Download Iron complexes of bidentate nitrogen donor ligands as

Iron complexes of bidentate nitrogen donor ligands as catalysts for Atom
Transfer Radical Polymerization of styrene
Rossella Ferro and Alfonso Grassi
Dipartimento di Chimica, Università di Salerno, 84084 Fisciano (Salerno)
The selective synthesis of polymeric materials with well designed properties and
molecular architecture is a topic of increasing interest. Living polymerization is a process
in which the rate of irreversible termination or chain transfer reactions is negligible in
comparison to chain growth rate, permitting a control of the average molecular weights of
the polymer synthesized. Among the polymerization techniques, the radical processes are
of increasing interest because they allow homo and co- polymerization, in rather mild
conditions, of a large variety of monomers ranging from unsaturated hydrocarbons to polar
monomers. Recently atom transfer radical polymerization (ATRP) is becoming worthy of
particular attention for the possibility of obtaining the living polymerization by means of a
simple and reversible reaction of the growing radical polymeryl with a metal halide.1
A typical ATRP mechanism is depicted in Scheme 1.
RX
kact
+ Mtn Y / Ligand
R
+
X
kdeact
Mtn+1 Y /Ligand
Monomer (M)
n
P X + Mt Y / Ligand
kact
P
kdeact
+
X
Mtn+1 Y /Ligand
+M
kp
Scheme 1
A metal mediated halide exchange process establishes a fast equilibrium between the
growing (Pn•) and dormant (Pn-X) polymer chains. Typically this equilibrium is shifted in
an ATRP process towards the dormant species: an extremely low radical concentration is
thus obtained which slows down the rate of the termination reactions characteristic of a
radical process, e.g. disproportionation and radical coupling. The fast halide exchange is
controlled by the electrochemical redox potential between the two redox states of the metal
centre, and ensures all polymer chains are growing at the same rate, furnishing excellent
control over the radical polymerization.
1
A different approach, leading to the same result, is the so called reverse ATRP.2 This
technique follows the same ATRP reactions of Scheme 1 but differs for the initiation
reaction. In this case a typical radical initiator (e.g. 2,2’-azo-bisisobutyronitrile (AIBN),
1,1,2,2-tetraphenyl-1,2-ethanediol (TPED)) can be used and left to react with the metal
halide in high oxidation state, permitting the polymerization to proceed later on as in
Scheme 1.
Reverse ATRP exhibits two advantages: i) the radical initiator is generally less toxic
and less expensive than the organic halide used in direct ATRP; ii) the metal catalyst is
more soluble, thermally and air-stable because of the highest oxidation state.
There is an increasing interest in the Iron mediated ATRP catalysis because of the
biocompatibility and low cost of the metal complexes, and the easy accessibility to a wide
number of complexes with different ligands.3 Previous studies by Gibson et al.4 showed
that bidentate ligands containing immino nitrogen donors provided stable complexes
suitable for polymerization of styrene and methyl methacrylate under ATRP protocol. At
redox potential higher than nearly 200 mV (vs Ag/AgCl; E1/2= 177 mV, ∆E=130 mV for
ferrocene used as external standard) a poor control over the radical polymerization was
actually found. However the average molecular weight and molecular weight distribution
of the polymer products were not found completely satisfactory: in particular the latter is
far from the typical value of 1-1.1 characteristic of a living process.
We investigated the performances in reverse ATRP of three Iron complexes, namely
Fe(bpzm)Cl3 (1), Fe(bipy)Cl3 (2) and Fe(box)Cl3 (3) bearing the bidentate neutral nitrogen
ligands, 3,5-dimethyl-bispirazolylmethane (bpzm), 2,2’-bipyridile (bipy), 1,1-bis[4,4dimethyl-1,3-oxazolin-2yl]ethane (box) respectively (Scheme 2). Aiming to assess the
electronic properties which assure good performances in ATRP of these complexes, we
choose the bpzm, bipy and box ligands as representative examples in which the steric and
Lewis base properties can be tuned with simple chemical modifications.
O
O
N
N
N
N
bpzm
N
N
N
N
bipy
box
Scheme 2.
The preliminary screening of 1-3 in ATRP of styrene in bulk showed that 3 is the best
one in terms of monomer conversion and polydispersity index: actually a 64% monomer
2
conversion in 18 h at 120°C was observed and the corresponding polymer exhibited a
Mw/Mn value of 1.30. Polymerization runs carried out in toluene solution to ensure the
homogeneous conditions, produced polystyrene with Mw/Mn of 1.15, that is one of the
lowest value for Iron based catalysts.
Steric and electronic effects have been invoked to justify the different performances
in ATRP of the Iron complexes bearing bi- and tri-dentate ligands with nitrogen donors. In
the reaction of the radical polymeryl with 1-3 we consider negligible the steric effects
because of the planar disposition of the ligand, and thus we focused our attention on the
electronic properties of the complexes.
The binding constant Kass of bpzm, bipy and box to FeCl3 were determined by UV-Vis
measurements. The stoichiometry of the complexes (ligand/FeCl3 ) was determined by
Job’s method5 and the Kass values were calculated using the Scatchard’s method:6 the
values are given in Table 1.
The highest affinity to the Fe(III) metal centre was observed for the box ligand which
produces stable complexes with 1:1 stoichiometry in acetonitrile solution at room
temperature. The bpzm, bipy ligands are weaker Lewis base and lead to an equilibrium
reaction between the free and coordinated ligand.
The electrochemical parameters ∆E and E1/2 of the Fe(II)/Fe(III) redox couple define
the reversibility of the redox process and the corresponding potential: both parameters
contribute in determining the molar concentration of the radical species in the ATRP
protocol. The electrochemical parameters E1/2 and ∆E of 1-3, measured by Ciclic
Voltammetry (VC), are reported in Table 1.
Table 1. Binding constants and electrochemical parameters for complexes 1-3
Complex Stoichiometrya
FeCl3/ligand
Kassb
E1/2
(V)
∆E
(V)
1
1/1
1980 ± 20
-0.865 0.110
2
1/1
6100 ± 200
-0.651 0.114
3
1/1
10100 ± 300 -0.552 0.125
a
Evaluated by Job’s method ([FeCl3]+[ligand]=1*10-4 M in CH3CN,
T=25°C). b Calculated by Scatchard’s method. ([FeCl3]=4*10-5 M in
CH3CN, T=25°C).
3
The box ligand produces the most stable complex 1 which is oxidized at higher
potential than 2 and 3: the latter complexes are easily oxidized and exhibit lower stability
and reversibility in agreement with the very modest performances in ATRP of styrene.
Matyjaszewsky et al.7 proposed an ideal range of E1/2 (-0.5 ÷ +0.4 V vs Ag/AgCl) for
the Copper catalysts to be used in ATRP. On the basis of Gibson and our results we are
thus able to define a range of electrochemical potential E1/2 between -0.55 and ~ +0.2 V vs
Ag/AgCl electrode (E1/2= 177 mV, ∆E=130 mV for ferrocene used as external standard) in
which good performances of the Iron complexes can be expected in ATRP of styrene.
To support the composition and chemical structure of the catalyst 1 we isolated and
characterized the structure of the Fe(box)Cl2 complex by single crystal X-ray diffraction
(see Figure 1). This complex, that is the reduced partner of 1, is still monomeric and
exhibit a pseudotetrahedal geometry with a 1:1 metal to ligand stoichiometry.
Figure 1. X-Ray crystal structure of Fe(box)Cl2.
To show the potentiality of combining the ATRP of polar monomers with metal
catalyzed polymerization of unsaturated hydrocarbons we synthesized diblock copolymer
of syndiotactic polystyrene with polymethylmethacrylate using a two steps procedure. The
first step is a typical Ziegler-Natta polymerization of styrene to yield syndiotactic
polystyrene using the Cp*TiBn3/B(C6F6)3 catalyst under pseudoliving conditions. The
polymerization run is terminated with 2-bromine-2-phenylethylisocyanate that produces a
polystyryl macromonomer with a benzyl bromide chain ending. In the second step methyl
methacrylate is polymerized using the living ATRP protocol with the CuBr/PMDETA8
catalyst in the presence of the brominated macroinitiator (Scheme 3).
sPS-PMMA diblock samples with different monomer block lengths were obtained
and characterized by NMR spectroscopy, thermal analysis and X-ray powder diffraction.
By slow cooling of hot THF solutions of the diblock copolymer, with block lengths of Mn
(PS) = 1.3x104 Da and Mn (PMMA)= 1.2x104 Da a gel was obtained at room temperature.
4
The treatment of this gel with supercritical CO2 removes the clathrated solvent leading to a
white solid sponge. A THF solution 0.04% of this sample was deposited by spin coating on
silica surface. The AFM image reveals the presence in the thin film of micelles of about 30
nm of diameters (see Figure 2).
Br
N
C
O
O
+
Br
+
N
H
Ti
Toluene
T = 0°C, t = 30 min
Bn
Ph
Bn
O
H 3 CO
Ph
Ph
CuBr / PMDETA
Anisolo
T = 90°C, t = 17h
O
N
H
Ph
Ph
Ph
Ph
R
R
R = COOCH 3
Scheme 3
The thermal analysis showed that the micella’s core consists of crystalline
syndiotactic polystyrene and the outer sphere of amorphous PMMA. Interestingly the Xray powder diffraction analysis suggested that the polystyrene core is in the nanoporose δ
crystalline form containing empty cavities in which small molecules can be hosted.9 Thus
the main challenge of this project is the possibility of using these micelles as shuttle of
small molecule to be used in drug delivery and analytical application.
Figure 2. AFM image of sPS-PMMA diblock copolymer.
5
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