHz, CDCl3, ppm):
(M+H)+: Calc.: 287.0215; Found: 287.9413.
d –17.26 (s); HRMS
2.4.9. (2,4,6-Br3C6H2O)2P(O)Cl (9)
Compound (9) was synthesized from 2,4,6-Br3C6H2OH (1.0 g,
3 mmol), POCl3 (0.92 g, 6 mmol) and LiCl (2 mg, 0.06 mmol). The
phosphor-chloridate (9) was not isolated because of its high boil-
ing point.
2.4.16. 4-FC6H4O)2P(O)OH (16)
Compound 16 was synthesized by the hydrolysis of 6 (0.57 g,
2 mmol); Reaction Time: 16 h; Yield = 0.33 g (60%); Physical state:
Reddish brown oily liquid; IR (neat, cmÀ1): 791 (P–O), 1246 (P@O),
3024 (C–H), 3451 (O–H); 1H NMR (400 MHz, CDCl3, ppm): d 6.42
(b, 1H, OH), 6.88–6.76 (m, 4H, Ar), 6.95–6.89 (m, 4H, Ar); 13C{1H}
NMR (100 MHz, CDCl3, ppm): d 111.63 (Ar–C), 116.31 (Ar–C),
151.64 (Ar–O), 158.56 (Ar–F); 31P{1H} NMR (161 MHz, CDCl3,
ppm): d –16.79 (s); HRMS (M+H)+: Calc.: 287.0215; Found:
287.9571.
2.4.10. 2.4.10. (2,4,6-I3C6H2O)2P(O)Cl (10)
Compound (10) was synthesized from 2,4,6-I3C6H2OH (1.0 g,
2.1 mmol), POCl3 (0.64 g, 4.2 mmol) and LiCl (2 mg, 0.01 mmol).
The phosphor-chloridate (10) was not isolated because of its high
boiling point.
2.4.11. (2-MeC6H4O)2P(O)OH (11)
Compound 11 was synthesized by the hydrolysis of 1 (0.60 g,
2 mmol); Reaction Time: 10 h; Yield = 0.33 g (61%); Physical state:
Reddish brown crystalline solid; m.p. = 105 °C; IR (neat, cmÀ1): 718
(P–O), 1214 (P@O), 2965 (C–H), 3343 (O–H); 1H NMR (400 MHz,
CDCl3, ppm): d 2.88 (s, 6H, Ar–Me), 7.83–7.70 (m, 8H, Ar), 8.84
(b, 1H, OH); 13C{1H} NMR (100 MHz, CDCl3, ppm): d 16.17 (Ar–C),
120.06 (Ar–C), 125.14 (Ar–C), 127.00 (Ar–C), 131.32 (Ar–C),
149.15 (Ar–O); 31P{1H} NMR (161 MHz, CDCl3, ppm): d À15.22
(s); HRMS (M+Na)+: Calc. 301.0601; Found: 301.0610.
2.4.17. (3-IC6H4O)2P(O)OH (17)
Compound 17 was synthesized by the hydrolysis of 7 (0.50 g,
1 mmol); Reaction Time: 11 h; Yield = 0.31 g (63%); Physical state:
Colourless oily liquid; IR (neat, cmÀ1): 763 (P–O), 1272 (P@O),
3086 (C–H), 3296 (O–H); 1H NMR (400 MHz, CDCl3, ppm): d 6.78
(d, 2H, Ar), 6.95–6.91 (m, 2H, Ar), 7.25–7.20 (m, 4H, Ar), 8.30 (b,
1H, OH); 13C{1H} NMR (100 MHz, CDCl3, ppm): d 94.45 (Ar–I),
115.06 (Ar–C), 124.72 (Ar–C), 130.14 (Ar–C), 131.15 (Ar–C),
156.09 (Ar–O); 31P{1H} NMR (161 MHz, CDCl3, ppm): d À16.72
(s); HRMS (M+H)+: Calc.: 502.8314; Found: 502.9846.
2.4.12. (4-MeOC6H4O)2P(O)OH (12)
Compound 12 was synthesized by the hydrolysis of 2 (0.66 g,
2 mmol); Reaction Time: 8 h; Yield = 0.38 g (63%); Physical state:
Colourless liquid; IR (neat, cmÀ1): 727 (P–O), 1189 (P@O), 2948
(C–H), 3313 (O–H); 1H NMR (400 MHz, CDCl3, ppm): d 3.76 (s,
6H, OCH3), 6.31 (b, 1H, OH), 6.79–6.67 (m, 8H, Ar); 13C{1H} NMR
(100 MHz, CDCl3, ppm): d 55.69 (Ar–OCH3), 115.18 (Ar–C), 121.53
(Ar–C), 143.35 (Ar–O), 158.31 (Ar–OCH3); 31P{1H} NMR (161 MHz,
CDCl3, ppm): d À16.35 (s); HRMS (M+H)+: Calc. 310.2471; Found:
311.9781.
2.4.18. (3-IC6H4O)2P(O)OH (18)
Compound 18 was synthesized by the hydrolysis of 8 (0.50 g,
1 mmol); Reaction Time: 14 h; Yield = 0.30 g (60%); Physical state:
Colourless viscous oily liquid; IR (neat, cmÀ1): 746 (P–O), 1209
(P@O), 2925 (C–H), 3290 (O–H); 1H NMR (400 MHz, CDCl3, ppm):
d 6.39 (m, 4H, Ar), 7.27 (d, 4H, Ar), 8.06 (b, 1H, OH); 13C{1H} NMR
(100 MHz, CDCl3, ppm): d 82.98 (Ar–I), 117.93 (Ar–C), 138.52
(Ar–C), 155.15 (Ar–O); 31P{1H} NMR, (161 MHz, CDCl3, ppm): d –
17.00 (s); HRMS (M+H)+: Calc.: 502.8314; Found: 502.9754.
2.4.13. (2-ClC6H4O)2P(O)OH (13)
2.4.19. (2,4,6-Br3C6H2O)2P(O)OH (19)
Compound 13 was synthesized by the hydrolysis of 3 (0.67 g,
2 mmol); Reaction Time: 13 h; Yield = 0.37 g (59%); Physical state:
Brown crystalline solid; m.p. = 115 °C; IR (neat, cmÀ1): 784 (P–O),
Compound 19 was synthesized by the hydrolysis of crude pro-
duct of 9 (0.74 g, 1 mmol); Reaction Time: 18 h; Yield = 0.48 g
(68%); Physical state: White solid; m. p. = 190 °C; IR (neat, cmÀ1):

P. Malik, D. Chakraborty / Inorganica Chimica Acta 400 (2013) 32–41
35
781 (P–O), 1284 (P@O), 3086 (C–H), 3431 (O–H); 1H NMR
(400 MHz, CDCl3, ppm): d 7.93 (s, 4H, Ar), 8.16 (b, 1H, OH);
13C{1H} NMR (100 MHz, CDCl3, ppm): d 117.88 (Ar–Br), 119.72
(Ar–Br), 135.42 (Ar–C), 148.07 (Ar–O); 31P{1H} NMR (161 MHz,
CDCl3, ppm): d À18.83 (s); GC–MS (M)+: Calc.:717.5011; Found:
716.2430.
CH3), 120.08 (Ar–C), 125.63 (Ar–C), 127.25 (Ar–C), 129.57 (Ar–C),
131.62 (Ar–C), 149.36 (Ar–O); 31P{1H} NMR (161 MHz, CDCl3,
ppm): d À10.05 (s); HRMS (M+H)+: Calc.: 189.0210; Found:
189.0316.
2.5.7. 2-FC6H4OP(O)(OH)2 (27)
Compound 27 was synthesized by the hydrolysis of 23 (0.42 g,
2 mmol); Reaction Time: 8 h; Yield = 0.25 g (65%); Physical state:
Colourless liquid; IR (neat, cmÀ1): 769 (P–O), 1256 (P@O), 3001
(C–H), 3458 (O–H); 1H NMR (400 MHz, CDCl3, ppm): d 7.31–7.15
(m, 3H, Ar), 7.46–7.43 (m, 1H, Ar), 8.00 (b, 2H, OH); 13C{1H} NMR
(100 MHz, CDCl3, ppm): d 117.20 (Ar–C), 122.43 (Ar–C), 124.62
(Ar–C), 126.40 (Ar–C), 138.31 (Ar–O), 152.43 (Ar–F); 31P{1H} NMR
(161 MHz, CDCl3, ppm): d À10.54 (s); HRMS (M+H)+: Calc.:
193.0811; Found: 193.4049.
2.4.20. (2,4,6-I3C6H2O)2P(O)OH (20)
Compound 20 was synthesized by the hydrolysis of crude pro-
duct of 10 (1 g, 2 mmol); Reaction Time: 15 h; Yield = 0.74 g
(70%); Physical state: White solid; m. p. = 203 °C; IR (neat, cmÀ1):
767 (P–O), 1253 (P@O), 3016 (C–H), 3454 (O–H); 1H NMR
(400 MHz, CD3OD, ppm): d 7.87 (s, 4H, Ar), 8.87 (b, 1H, OH);
13C{1H} NMR (100 MHz, CD3OD, ppm): d 83.70 (Ar–I), 86.56 (Ar–
I), 147.93 (Ar–C), 157.28 (Ar–O); 31P{1H} NMR (161 MHz, CDCl3,
ppm):
1005.5964.
d
À22.82 (s); GC–MS (M)+: Calc.:1005.5701; Found:
2.5.8. 3-CF3C6H4OP(O)(OH)2 (28)
2.5. Synthesis of aryldihydrogenphosphates
Compound 28 was synthesized by the hydrolysis of 24 (0.52 g,
2 mmol); Reaction Time: 5 h; Yield = 0.29 g (60%); Physical State:
Colourless oily liquid; IR (neat, cmÀ1): 794 (P–O), 1236 (P@O),
2985 (C–H), 3385 (O–H); 1H NMR (400 MHz, CDCl3, ppm): d
7.58–7.28 (m, 4H, Ar), 8.19 (b, 2H, OH); 13C{1H} NMR (100 MHz,
CDCl3, ppm): d 112.46 (Ar–C), 117.87 (Ar–C), 118.99 (Ar–C),
122.59 (Ar–C), 130.45 (Ar–CF3), 131.67 (Ar–C), 132.11 (Ar–CF3),
155.60 (Ar–O); 31P{1H} NMR (161 MHz, CDCl3, ppm): d À10.81
(s); HRMS (M+H)+: Calc.: 243.0912; Found: 243.0037.
Phenol (1 mmol) was stirred under reflux with POCl3 (1 mmol)
and LiCl (0.02 mmol) under a nitrogen atmosphere to give the ar-
ylphosphodichloridates. The reaction mixture was purified by dis-
tillation under reduced pressure. The reactions were monitored
using TLC (80% EtOAc–hexane solution) against the respective phe-
nols. For the synthesis of aryldihydrogenphosphates, the arylpho-
sphodichloridates were subjected to hydrolysis and the work-up
was done by the same method as arylhydrogenphosphates.
2.6. Procedure for the bulk polymerization of CL and VL
2.5.1. 2-ClC6H4OP(O)Cl2 (21)
See the Ref. [45].
For CL polymerization, 23.6 lmol of the 11–20/24–28 was ta-
ken in a flask under a nitrogen atmosphere, subsequently 0.5 mL
CL (0.54 g, 4.71 mmol) was added neat and the mixture was rapidly
stirred at 80 °C. A rise in viscosity was observed and finally the stir-
ring ceased. For VL same procedure was followed. The progress of
the polymerization was followed by monitoring the disappearance
of the monomer using TLC technique. The polymerizations were
quenched by pouring the contents into cold heptane. The polymers
were isolated by filtration and dried till constant weight.
2.5.2. 2-MeC6H4OP(O)Cl2 (22)
See the Ref. [45].
2.5.3. 2-ClC6H4OP(O)Cl2 (23)
See the Ref. [45].
2.5.4. 3-CF3C6H4OP(O)Cl2 (24)
Compound 24 was synthesized from 3-CF3C6H4OH (1.0 g,
9.2 mmol), POCl3 (5.6 g, 36.8 mmol) and LiCl (8.7 mg, 0.2 mmol);
Reaction Time: 20 h ;Yield = 1.3 g (60%); Physical State: Colourless
liquid; 1H NMR (400 MHz, CDCl3, ppm): d 7.61–7.50 (m, 4H, Ar);
13C{1H} NMR (100 MHz, CDCl3, ppm): d 112.42 (Ar–C), 118.99
(Ar–C), 122.60 (Ar–C), 125.31 (Ar–CF3), 130.45 (Ar–CF3), 155.59
(Ar–O); 31P{1H} NMR (161 MHz, CDCl3, ppm): d 1.31; Anal. Calc.
for C7H4Cl2F3O2P: C, 30.14; H, 1.45, Found: C, 30.87; H, 1.12%.
2.7. Characterization of polymers
Data concerning number average molecular weight (Mn),
weight average molecular weight (Mw) and molecular weight dis-
tributions (Mw/Mn) (MWDs) of the polymer samples obtained by
the ring-opening polymerization of various cyclic ester monomers
were determined by using a GPC instrument with Waters 510
pump and Waters 410 differential refractometer as the detector.
Three columns namely WATERS STRYGEL-HR5, STRYGEL-HR4 and
STRYGEL-HR3 each of dimensions (7.86 Â 300 mm) were con-
nected in series. Measurements were done in THF at 27 °C. Mn
and MWDs of polymers were measured relative to polystyrene
standards.
2.5.5. 2-ClC6H4OP(O)(OH)2 (25)
Compound 25 was synthesized by the hydrolysis of 21 (0.45 g,
2 mmol); Reaction Time: 6 h; Yield = 0.28 g (68%); Physical state:
Colourless viscous oily liquid; IR (neat, cmÀ1): 787 (P–O), 1246
(P@O), 3023 (C–H), 3363 (O–H); 1H NMR (400 MHz, CDCl3, ppm):
d 7.39–7.25 (m, 2H, Ar), 7.53–7.45 (m, 2H, Ar), 11.72 (b, 2H, OH);
13C{1H} NMR (100 MHz, CDCl3, ppm): d 120.67 (Ar–C), 126.15
(Ar–C), 126.48 (Ar–C), 127.98 (Ar–C), 130.83 (Ar–Cl), 146.94 (Ar–
O); 31P{1H} NMR (161 MHz, CDCl3, ppm): d À10.14 (s); HRMS
(M+H)+: Calc.: 209.5404; Found: 209.5048.
3. Results and discussion
The evolution of metal free ring-opening polymerization of cyc-
lic esters has motivated us to investigate the catalytic activities of
arylhydrogenphosphates and aryldihydrogenphosphates for the
ring-opening polymerization of cyclic esters. In the initial studies,
the arylhydrogenphosphates and aryldihydrogenphosphates were
synthesized and characterized and subsequently these compounds
were used as initiator for the ring-opening polymerization of CL
and VL.
2.5.6. 2-MeC6H4OP(O)(OH)2 (26)
Compound 26 was synthesized by the hydrolysis of 22 (0.41 g,
2 mmol); Reaction Time: 5 h; Yield = 0.23 g (61%); Physical state:
Colourless liquid; IR (neat, cmÀ1): 769 (P–O), 1287 (P@O), 2967
(C–H), 3404 (O–H); 1H NMR (400 MHz, CDCl3, ppm): d 2.28 (s,
3H, Ar–CH3), 7.15–7.10 (m, 2H, Ar), 7.30–7.21 (m, 2H, Ar), 10.54
(b, 2H, OH); 13C{1H} NMR (100 MHz, CDCl3, ppm): d 16.30 (Ar–

36
P. Malik, D. Chakraborty / Inorganica Chimica Acta 400 (2013) 32–41
3.1. Synthesis and characterization of arylhydrogenphosphates and
aryldihydrogenpho-sphates
arylhydrogenphosphates, the P–OH proton is more downfield
shifted than the phenolic –OH proton. The 13C NMR spectra of
these compounds show all the structural fragments. Indeed, there
was no appreciable difference between the chemical shifts of phe-
nol and arylhydrogenphosphates. The signals for ipso carbon of the
aryl rings are more downfield shifted as compared to the other sig-
nal constituting the aryl ring. The 31P NMR spectra of these com-
pounds have shown a sharp singlet in the upfield region. For the
The synthesis of arylhydrogenphosphates is a two step process.
In the first step phosphorochloridates were synthesized by litera-
ture method [46] and further in the second step these phosphoro-
chloridates subjected to hydrolysis in presence of water at 90 °C.
The compound 1–10 were prepared by the reaction corresponding
phenols (2 mmol) with POCl3 (1 mmol) in 2:1 stoichiometric ratio
in the presence of LiCl (0.02 mmol) under a nitrogen atmosphere
(Scheme 1). The reactions were more facile in case of the electron
releasing group containing phenols as compare to the electron
withdrawing group containing phenols. The tri-substituted haloar-
ylphosphorochloridates could not be isolated from the reaction
mixture due to their very high temperature of boiling. Moreover
in second step, the chloridates/crude reaction mixtures were trea-
ted with water and subjected to heat at 90 °C to obtain the corre-
sponding arylhydrogenphosphates 11–20.
The aryldihydrogenphosphates 25–28 were synthesized by the
reaction of corresponding phenols (1 mmol) with POCl3 (1 mmol)
in 1:1 stoichiometric ratio in presence of LiCl (0.02 mmol) under
a nitrogen atmosphere, followed by hydrolysis (Scheme 2). As a
matter of fact, in literature few related reports are available, where
these compounds have used in some important biological synth-
esis and catalysis [47–54].
In 1979, Crano has synthesized bis(polyhalophenyl)hydrogen-
phosphate by the selective hydrolysis of tris(polyhalophenyl)
phosphate with H2O in the presence of an inert organic solvent
at pH 1–9. In fact, only few compounds were characterized. In this
regard, our contribution is that we have established a reliable route
for synthesis these compounds and all the compounds unambigu-
ously characterized by various spectroscopic methods.
The aforementioned compounds 1–28 have been characterized
by IR and 1H, 13C, 31P NMR spectroscopy and ESI-MS/GC–MS. The
expected signals with appropriate ratio of integrations in 1H
NMR spectra revealed the formation of desired products. The P–
OH proton signal appears broad in all the products. In the case of
arylhydrogenphosphates, 31P is more upfield shifted (ca.
DdP = 5–
6 ppm) as compared to the aryldihydrogenphosphates. In addition,
the most shielding effect was observed in the 31P NMR spectrum of
tri-iodoarylhydrogenphosphate which appears as a singlet at
À22.82 ppm. The tri-haloarylhydrogenphosphates have shown
poor solubility behaviour in CDCl3. To overcome the solubility pro-
blem, the 1H, 13C and 31P NMR were measured in CD3OD. Measure-
ment of the HRMS/GC–MS of arylhydrogenphosphates and
aryldihydrogenphosphates revealed the molecular ion as a proton
or sodium (11) adducts with extremely close agreement to the the-
oretical values. Purity was assured with the correct elemental ana-
lysis results.
3.2. Single crystal X-ray diffraction studies
Among the several compounds, suitable crystals for X-ray dif-
fraction studies could be obtained from 13 and 14. The single crys-
tals were grown at 0 °C form toluene solution by slow evaporation.
Selected bond lengths and angles are delineated in Table 1 and the
molecular structures of 13 and 14 are depicted in Fig. 1. The crystal
data is illustrated in Table 1. These compounds exhibit centrosym-
metric structure in their solid state. The bond angles disclosed that
the phosphorus centres are in distorted pyramidal environment.
The crystal structure analysis revealed that infinite one-dimen-
sional polymer chains packed via intermolecular hydrogen bond-
ing between the hydrogen of –OH group and oxygen of P@O in a
linear fashion (Fig. 2). The double bond character of P@O reduced
due to the extended hydrogen bonding as a result of that both
the P–O bonds are equidistant. In case of 13 and 14, the observed
O
O
P
LiCl (cat)
H2O
ArOH
(2 equiv.)
+
P
POCl3
ArO
OAr
ArO
OAr
90 °C
reflux
(1 equiv.)
OH
Cl
Ar = 2-MeC6H4
2-OMeC6H4
2-ClC6H4
Ar = 2-MeC6H4
2-OMeC6H4
2-ClC6H4
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
2-BrC6H4
2-BrC6H4
3-FC6H4
3-FC6H4
4-FC6H4
4-FC6H4
3-IC6H4
3-IC6H4
4-IC6H4
4-IC6H4
2,4,6-I3C6H2
2,4,6-Br3C6H2
2,4,6-I3C6H2
2,4,6-Br3C6H2
Scheme 1. Synthesis of arylhydrogenphosphate derivatives.
O
O
P
LiCl (cat)
reflux
H2O
ArOH
(1 equiv.)
+
POCl3
(1 equiv.)
P
ArO
Cl
ArO
OH
60 °C
Cl
OH
2-MeC H
Ar =
6
4
21
22
23
24
Ar = 2-MeC6H4
2-ClC6H4
25
26
27
28
2-ClC6H4
4-FC6H4
3-CF3C6H4
4-FC6H4
3-CF3C6H4
Scheme 2. Synthesis of aryldihydrogenphosphates derivatives.

P. Malik, D. Chakraborty / Inorganica Chimica Acta 400 (2013) 32–41
37
Table 1
Crystal data for 13 and 14.
13
14
Empirical formula
Formula weight
Space group
Crystal system
T (K)
C
12H9Cl2O4P
C6 H4.50 Br O2 P0.50
203.99
P21/c
monoclinic
298(2)
319.06
Pccn
orthorhombic
298(2)
k (Å)
a (Å)
b (Å)
c (Å)
0.71073
22.0545(9)
4.6788(2)
12.8346(5)
90
0.71073
11.7769(11)
4.7093(4)
13.1053(11)
90
Fig. 2. One-dimensional structure of 14.
a
(°)
b (°)
90
90
109.462(5)
90
685.30(10)
4
c
(°)
Table 2
V (Å3)
1324.38(9)
4
1.600
Results of CL ring-opening polymerization using various substituted arylhydrogen-
phosphates in 200:1 ratio at 80 °C.
Z
Dcalc (g/cm3)
1.977
c
Reflections collected
No. of independent reflections
Goodness-of-fit (GOF) on F2
1670
1163
1.027
R1 = 0.0424
wR1 = 0.1017
R1 = 0.0699
wR1 = 0.1134
2606
1941
0.974
R1 = 0.0381
wR1 = 0.0827
R1 = 0.0592
wR1 = 0.0903
Entry
cat
ta (min)
Yieldb (%)
Mn (kg/mol)
MWD
1
2
3
4
5
6
7
8
9
11
12
13
14
15
16
17
18
19
20
35
22
24
18
30
31
27
23
28
28
95
94
97
98
95
98
94
96
98
97
44.0
40.2
47.5
31.3
45.5
36.1
80.7
90.7
46.0
33.5
1.21
1.23
1.12
1.05
1.23
1.30
1.24
1.18
1.16
1.28
Final R indices (I > 2
r
(I))
R indices (all data)
P
P
P
P
|F0| À |Fc|/ |F0|, wR2 = [ (F0 À Fc2)2/ w(F02)2]1/2
.
2
R1
=
10
P–O bond lengths are 1.48 and 1.47 Å, respectively, which is short-
er than the normal P–O length and longer than the P@O (1.45 Å)
bond [44].
a
Time of polymerization measured by quenching the polymerization reaction
when all monomer was found consumed.
b
Isolated yield.
c
Measured by GPC at 27 °C in THF relative to polystyrene standards with Mark–
3.3. Polymerization studies
Houwink correlation for Mn.
To explore the feasibility and efficacy of compounds 10–20 and
24–28 as organocatalysts, polymerizations studies were carried
out. All the compounds were tested for the bulk ring-opening poly-
merization of CL and VL in the [M]o/[Int]o ratio of 200:1 at 80 °C
under ambient pressure. These arylhydrogenphosphates and aryl-
dihydrogenphosphates have proved to be efficient initiators for
the ring opening polymerization. From the sustainability view-
point, the polymerizations were performed in solvent-free condi-
tions. Therefore, the approach minimizes sensitive aspects
related to the use of solvents, waste formation or leaching of toxic
catalysts. First time we have used the arylhydrogenphosphates and
aryldihydrogenphosphates as self initiators for the ring-opening
polymerizations of cyclic esters, in this regard there are no reports
available in the literature. The polymerization results described in
Tables 2 and 3 confirmed that the polymerization proceeds in a
controlled fashion with high polymer yields. In addition, MWDs
for the obtained polycaprolactone (PCL) and polyvalerolactone
(PVL) were low in the range of 1.05–1.28 and from 1.08 to 1.35, re-
spectively, the MWDs range is evident of the more controlled poly-
merization in case of CL (Table 2 versus Table 3). The highest Mn of
90.7 kg/mol with 96% yield for PCL (Table 2 Entry 8) and the Mn of
90.4 kg/mol with 97% yield for PVL (Table 3 Entry 9) were obtained
after the time interval of 23 and 20 min, respectively. Observations
depicted that the shorter reaction time allows greater control over
the MWDs since the PCL has Mn of 31.3 kg/mol with MWD 1.05 and
for PVL Mn 81.5 kg/mol with 1.08 MWD (Table 2 entry 4 and Table
3 entry 8). In fact, in the prolonged reactions, the tendency of chain
trans-esterification reactions is responsible for the broadening of
the MWDs. For the aryldihydrogenphosphates, Mn of the resultant
Fig. 1. Molecular structures of (a) 13 and (b) 14; thermal ellipsoids were drawn at 30% probability level. Selected bond lengths (Å) and bond angles (°) for 13 P(1)–
O(2) = 1.4876(16); P(1)–O(2)#1 = 1.4876(16); P(1)–O(1) = 1.5754(15); P(1)–O(1)#1 = 1.5754(15); O(2)–P(1)–O(2)#1 = 115.44(14); O(2)–P(1)–O(1) = 110.31(8); O(2)#1–
P(1)–O(1) = 106.77(8); O(2)#1–P(1)–O(1)#1 = 110.31(8); C(1)–O(1)–P(1) = 121.33(13). Selected bond lengths (Å) and bond angles (°) for 14 P(1)–O(3) = 1.495(4); P(1)–
O(1) = 1.572(3); P(1)–O(2) = 1.579(4); C(1)–O(1) = 1.411(6); O(1)–P(1)–O(2) = 107.0(2); O(3)–P(1)–O(1) = 105.93(19); O(4)–P(1)–O(1) = 111.9(2); O(4)–P(1)–O(3) = 115.4(2);
C(8)–C(7)–O(2) = 120.0(4).

38
P. Malik, D. Chakraborty / Inorganica Chimica Acta 400 (2013) 32–41
Table 3
2.0
1.5
1.0
0.5
0.0
100
Results of VL ring-opening polymerization using various substituted arylhydrogen-
phosphates in 200:1 ratio at 80 °C.
(4-FC6H4O)2P(O)OH
90
80
70
60
50
40
30
20
10
0
(2,4,6-Br3C6H2O)2P(O)(OH)
ta (min)
Yieldb (%)
Mn (kg/mol)
MWD
c
Entry
cat
1
2
3
4
5
6
7
8
9
11
12
13
14
15
16
17
18
19
20
28.5
25
24
22
25
30
22
19
20
23
96
91
94
97
96
95
94
92
97
96
38.5
32.4
52.5
54.5
28.9
38.4
85.1
81.5
90.4
66.2
1.12
1.21
1.27
1.31
1.16
1.26
1.21
1.08
1.27
1.23
n
M
10
a
Time of polymerization measured by quenching the polymerization reaction
when all monomer was found consumed.
b
Isolated yield.
0
200
400
600
800
1000
c
Measured by GPC at 27 °C in THF relative to polystyrene standards Mn.
[CL]o/[Int]o
Fig. 3. Plot of Mn and MWD vs. [M]o/[Int]o for CL polymerization at 80 °C using 16
and 19.
Table 4
Results of CL ring-opening polymerization using various substituted aryldihydrogen-
phosphates in 200:1 ratio at 80 °C.
(2-CH3C6H4O)2P(O)OH (11)
(4-FC6H4O)2P(O)OH (16)
c
Entry
cat
ta (min)
Yieldb (%)
Mn (kg/mol)
MWD
1
2
3
4
25
26
27
28
29
30
30
21
95
98
94
97
29.4
34.0
58.5
31.7
1.35
1.19
1.26
1.22
(3-IC6H4O)2P(O)(OH) (17)
4.5
(2,4,6I3C6H2O)2P(O)OH (19)
Y = -0.037 + 0.165 X
R = 0.9967
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
2-ClC6H5OP(O)(OH)2 (26)
(3-CF3C6H4O)P(O)(OH)2 (28)
Y = -0.020 + 0.092 X
a
Time of polymerization measured by quenching the polymerization reaction
R = 0.9988
when all monomer was found consumed.
b
Isolated yield.
Y = -0.019 + 0.084 X
R = 0.9764
c
R = 0.9990
Y = 0.053 + 0.095 X
Measured by GPC at 27 °C in THF relative to polystyrene standards with Mark–
Houwink correlation for Mn.
Y = 0.012 + 0.104 X
R = 0.9963
Y = -0.005 + 0.083 X
R = 0.9993
Table 5
Results of VL ring-opening polymerization using various substituted aryldihydrogen-
phosphates in 200:1 ratio at 80 °C.
c
Entry
cat
ta (min)
Yieldb (%)
Mn (kg/mol)
MWD
0
5
10
15
20
25
30
35
1
2
3
4
25
26
27
28
27
24
26
17
96
95
93
95
40.0
29.8
38.4
45.7
1.32
1.25
1.27
1.20
Time (min)
Fig. 4. Semilogarithmic plots of CL conversion in time initiated by 11, 16, 17, 19, 26
and 28: [M]o/[Int]o = 200 at 80 °C.
a
Time of polymerization measured by quenching the polymerization reaction
first-order relationship with the reaction time and monomer con-
version which indicates that the monomer consumption rate was
constant during the polymerization. In addition, Mn of the resultant
polymers linearly increased with the reaction time and the mono-
mer conversion was as high as ꢀ93% (see Supporting information).
The ln[M]o/[M]t versus time plots (Fig. 4) exhibit linear variation.
From the slope of the plots, the values of the apparent rate constant
(kapp) for CL polymerizations initiated by 11, 16, 17, 19, 26 and 28
when all monomer was found consumed.
b
Isolated yield.
c
Measured by GPC at 27 °C in THF relative to polystyrene standards for Mn.
polymers is lower as compared to the arylhydrogenphosphates
(Tables 2 and 4 versus Tables 4 and 5) and results revealed that
the aryldihydrogenphosphates initiated VL polymerization pro-
ceeds faster than CL (Table 4 versus Table 5). To analyse the solvent
effect on polymerization the polymerizations were performed in
toluene. It was found that the solvent does not have any significant
effect on the Mn and MWDs.
To confirm that the polymerization system proceeds in a con-
trolled fashion, the ring-opening polymerizations of CL and VL were
carried out by using compound 16, 19, 26 and 28 with varying [M]o/
[Int]o ratios from 200–1000 at 80 °C under a nitrogen atmosphere.
The molecular weights of the obtained PCL and PVL linearly in-
creased with the increasing initial ratio of [M]o/[Int]o (Fig. 3 and
see supporting information). The small change in the MWDs proved
the absence of termination or chain transfer reactions. The results
suggest the controlled nature of these polymerizations. The kinetic
studies of ring-opening polymerization for CL and VL performed by
using 11, 16, 17, 19, 26 and 28 in ratio of [M]o/[Int]o = 200 at 80 °C
under a nitrogen atmosphere. The kinetic data shows a distinct
were found to be 8.3 Â 10À2, 8.4 Â 10À2, 10.5 Â 10À2, 9.2 Â 10À2
,
9.5 Â 10À2 and 9.6 Â 10À2 minÀ1, respectively. For VL, the results
are similar and the apparent rate constant (kapp) polymerizations
initiated by 11, 16, 17, 19, 26 and 28 were found to be 9.6 Â 10À2
,
9.3 Â 10À2, 14.9 Â 10À2, 12.1 Â 10À2, 12.5 Â 10À2 and 8.2 Â 10ÀÀ2
minÀ1, respectively (see Supporting information). Notably, the
initiator efficiency (f) of arylhydrogenphosphates and aryldihydro-
genphosphates lies in the range of 0.6–0.2.
To understand the polymerization mechanism, low molecular
weight oligomers of CL were synthesized by stirring CL with 11
or 26 in 20:1 molar ratio under neat conditions at 80 °C. The pro-
ducts were extracted with cold heptanes. The oligomers were char-
acterized by 1H, 31P NMR spectroscopy and MALDI-TOF. The 1H and
31P NMR measurement of oligomer revealed that the arylhydro-
genphosphate and aryldihydrogenphosphate residues are present
in the obtained PCL oligomers (Figs. 5 and 6). The characteristic

P. Malik, D. Chakraborty / Inorganica Chimica Acta 400 (2013) 32–41
39
g
O
O
P
d'
c
b'
a
e
O
O
H
O
g
O
n
a'
b
c'
d
e'
O
O
f
f
Fig. 5. 1H and 31P NMR spectra of the low molecular weight oligomer of CL and 11 in 20:1 ratio.
g
O
O
P
d'
c
b'
a
e
O
O
H
O
O
n
a'
b
c'
d
e'
O
OH
f
Fig. 6. 1H and 31P NMR spectra of the low molecular weight oligomer of CL and 26 in 20:1 ratio.
peaks of the catalysts observed in the range of 6.93–7.01, 7.19–
7.25 and 2.22 ppm which can be assigned as phenyl protons and
–CH3 protons respectively and the remaining peaks assigned as es-
ter methylene protons. In addition, sharp singlets of their 31P NMR
spectra at À17.17 and À11.12 ppm also corroborated the presence
of arylhydrogenphosphate and aryldihydrogenphosphates moi-
eties as end terminal groups. Indeed, the aforementioned 1H
NMR spectrum does not give any information about the –OH
groups of the aryldihydrogenphosphates whether, only one or both
the –OH groups are involved in the initiation of the polymerization.

40
P. Malik, D. Chakraborty / Inorganica Chimica Acta 400 (2013) 32–41
O
O
H
n
O
n
O
P
O
OH
mass + CH2Cl2
1
2
3
4
5
6
7
8
9
387.28
508.44
615.06
729.76
843.92
958.08
1072.46
1186.40
1300.78
1414.94
1529.10
1643.26
1757.36
1871.58
1985.52
2099.90
2214.06
2328.22
2441.52
10
11
12
13
14
15
16
17
18
19
Fig. 7. MALDI-TOF of the crude product obtained from a reaction between CL and 26 in the ratio of 20:1.
R
O
P
O
OH
R
O
R
H
O
P
O
P
O
O
O
O
O
O
OH
nH
O
O
O
HO
HO
Intermediate
R
R
O
P
O
O
O
P
O
O
H
O
H
O
n
O
HO
O
O
n
HO
O
Scheme 3. Cationic mechanism for the arylhydrogenphosphates and aryldihydrogenphosphates catalyzed the ring-opening polymerization of CL and VL.
Furthermore, to resolve this issue the MALDI-TOF of the PCL
in the same manner and eventually leads to the formation of
product.
was measured, as shown in Fig. 7, series of peaks appeared at reg-
ular intervals corresponding to the monomer dichloromethane ad-
duct units. The observed molecular weights agreed well with the
calculated ones of the PCL. The result unveiled that the only one
–OH group directly involved in the polymerization.
4. Conclusions
All the results of 1H, 31P NMR and MALDI-TOF measurements
strongly indicated that the aryldihydrogenphosphate acts as the
initiator for the ring-opening polymerization. Thus on the basis
of results the cationic mechanism was proposed for the arylhydro-
genphosphates and aryldihydrogenphosphates initiated the ring-
opening polymerization of CL and VL as shown in Scheme 3. The
polymerization process began with the protogenic activation of
the monomer by compound 26. The counter anion attacks on the
carbonyl carbon which leads to the formation of intermediate
(Scheme 3). The intermediate initiates the next ring-opening cycle
In conclusion, we have synthesized a series of arylhydrogen-
phosphates and aryldihydrogen-phosphates. The arylhydrogen-
phosphates and aryldihydrogenphosphates are potent self
initiating for the bulk ring-opening polymerization of CL and VL.
The polymerization proceeds in a controlled fashion which leads
to well defined polyesters with high molecular weights and narrow
MWDs. The post polymerization results confirmed that the arylhy-
drogenphosphates and aryldihydrogenphosphates initiated the
ring-opening polymerization of CL and VL which proceeds through
a cationic mechanism.

P. Malik, D. Chakraborty / Inorganica Chimica Acta 400 (2013) 32–41
[21] V.C. Gibson, E.L. Marshall, Compr. Coord. Chem. II (9) (2004) 1.
41
Acknowledgements
[22] R.C. Pratt, B.G.G. Lohmeijer, D.A. Long, R.M. Waymouth, J.L. Hedrick, J. Am.
Chem. Soc. 128 (2006) 4556.
The authors thank the Department of Science and Technology
New Delhi and Department of Chemistry, Indian Institute of Tech-
nology Madras for their support. The authors are grateful to the re-
viewers for their suggestions and comments.
[23] H.R. Kricheldorf, Chem. Rev. 109 (2009) 5579.
[24] A.-C. Albertsson, I.K. Varma, Biomacromolecules 4 (2003) 1466.
[25] J.L. Hedrick, T. Magbitang, E.F. Connor, T. Glauser, W. Volksen, C.J. Hawker, V.Y.
Lee, R.D. Miller, Chem. Eur. J. 8 (2002) 3308.
[26] P.J. Flory, J. Am. Chem. Soc. 62 (1940) 1561.
[27] F. Sanda, H. Sanada, Y. Shibasaki, T. Endo, Macromolecules 35 (2002) 680.
[28] S. Tempelaar, L. Mespouille, P. Dubois, A.P. Dove, Macromolecules 44 (2011)
2084.
Appendix A. Supplementary material
[29] X. Lou, C. Detrembleur, R. Jérôme, Macromolecules 35 (2002) 1190.
[30] C.K. Ober, S.Z.D. Cheng, P.T. Hammond, M. Muthukumar, E. Reichmanis, K.L.
Wooley, T.P. Lodge, Macromolecules 42 (2009) 465.
[31] M.K. Kiesewetter, E.J. Shin, J.L. Hedrick, R.M. Waymouth, Macromolecules 43
(2010) 2093.
[32] S. Csihony, D.A. Culkin, A.C. Sentman, A.P. Dove, R.M. Waymouth, J.L. Hedrick, J.
Am. Chem. Soc. 127 (2005) 9079.
[33] O. Coulembier, P. Degée, J.L. Hedrick, P. Duboi, Prog. Polym. Sci. 31 (2006) 723.
[34] N.E. Kamber, W. Jeong, R.M. Waymouth, R.C. Pratt, B.G.G. Lohmeijer, J.L.
Hedrick, Chem. Rev. 107 (2007) 5813.
CCDC 905871 and 905872 contain the supplementary crystallo-
graphic data for compounds 13 and 14. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data as-
sociated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.ica.2013.02.006.
[35] A. Chuma, H.W. Horn, W.C. Swope, R.C. Pratt, L. Zhang, B.G.G. Lohmeijer, C.G.
Wade, R.M. Waymouth, J.L. Hedrick, J.E. Rice, J. Am. Chem. Soc. 130 (2008)
6749.
[36] O. Coulembier, D.P. Sanders, A. Nelson, A.N. Hollenbeck, H.W. Horn, J.E. Rice, M.
Fujiwara, P. Dubois, J.L. Hedrick, Angew. Chem., Int. Ed. 48 (2009) 5170.
[37] O. Coulembier, M.K. Kiesewetter, A. Mason, P. Dubois, J.L. Hedrick, R.M.
Waymouth, Angew. Chem., Int. Ed. 46 (2007) 4719.
[38] P.D. María, ChemCatChem. 2 (2010) 487.
[39] D. Bourissou, B. Martín-Vaca, A. Dumitrescu, M. Graullier, F. Lacombe,
Macromolecules 38 (2005) 9993.
[40] K. Makiguchi, T. Satoh, T. Kakuchi, J. Polym. Sci., Part A: Polym. Chem. 49
(2011) 3769.
[41] Y. Shibasaki, H. Sanada, M. Yokoi, F. Sanda, T. Endo, Macromolecules 33 (2000)
4316.
[42] J. Casas, P.V. Persson, T. Iversen, A. Córdova, Adv. Synth. Catal. 346 (2004) 1087.
[43] K. Makiguchi, T. Satoh, T. Kakuchi, Macromolecules 44 (2011) 1999.
[44] G.M. Sheldrick, Programs SHELXS-97 (crystal structure solution) and SHELXL-97
(crystal structure refinement), University of Göttingen, Germany, 1997.
[45] P. Malik, D. Chakraborty, Polyhedron 29 (2010) 2142.
[46] G.M. Kosolapoff, C.K. Arpke, R.W. Lamb, H. Reich, J. Chem. Soc. C (1968) 815.
[47] N. Bojars, Technikas Apskats 52 (1966) 7.
References
[1] C.J. Hawker, K.L. Wooley, Science 309 (2005) 1200.
[2] C.W. Bielawski, R.H. Grubbs, Prog. Polym. Sci. 32 (2007) 1.
[3] D.A. Tomalia, J.M.J. Frechet, J. Polym. Sci., Part A 40 (2002) 2719.
[4] S.I. Stupp, V. Lebonheur, K. Walker, L.S. Li, K.E. Huggins, M. Keser, A. Amstutz,
Science 276 (1997) 384.
[5] G.J. Domski, J.M. Rose, G.W. Coates, A.D. Bolig, M. Brookhart, Prog. Polym. Sci.
32 (2007) 30.
[6] K.M. Stridsberg, M. Ryner, A.C. Albertsson, Advances in Polymers Science,
Springer-Verlag, Berlin, 2002. p. 41.
[7] F. Nederberg, B.G.G. Lohmeijer, F. Leibfarth, R.C. Pratt, J. Choi, A.P. Dove, R.M.
Waymouth, J.L. Hedrick, Biomacromolecules 8 (2007) 153.
[8] F. Nederberg, V. Trang, R.C. Pratt, A.F. Mason, C.W. Frank, R.M. Waymouth, J.L.
Hedrick, Biomacromolecules 8 (2007) 3294.
[9] D.G. Barrett, M.N. Yousaf, Molecules 14 (2009) 4022.
[10] K. Fukushima, R.C. Pratt, F. Nederberg, J.P.K. Tan, Y.Y. Yang, R.M. Waymouth,
J.L. Hedrick, Biomacromolecules 9 (2008) 3051.
[11] E.F. Connor, W.N. Gregory, M. Myers, A. Möck, J.L. Hedrick, J. Am. Chem. Soc.
124 (2002) 914.
[12] J. Wu, T. Yu, C. Chen, C. Lin, Coord. Chem. Rev. 250 (2006) 602.
[13] M. Kamigaito, T. Ando, M. Sawamoto, Chem. Rev. 101 (2001) 3689.
[14] O. Dechy-Cabaret, B. Martin-Vaca, D. Bourissou, Chem. Rev. 104 (2004) 6147.
[15] M.J. Stanford, A.P. Dove, Chem. Soc. Rev. 39 (2010) 486.
[16] R.J. Pounder, A.P. Dove, Polym. Chem. 1 (2010) 260.
[17] S.H. Kim, F. Nederberg, L. Zhang, C.G. Wade, R.M. Waymouth, J.L. Hedrick, Nano
Lett. 8 (2008) 294.
[18] S.H. Kim, O.H. Park, F. Nederberg, T. Topuria, L.E. Krupp, H.C. Kim, R.M.
Waymouth, J.L. Hedrick, Small 4 (2008) 2162.
[19] R.H. Platel, L.M. Hodgson, C.K. Williams, Polym. Rev. 48 (2008) 11.
[20] B.J. O’Keefe, M.A. Hillmyer, W.B.J. Tolman, J. Chem. Soc., Dalton Trans. 11
(2001) 2215.
[48] C.J. Crano, US 4170614 19791009, 1979.
[49] M. Rathore, A. Kabara, P. Rani, C.K. Narang, N.K. Mathur, Indian J. Chem. B 32
(1993) 1066.
[50] Fr. Demande, FR 2004072 19691121, 1969.
[51] D. Noort, G.A. Van der Marel, A. Van der Gen, G.J. Mulder, J.H. Van Boom, Recl.
Trav. Chim. Pays-Bas 110 (1991) 53.
[52] S.N. Munik, V.N. Eliseenkov, B.E. Ivanov, Zh. Obshch. Khim. 65 (1995) 396.
[53] A. Chaturvedi, S. Perbha, Orient. J. Chem. 11 (1995) 149.
[54] P. Deprez, E. Mandine, D. Gofflo, S. Meunier, D. Lesuisse, Bioorg. Med. Chem.
Lett. 12 (2002) 1295.