SK, Bhagavath P. Internal donors on supported Ziegler Natta catalysts for isotactic polypropylene: a brief tutorial review

The scientific and technical advances in the field of polymer science has been abundant in recent years. Amongst the various polymeric materials available in market, synthesis of polyolefins has been in the forefront since decades. A major challenge in this domain remains in attaining stereoregular polyolefins especially polypropylene (PP) and significant efforts were carried out by synthesizing various internal donors (ID) aiding the catalysts involved in producing them. This short review gives an overview of i) various generations of Ziegler–Natta (ZN) catalyst systems ii) general classes of ID that has been demonstrated by the researchers over the past decades iii) their influence on PP isotacticity and polymer properties. The coordination modes of different donor classes on supported ZN system and comparative study especially between phthalate and diether ID classes were also addressed here. This review also presents the studies carried out on phthalate catalyst structure analysis, detailed comparison study on phthalate and diether IDs in terms of PP isotacticity, regioselectivity, hydrogen response, and also their cross combination study and competitive behavior. Further a brief description on other structurally varied IDs like malonates, maleates, silyl diol esters, bifunctional donors, multi ether donors demonstrated for isotactic PP were also presented. Studies conducted on compatibility of incorporation of two different classes of IDs on a single supported ZN system for the fundamental understanding of the catalyst behavior; and also on how mixed donor approach enables in tuning the catalyst for polymer properties were also presented. This review also provides an opportunity to the young minds and the basic researchers from academic point of view by and large to create new polymeric materials with useful properties or modify the existing materials for new applications by incorporating new IDs for further improvisation of the stereo regularity in obtaining the polymers.

Graphic Abstract

Roles of Salicylate Donors in Enhancement of Productivity and Isotacticity of Ziegler-Natta Catalyzed Propylene Polymerization

Roles of internal salicylate donors (SID) in enhancing activity and stereoselectivity of Ziegler–Natta catalyzed propylene (PP) polymerization were examined using DFT calculations. Five salicylate donors were studied. The chelate mode is the preferred adsorption mode. The linear relationship (R² = 0.96) between calculated adsorption energies (E_ads) of five SIDs and the experimental PP activities was observed. Thus, the SID with the strongest adsorption energy will provide the highest activity in agreement with our previous studies. Compared with diisobutyl phthalate (DIBP), which is the industrial electron donor, SID has stronger E_ads. The insertion step, which involves the π-complex formation (∆E_π) and the insertion activation or intrinsic activation energy (E_a) for PP polymerization was also examined. The relation between ln(activity) and apparent activation energy (E_a(app)), which is ∆E_π + E_a for the primary(1,2)-re insertion with R² = 0.99, was observed. The salicylate donor also has a lower E_a(app) than that of DIBP. This explains the better catalytic performance of SID. Our results also demonstrated that the size and the type of hydrocarbon substituents play a key role in controlling stereoselectivity and activity. In addition, we found a good relationship between E_ads and both intrinsic (E_a) and apparent (E_a(app)) activation energies of five salicylate donors with R² of 0.90 and 0.97, respectively.

Fundamental aspects of heterogeneous Ziegler–Natta olefin polymerization catalysis: an experimental and computational overview

Using an internal/external donor containing MgCl₂-supported Ziegler–Natta catalyst is one of the important processes for polyolefin production.

Towards the design of a mixture of diether and succinate as an internal donor in a MgCl2-supported Ziegler–Natta catalyst

An investigation into competitions of mixtures of internal donors for interaction with MgCl₂was carried out and the subsequent influence on catalyst performance was studied.

Silyl diol ester as a new selectivity control agent in MgCl2-supported Ziegler-Natta systems for propylene polymerization: catalyst structure and polymer properties

In this study, bis(benzoyloxy)dimethylsilane (SDE) was developed as a non-phthalate selectivity control agent (internal donor (ID) and external donor (ED)) in MgCl₂-supported Ziegler–Natta (ZN) systems. The impact of SDE as an ID was investigated in regards to chemical composition, morphology, adsorption behavior (using FTIR spectroscopy, WAXD and SEM studies) and the propylene polymerization performance of the catalyst. The results of the adsorption behavior of SDE revealed that SDE, while able to stabilize the electronically unsaturated surfaces of MgCl₂ [(104) and (110)], has a rather high tendency to be absorbed on strongly acidic Mg ions at the corners of these crystals. The catalyst with optimum SDE content as an ID afforded a system with reasonable activity and isotacticity (even in the absence of ED) along with broader molecular weight distribution (PDI: 6.2–10) and greater flexural modulus than phthalate-based ZN systems. SDE was also used as a new ED in most widely used fourth- and fifth-generation catalyst systems. The results revealed that in fifth-generation catalyst systems, there was an increase in hydrogen response (>40%) along with an increase of activity without any change in the thermal properties of the final polymer in comparison to a conventional ED (alkoxy silane).

In this study, bis(benzoyloxy)dimethylsilane (SDE) was developed as a non-phthalate selectivity control agent (internal donor (ID) and external donor (ED)) in MgCl₂-supported Ziegler–Natta (ZN) systems for polypropylene polymerization.

Crystallization and melting behavior of i-PP: a perspective from Flory's thermodynamic equilibrium theory and DSC experiment

A novel conceptual framework that generates insightful new results about crystallization and melting behaviors of i-PP.

Application of the correct design of successive self-nucleation and annealing (SSA) to study the stereo-defects and its distribution of homo- and co-polypropylene

The SSA revealed the differences in molecular chain microstructure of different PP samples when the preliminary characterization found that the homo-polypropylene or co-polypropylene samples had the similar mechanical and thermal properties.