Synthesis and polymerization kinetics of ε-caprolactone and ?-lactide to low molecular weight polyesters catalyzed by Zn(II) and Cu(II) N-hydroxy-N,N′-diarylformamidine complexes

Article abstract of DOI:10.1016/j.poly.2017.09.029

Discrete Zn(II) and Cu(II) complexes have been synthesized by using N-hydroxy-N,N′-diarylformamidine ligands: N-hydroxy-N,N′-bis(2,6-diisopropylphenyl)formamidine (L1), N-hydroxy-N,N′-bis(2,6-dimethyl)formamidine (L2), N-hydroxy-N-(2-methoxyphenyl)-N′-(2,

Full text of DOI:10.1016/j.poly.2017.09.029

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Accepted Manuscript
Kinetics and synthesis of low molecular weight polyesters from ε-caprolactone
and ʟ-lactide catalyzed by Zn(II) and Cu(II) N-Hydroxy N,N ’-diarylformami-
dine complexes
Wisdom A. Munzeiwa, Bernard Omondi, Vincent O. Nyamori
PII:
S0277-5387(17)30606-X
https://doi.org/10.1016/j.poly.2017.09.029
POLY 12839
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
1 July 2017
17 September 2017
Please cite this article as: W.A. Munzeiwa, B. Omondi, V.O. Nyamori, Kinetics and synthesis of low molecular
weight polyesters from ε-caprolactone and ʟ-lactide catalyzed by Zn(II) and Cu(II) N-Hydroxy N,N ’-
diarylformamidine complexes, Polyhedron (2017), doi: https://doi.org/10.1016/j.poly.2017.09.029
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Kinetics and synthesis of low molecular weight polyesters from ε-caprolactone and ʟ-
lactide catalyzed by Zn(II) and Cu(II) N-Hydroxy N,N’-diarylformamidine complexes
Wisdom A. Munzeiwa, Bernard Omondi* and Vincent O. Nyamori
School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Private Bag
X54001, Durban 4000, South Africa
*E-mails: owaga@ukzn.ac.za
ABSTRACT
Discrete Zn(II) and Cu(II) complexes have been synthesized using N-hydroxy-N,N’-
diarylformamidine ligands: N-hydroxy-N,N’-bis(2,6-diisopropylphenyl)formamidine (L1), N-
hydroxy-N,N’-bis(2,6-dimethyl)formamidine (L2), N-hydroxy-N-(2-methoxyphenyl)-N’-(2,6-
diisopropylphenyl)formamidine
(L3),
and
N-hydroxy-N-(2-methoxyphenyl)-N’-(2,6-
dimethylphenyl)formamidine (L4). Reaction of ligands L1 - L4 with either ZnOAc2.2H2O or
CuOAc2.2H2O in aqueous ethanol gave mononuclear complexes [Zn-(L1)2] (1), Zn-(L2)2] (2),
Zn-(L3)2] (3), [Zn-(L4)2] (4), and [Cu-(L1)2] (5), [Cu-(L2)2] (6), [Cu-(L3)2] (7), [Cu-(L4)2] (8),
as Zn(II) and Cu(II) complexes, respectively, with high yields of up to 84%. All the complexes
were characterized by elemental analysis, IR, NMR and mass spectroscopies. The molecular
structures of complexes 3 and 7 were determined by single crystal X-ray diffraction analyses.
The Zn(II) center in complex 3 exhibited a distorted tetrahedral geometry while in complex 7,
the Cu(II) center had a square planar geometry with near C2 symmetry. In both structures, the
coordination sites are occupied by imino N and hydroxyl O donor atoms from the chelating
ligands. All complexes showed catalytic activity in ring-opening polymerization of ε-
caprolactone and ʟ-lactide in the presence of a co-initiator and exhibited well-controlled living
polymerization process. The molecular weights were found to be low ranging from 1855 - 3999
Da for polycaprolactone (PCL) and up to 1720 Da for polylactic acid (PLA). The Zn(II)
catalysts were found to be more active than Cu(II) catalysts with complex 2 (kapp = 0.1751 h-1)
being the most active.
Keywords:
Zn(II), Cu(II), polyesters, ROP, kinetics, N-Hydroxy-N,N’-diarylformamidine
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1.0 Introduction
Petroleum based-polymers that are derived from nonrenewable fossils have low biodegradability
and cause environmental pollution after disposal [1]. In addressing these challenges, modern-
day polymer research is geared towards developing economically viable, recyclable and
biodegradable polymers derived from renewable resources. Aliphatic polyesters have emerged
as better surrogates and they present many traits. Poly(lactic acid) (PLAs) and poly(ε-
caprolactone) (PCLs) are interesting candidates because they are bio-compatible, bio-degradable
and can be bio-derived. Owing to their diverse applications in the field of medicine [2],
packaging [3] and electronic devises [4], their demand has increased over the past decades.
Metal-based ring opening polymerization (ROP) has indisputably proved to be a more proficient
method for the synthesis of polyesters.
Commercially, tin(II) complexes are used as catalyst or initiators for the synthesis of PCLs and
PLAs, however their toxicities limits their use in the production of polymers for medical
applications since the elimination of remnant catalyst from the polymer is a challenge [5].
Coordination complexes of cheap and bio-compactable metals with reasonable toxicity are now
being investigated as replacements. Metals such as zinc [6], magnesium [7], calcium [8],
aluminum [9] and copper [10] have been investigated and have shown promising results towards
ROP of cyclic esters. Alkali earth metals [11] and lanthanides [12] have also been explored for
catalytic activity in ROP. More recently, silver which is known to have antimicrobial properties
have been found to be active towards ε-caprolactone (ε-CL) polymerization [13].
A metal-oxygen bond is a prerequisite for effective initiation of the polymerization process.
Many metal-based ROP systems are dominated by nitrogen and/or oxygen as metal stabilizing
ligands which have a positive influence towards catalytic activity [15]. Ligand-supported
single site metal alkoxide initiators have been shown to control the polymerization process and
yield polymers with controlled molecular weights (Mw), polydispersity indices (PDIs),
architecture and end groups [14]. This has attracted much interest and prompted researchers to
design more ligands to support metal-alkoxide based catalyst for ROP. Salen-type ligand
supported metal initiators have been systematically studied and used effectively in ROP of
lactides with high stereo-control toward, either isotactic or heterotactic PLA polymers [16].
2

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Apart from the chemical properties, physical properties of auxiliary ligands also impact strongly
on the catalytic activity of metal based catalyst. Thus, it is paramount to probe correlation
between ligand structure and catalytic activity. From previous work reported by our group,
N,N’-diarylformamidine ligands have been used to support Zn(II) and Cu(II) complexes which
were active in ROP ε-CL and ʟ-LA. Herein, the N,N’-diarylformamidines ligands were modified
to N-hydroxy-N,N’-diarylformamidine ligands which potentially introduce M—O bonds as part
of the catalyst. We hypothesized that the presence of M—O bonds will bring enhanced catalytic
activity as well as polymer characteristics.
2.0 Experimental section
2.1 Materials
All experiments were carried out under argon, 5.0 technical grade, (Airflex Industrial Gases,
South Africa) using Schlenk techniques. All solvents were obtained from Sigma-Aldrich.
Reagent grade absolute ethanol (98%) was distilled and dried from magnesium turnings;
dichloromethane (DCM), (99%) and hexane (98%) were dried from sodium–benzophenone
mixture. Reagents, Cu(OAc)2.H2O (98%), Zn(OAc)2.2H2O (97%), ε-caprolactone (ε-CL) (98%)
and L-lactide (L-LA) 97% and 3-chloroperoxybenzoic acid, (MCPBA) (77%) were obtained
from Sigma-Aldrich. Anhydrous MgSO4 (98%), NaOH (99%), anhydrous NaHCO3 (97%) and
anhydrous K2CO3 (99%) were obtained from Promark Chemicals, South Africa.
2.2 Instrumentation
13
1H and C NMR spectra were measured at room temperature on a Bruker AvanceIII 400MHz
1
13
spectrometer. Both H NMR and C NMR data was recorded in CDCl3 and referenced to the
residual CDCl3 peaks at δ 7.26 and δ 77.00, respectively. IR spectra were obtained on a
PerkinElmer Universal ATR spectrum 100 FTIR spectrometer. Mass spectra of complexes were
obtained from a Water synapt GR electrospray positive spectrometer.
3