Structural Characterization of an ATRP Catalyst Complex - Matyjaszewski Polymer Group - Carnegie Mellon University

Structural Characterization of an ATRP Catalyst Complex

Structure of a Catalyst Complex

Effect of Substituents on the Ligands:

Structure of a Catalyst Complex

The primary roles of the ligand in an ATRP catalyst complex is to solubilize the transition metal salts in the polymerization medium and to adjust the redox potential of the metal center to provide an appropriate activity and dynamics for the repetitive halogen exchange reaction. The electron donating ability of the ligand can greatly affect the redox potential of the transition metal complex and influence the reactivity of the metal center in halogen abstraction and transfer meaning that the structure of the catalyst selected for a given reaction affects the kinetics of an ATRP and hence the degree of control over the polymerization reaction.

Equilibrium

In the above scheme representing an ATRP equilibrium Mt is a transition metal with two stable oxidation states differing by 1 (m <-> m+1).  As noted elsewhere on this web site the bulk of the work on ATRP conducted by the Matyjaszewski group has employed copper as the transition metal since it has provided a wide range of transition metal complexes with a range of accessible nitrogen containing ligands that have been shown to be suitable for controlled polymerization of a broad range of radically copolymerizable monomers.  Therefore, structural characterization of a series ATRP active copperI and copperII complexes with a spectrum of ligands continues to be studied within the Matyjaszewski group using a variety of analytical tools. 

Recent reviews on the structural aspects of copper catalyzed ATRP(1-4) provide some background on the fundamentals of transition metal catalyzed atom transfer reactions, including ATRA and ATRP.  The reviews focus on the structure of a catalyst complex formed with bidentate, tridentate and tetradentate nitrogen ligands that generally work well for Cu-mediated ATRP.  The choice of ligand greatly influences the effectiveness of the catalyst in a specific polymerization reaction.  One ligand does not work for every copolymerization since catalyst activity span seven orders of magnitude.  Within the review articles a broad series of ligands forming catalyst complexes with an expansive range of activity are discussed including:

2,2'-bipyridine   (bpy),

4,4'-di(5-nonyl)-2,2'-bipyridine   (dNbpy),

N,N,N',N'-tetramethylethylenediamine   (TMEDA),

N-propyl(2-pyridyl)methanimine   (NPrPMI),

2,2':6',2''-terpyridine   (tpy),

4,4',4''-tris(5-nonyl)- 2,2':6',2''-terpyridine   (tNtpy),

N,N,N',N'',N''-pentamethyldiethylenetriamine   (PMDETA),

N,N-bis(2-pyridylmethyl)octylamine   (BPMOA),

1,1,4,7,10,10-hexamethyltriethylenetetramine   (HMTETA),

tris[2-(dimethylamino)ethyl]amine   (Me6TREN),

tris[(2-pyridyl)methyl]amine   (TPMA),

1,4,8,11-tetraaza-1,4,8,11-tetramethylcyclotetradecane   (Me4CYCLAM) and

N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine   (TPEN)

The structures of copperI and copperII complexes with the following ligands are discussed in the context of how structure affects catalyst activity in addition to solvent and temperature.

diethylenetriamine                                                           (DETA),

triethylenetetramine                                                        (TETA),

N,N-bis(2-pyridylmethyl)amine                                         (BPMA),

tris[2-aminoethyl]amine                                                   (TREN) and

1,4,8,11-tetraazacyclotetradecane                                  (CYCLAM)

N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine              (TPEN)

The ligands most frequently employed for copper based catalysts are presented below:

ligands

Aspects of the structural studies concentrate on the stoichiometry between the complex forming ligand and the copper centers,(5-6) determination of the geometry of the formed complexes, and their solution behavior.(7-9)  Techniques used include solid state X-ray crystallography, Extended X-ray Absorption Fine Structure (EXAFS),(7) Electrospray Ionization Mass Spectrometry (ESI-MS),(9) and UV-Vis, Raman and Far IR spectroscopy.

A series of Cu(II) complexes were examined and they adopted either a trigonal bipyramidal structure, as in the case of the dNbpy ligand, or a distorted square pyramidal coordination, in the case of triamines and tetramines.(10)

complex

Experimental (dotted line) and calculated (solid line) (k)3χ(k) functions (a) (k range: 4.20 - 14.6 Å-1) and their Fourier transforms (b) for CuIBr/Me6TREN in toluene at room temperature.

Depending on the type of amine ligand, the complexes were either neutral (triamines) or ionic (bpy and tetramines). The counterions in the case of the ionic complexes were either a bromide anion (Me4Cyclam and HMTETA) or the linear [CuIBr2]- anion (dNbpy).

No direct correlation was found between the length of the CuII-Br bond and the deactivation rate constant in an ATRP, which suggests that other parameters such as the entropy for the structural reorganization between the CuI and CuII complexes might play an important role in determining the overall activity of the catalyst in ATRP.(1)

Effect of Substituents on the Ligands:

Since the first employed CuX(bpy)2 catalytic system was heterogeneous in non-polar media early work on incorporation of substituents into ligand for an ATRP focused on controlling the solubility of the catalyst complex. Incorporation of alkyl-substituents in the Bpy ligand provided soluble catalyst complexes and polymers with low dispersity and a means to determine the kinetics of the reaction.(11-12) 4,4'-Di-n-heptyl-2,2'-bipyridine (dHbpy), 4,4'-di(5-nonyl)-2,2'-bipyridine (dNbpy) and 4,4'-di-5-nonyl-2,2'-bipyridine (bpy9) were synthesized to provide homogeneous copper complexes in ATRP.(11,13)  However as noted above in addition to modifying the solubility of the catalyst complex the electron donating ability of the ligands greatly affect the redox potential of the transition metal complex and influence the reactivity of the metal center in halogen abstraction and transfer.(14)  Catalysts therefore provide synthetic freedom in ligand design to manipulate and tune catalytic properties, which had not been truly exploited in ATRP.

Recent work has systematically examined the effect of incorporating electron donor groups (EDG) into potential ATRP ligands.(15)  This initial paper focused on examining the importance of electronic effects when employing various para substituents (R) in bpy ligands.  The substituents ranged from electron withdrawing groups (EWGs: Cl) to electron donating groups (EDGs: Me, MeO, (Me)2N), and represented the first systematic study seeking to correlate structure-activity of substituents bearing variable Hammett parameters (σρ) to E1/2  (KATRP) and polymerization rate, see following table.


Table 1: CV of 4,4'-Substituted Bipyridyl Ligands.

Entry

R

σρa

E1/2 (V)b

Δ Ep (mV)

1

Cl

0.23

0.270

130

2

H

0

0.055

110

3

Me

-0.17

-0.048

125

4

dN

n/a

-0.055

120

5

MeO

-0.27

-0.088

105

6

(Me)2N

-0.83

-0.313

14

aLiterature values,(16) bV vs. Saturated Calomel Electrode (SCE)

It was expected that electron withdrawing groups (EWG) would stabilize CuI while electron donating groups (EDG) would stabilize   and indeed when values for E1/2  are considered changing the substituents significantly changed the activity of the catalyst complexes; ∆ (-Cl to -NMe2) ≈ 600 mV, Figure A, which means an 1010 range in activity for Bpy based catalysts. The only Bpy ligand evaluated in the study that is currently not commercially available is the most active (Me)2N-Bpy.  Changing from -H to -NMe2 provides a difference in KATRP of a million, see Figure B, making the p-(Me)2N-Bpy similar in activity to one of the most active complexes in ATRP, namely Cu/Me6TREN.
5Aligandsubs5AEGDBpy

The increased concentration of the CuII/L complex in the reaction medium means that lower concentrations of catalyst can be expected to provide good control over the instantaneous increase in molecular weight for each polymer chain with conversion providing a polymer with narrow Mw/Mn.  Therefore the highly active R-bpy ligands were investigated to determine their ability to maintain CLRP behavior with ppm concentrations of catalyst.  ATRP conducted with 500 ppm of CuII showed that increasing electron donating character of the substituents resulted in higher rates of polymerizations and polymers of similar Mn had lower Mw/Mn values when prepared using more active catalysts.


Catalyst development plays a pivotal role in the endeavor to reduce catalyst loadings, overcome limitations in monomer selection and to achieve high control in ATRP systems. Based on rational design, introduced above, the currently most active copper catalyst for atom transfer radical polymerization (ATRP) was prepared.(17)  The performance of these copper and tris(2-pyridylmethyl)amine based catalysts was evaluated by cyclic voltammetry, stopped-flow measurements, and simulations. The new ligands, containing up to nine EDGs in each ligand, were employed to form catalyst evaluated in various ATRP methods including normal ATRP, ARGET, ICAR or eATRP which provided important insights into how to use very active catalysts in ATRP.


5AKSligands

Counter intuitively the standard ATRP reaction was slow with the ligand containing all the activating EDGs, only low conversions and moderate control were observed. After 1 h, the monomer conversion was 7% and the resulting polymers had a high dispersity, Mw/Mn= 1.34. At longer reaction times no significant conversion increase was achieved, i.e. after 5.15 hours, conversion was only 13.5 % and no significant conversion at longer reaction times and provided polymer with moderately broad dispersity, 1.25-1.35.
This behaviour can be attributed to the highly active nature of CuI/TPMA* catalyst complex.  The active catalyst generates high radical concentrations resulting in significant termination reactions between oligomeric products and rapid conversion of CuI/TPMA* to X-CuII/TPMA*. As a result, a strong decrease in the polymerization rate occurred.  While ARGET ATRP with 50 ppm catalyst activated by reaction with different mole ratio’s of Sn(EH)2 provided linear first order reaction kinetics and reached 90% conversion with dispersity 1.1.  Similar results were obtained with e(ATRP), SARA and ICAR ATRP reactions with the catalyst complexes containing the EDGs providing better control than with the “old” very active TPMA ligand.  The catalyst complex formed with TPMA* is three orders of magnitude more active than the complex formed with TPMA and provided a well controlled reaction with only 5 ppm catalyst.
A similar improvement in the activity of iron based catalysts for the polymerization of styrene with phosphine based ligands containing electron donating groups was also observed.(18)  In this paper the only EDG evaluated was a –OMe group and it was determined that FeIIIBr3 in the presence of tris(2,4,6-trimethoxyphenyl)-phosphine (TTMPP) provided faster ATRP of St than in the presence of tris(4-methoxyphenyl)phosphine and much faster than with triphenylphosphine under identical conditions.  A ratio of ligand to iron of 2:1 provided a more active catalyst and the reaction progressed without the addition of any reducing agent indicating that the phosphines could directly reduce FeIII to FeII while also acting as ligands to form a complex with the transition metals and form efficient ATRP catalysts.  However with a ratio on 2:1 with TTMPP ligand the molecular weight became higher than the theoretical value after 70% conversion, indicating some plausible contribution of termination by coupling.  The reducing effect of the phosphine ligands and the possibility of coupling was confirmed by activation of PSt-Br macroinitiator with FeIIIBr3/TTMPP catalyst to form PSt with doubled molecular weight via coupling.  ATRP of BA and MMA with the same catalysts containing the same phosphine ligands also provided well controlled reactions under proper conditions.
In conclusion, very active catalysts formed by incorporating EDGs into aromatic based ligands provide significantly better performance when combined with methods which require only ppm of catalyst and regenerate the active Cu(I)/ligand species during the polymerization, than with normal ATRP.

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