The Catalyst Review June 2019 - 8

SPECIAL FEATURE

A Titanium Organometallic Complex Coordinated on MgCl2,
Improving Polypropylene Catalyst Performance:
A Replacement for the TiCl4/Internal Modifier?
By Gregory G. Arzoumanidis, PhD
Significant research effort has been devoted for the replacement of the original butyl phthalates internal modifiers (IM) of MgCl2
supported polypropylene catalysts, primarily due to safety concerns. A new approach has now been developed, by which the IM may
be replaced in part by a Ti or Zr organometallic complex, imparting enhanced catalytic performance.
Internal Modifiers and Their Role
Ninety five percent of the world's production of polypropylene (PP), and ninety percent of polyethylene (PE) is presently carried
out by using MgCl2/TiCl4 supported catalysts, activated by triethylaluminum (TEA). For PP, the catalytic system incorporates internal
donors (ID, electron-donating organic compounds) adsorbed on the surface of MgCl2, and external donors (ED, added to the catalyst
mix), necessary for stereoregulation. A key role is played by the ID. It blocks nonstereospecific active sites, competing with TiCl4
molecules for adsorption sites on MgCl2 faces. IDs affect the molecular weight (MW) and the molecular weight distribution (MWD),
two fundamental parameters controlling polymer properties (Groppo et al. 2013). The most common IDs are phthalate and succinate
diesters, as well as diethers (Albizzati et al. 1995). The latter is coordinating strongly on the MgCl2 surfaces and it cannot be removed
by the TEA, unlike the former two IDs that may be partially removed by it. In this case the role of the ED is to replenish the removed
ID during the polymerization process and maintain the stereoregularity of the catalytic system. It is also the reason why no ED is
necessary with diether MgCl2 catalysts. Common EDs are various dimethoxysilanes (DMS). One of their advantages is the stability of
their complex with TEA, which does not deteriorate to yield unwanted byproducts.
The most common commercially used system is
MgCl2/TiCl4/Dibutylphthalate (DBP)/DMS. In the
late 2000s safety issues were raised for the use
of DBP. A sudden surge of industrial research activity
was initiated to find good substitute IDs. A second
incentive for this activity was for industrial concerns
to establish proprietary position. Table 1 summarizes
some of the newer IDs.

Table 1. Internal donors claimed by the industry.

The majority of well performing IDs are bidendate,
attached to the well-established (100) and (110)
Source: Author 2019
surfaces of MgCl2, either in bridge or chelate bonding
mode. There is also the zip coordination mode, when the two oxygen atoms of the ID are coordinated to two Mg atoms on different
adjacent MgCl2 layers.
The adsorption mode of various IDs and the TiCl4 on the (100) and (110)
Figure 1.
lateral cuts of MgCl2, along with the implication of their coordination mode
to catalyst performance has been studied continuously mainly by various
academic groups since the 1990s, an activity still going on to-date (Bukatov
et al. 2019; Stukalov et al. 2009). In the (110) cut, the Mg and Cl atoms are
all on the same plane (level). Here the Mg atom is tetracordinated and
therefore has two coordination sites open, which can accept both chelate
and bridge ID bonding modes. On the 110 cut, TiCl4 coordinates only as
a monomer. The (110) cut is more acidic than the (100). In the (100) cut,
however, only the Cl atoms are on the surface, and the Mg atoms are at a
layer below. The Mg now is pentacoordinated, thus has one coordination
Source: Author 2019
site open, which allows bridge bonding only. On the 100 cut, TiCl4 may
coordinate as monomer, dimer or even oligomer. The TiCl4 dimer/oligomer after reduction with TEA resembles and mimics the purple
unsupported (Solvay) TiCl3 catalyst.

The Catalyst Review

June 2019

The Catalyst Review June 2019

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