Starting from the discovery by Ziegler and Natta of the first catalyst for controlled alkene polymerization, the development of catalysts for the synthesis of polymeric materials has been an impressively growing research field. Nowadays, it is possible to state that single-site metal catalyzed polymerization represents a powerful tool for the controlled, environmentally friendly, synthesis of new macromolecules. Indeed, thanks to the capability of tailoring the chemical environment on the metal, it is possible to tune the electronic and steric properties of the catalytic centre, allowing the control of the properties of the synthesized macromolecules, i.e. molecular weight and molecular weight distribution, nature of the end groups, stereochemistry, comonomer insertion and its distribution.

This is the main research topic of Dr. Milani and two research lines are currently under investigation:

1. Development of catalysts for CO/vinyl arene copolymerization;


2. Development of catalysts for ethylene/polar monomer copolymerization.

From a general point of view the research activity consists of the typical steps of projects in homogeneous catalysis, that are: i. synthesis and characterization of ligands; ii. synthesis and characterization of the related complexes; iii. study of the catalytic behavior in the target reactions; iv. mechanistic investigation; v. characterization of the synthesized macromolecules. In particular, for points i. and v. dr. Milani takes advantage of the many National and International collaborations established along the years.

1. Development of catalysts for CO/vinyl arene copolymerization

Dr. Milani has a consolidate experience in the development of homogeneous catalysts for the controlled synthesis of CO/vinyl arene polyketones via the direct copolymerization of the two comonomers (Figure 1).

Figure 1. The CO/vinyl arene copolymerization.

In the case of vinyl arenes as alkene comonomers the best performing catalysts are based on palladium complexes with bidentate nitrogen-donor ligands (N-N) and two major issues have been addressed: catalyst stability and polymer stereochemistry.
Two families of precatalysts have been developed, dicationic derivatives of general formula [Pd(N-N)₂][X]₂, and monocationic species [Pd(CH₃)Cl(N-N)][X]. In particular, it was found that an important contribution to the catalyst stability comes from the reaction medium, and 2,2,2-trifluorethanol is the solvent choice.¹ A strict relationship between ligand symmetry and polymer stereochemistry was established.²
The best performing catalysts for the synthesis of highly syndiotactic copolymer are based on ligands with the 1,10-phenanthroline skeleton: in particular the active species containing the 5,5,6,6-tetrafluoro-5,6-dihydro-1,10-phenanthroline (F₄-phen, Figure 2) is active for at least 96 h, leading to the CO/4-methyl styrene polyketone with a Mw = 1000000 (Mw/Mn = 2.3) and with a content of the uu triad of 96 %. The polymer is obtained as a white solid (Figure 2), indicating that no evident decomposition of the catalyst to inactive Pd metal takes place.³

Figure 2. The F₄-phen (left) and a sample of CO/4-methyl styrene polyketone (right).

For the synthesis of copolymers with a fully isotactic microstructure, catalysts based on aza-bis-oxazolines (Figure 3) represents a good compromisd between productivity, degree of isotacticity and copolymer molecular weight.⁴

Figure 3. The aza-bis-oxazoline (left) and an example of a nonsymmetrically substituted Ar-BIAN (right).

On the other hand, catalysts based on -diimines with an acenaphthene skeleton and with aryl rings symmetrically and nonsymmetrically substituted in meta positions (Ar-BIAN, Figure 3) lead to polyketones with an atactic microstructure regardless to the symmetry of the ligand bonded to palladium.⁵
Finally, we also discovered that when the ligands are substituted in close proximity to the nitrogen-donors the catalytic products are brown oils, CO/vinyl arene cooligomers and not the expected copolymers (Figure 4).⁶

Figure 4. An example of ortho substituted N-ligand (left) and a sample of CO/ styrene oligoketone (right).

We are currently studying catalysts based on N-donor ligands that should drive the stereochemistry of the copolymerization reaction toward the synthesis of stereoblock copolymers.

2. Development of catalysts for ethylene/polar monomer copolymerization.

One of the major unsolved problems in the field of polymer chemistry is represented by the synthesis of ethylene/polar monomer copolymers via the homogeneously catalyzed, direct copolymerization of the two comonomers (Figure 5).

Figure 5. The ethylene/polar monomer copolymerization.

The aim of this research deals with the development of homogeneous catalysts that are able to efficiently carry out the reaction under mild conditions of temperature and pressure leading to the copolymer with a content of the polar monomer of at least 20 % and with the polar monomer inserted into the main polymer chain.
The research currently pursued is based on the investigation of Pd-complexes with N-donor ligands.
Promising results are obtained, but they have not been published yet, so no detail is reported.