Heterogeneous Ziegler-Natta and Phillips-type olefin polymerization catalysts have the monopoly of isotactic polypropylene production and a large share in the market of high density polyethylene, respectively. Their high industrial impact and the relatively mild conditions under which they work explain why both catalysts have been the subject of an intense research. The industrially adopted strategy to improve catalyst's formulation is still based on a trial-and-error procedure; however, a rational design of new and more efficient catalysts (which is the key to produce polyolefins having a specific architecture) necessarily implies to achieve a detailed understanding of the structure of the active sites at a molecular level. Herein, it is shown that spectroscopic methods have this potential, especially when several complementary techniques are adopted and coupled with theoretical calculations. This is valid for both Phillips-type and Ziegler-Natta catalysts, because most of the problems encountered in their characterization and understanding are common, although for decades they were not considered to be closely related. The main advantages and disadvantages of several spectroscopies in the investigation of both categories of catalysts are critically analyzed, by discussing many examples taken from the recent literature.

Operando-sensitive spectroscopic techniques were employed for investigating the changes in the molecular structure of the Cr sites in the Cr/SiO Phillips catalyst during ethylene polymerization. Practically, the most arduous barrier to be overcome was the separation of the chromates reduction carried out by ethylene from the subsequent polymerization. By carefully tuning the experimental parameters we succeeded in observing these two events separately. We found that the sites involved in ethylene polymerization are mainly divalent Cr ions in a 6-fold coordination, in interaction with the oxygenated byproduct (mostly methylformate, generated from the disproportionation of two formaldehyde molecules). Unreduced Cr species are also present during ethylene polymerization as well as reduced Cr species (either Cr or Cr) acting as spectators. Our results challenge the old vision of "naked" chromium species (i.e., low coordinated) as the active sites and attribute a fundamental role to external (and flexible) oxygenated ligands that resemble the ancillary ligands in homogeneous polymerization catalysis.

In this work, we summarize and critically compare some of the experimental results recently published on the Phillips catalyst, in the attempt to make the point on a few particularly debated questions that have recently animated the specialized literature; in particular, we discuss the structure of the active chromium sites and how ethylene polymerization initiates on them. The data collected in this article unequivocally demonstrate that the structural and electronic properties of the chromium sites strongly depend on the strain of the silica surface, which in turns is affected by both the activation treatment and the chromium loading. This explains, at least partially, the differences of results obtained in different research groups. Another fundamental message is the need of applying the largest possible set of characterization methods, including theoretical calculation on large and flexible models. Our final purpose (and hope) is to promote a positive and constructing discussion on this catalyst, as a premise to create a solid scientific base useful to both the young researchers approaching this field and the industrial researchers who daily work with it.

We show the unprecedented potential of commercially available TiO 2 materials reduced in H 2 (H 2reduced TiO 2 ) in the conversion of ethylene to high density polyethylene (HDPE) under mild conditions (room temperature, low pressure, absence of any activator), with the consequent formation of HDPE/TiO 2 composites, which have been characterized by electron microscopy. Combination of UV−vis and IR spectroscopies allows one to demonstrate that ethylene polymerization occurs on Ti 4−n defect sites, which behave as shallow-trap defects located in the band gap and, differently from the active sites in the widely used Ziegler−Natta catalysts, do not contain any alkyl (Ti−R) or hydride (Ti−H) ligands. These results represent a step forward the understanding of ethylene polymerization mechanism and open valuable perspectives for commercial TiO 2 materials as catalysts for polyethylene production under mild conditions.

Ethylene polymerization on a model Cr(II)/SiO(2) Phillips catalyst modified with gas phase SiH(4) leads to a waxy product containing a bimodal MW distribution of α-olefins (M(w) < 3000 g mol(-1)) and a highly branched polyethylene, LLDPE (M(w) ≈ 10(5) g mol(-1), T(m) = 123 °C), contrary to the unmodified catalyst which gives a linear and more dense PE, HDPE (M(w) = 86,000 g mol(-1) (PDI = 7), T(m) = 134 °C). Pressure and temperature resolved FT-IR spectroscopy under operando conditions (T = 130-230 K) allows us to detect α-olefins, and in particular 1-hexene and 1-butene (characteristic IR absorption bands at 3581-3574, 1638 and 1598 cm(-1)) as intermediate species before their incorporation in the polymer chains. The polymerization rate is estimated, using time resolved FT-IR spectroscopy, to be 7 times higher on the SiH(4)-modified Phillips catalyst with respect to the unmodified one.

No abstract

We present an X-and Q-band electron paramagnetic resonance (EPR) investigation of a 0.5 wt % Cr/SiO 2 Phillips catalyst, aimed at establishing the Cr oxidation and spin state under different ethylene polymerization conditions. Our data indicate the formation of different Cr(III) species upon reaction of Cr(VI)/SiO 2 with ethylene at 150 °C, which is correlated to the disappearance of Cr(V) sites originally present in the catalyst. These species are characterized by unusually large zero field splitting (ZFS). No EPR active species are formed when ethylene is reacted at room temperature with Cr(II)/SiO 2 , despite clear evidence of a consistent ethylene polymerization activity.

Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303156. Scheme 1. Schematic representation of the local structure of starting Cr sites in Cr II /SiO 2 (1) and possible structures for the hydrosilane-modified Cr sites (a, b, and c). Note that in (1) the active Cr sites are considered to be in + 2 oxidation state, as accepted by the majority of authors, although some works are in favor of + 3 oxidation state. [1]