2.2 Polyethylenes from Iron Catalysts
The polyethylene products from (many) bis(imino)pyridyl iron dichloride/aluminum alkyl catalyst systems are bimodal and broadly distributed [35, 44]. This
results from at least two different chain termination (transfer) reactions (Scheme 1).
Next to the common β-hydrogen elimination of basically all polymerization
catalysts, alkyl exchange between aluminum and iron terminates chain growth by
a formal chain transfer reaction [27, 28]. The ease of such a reaction is easily
envisioned in the bimetallic complexes of (1) and (2). The polyethylene obtained by
bis(imino)pyridine iron catalysts after hydrolysis is thus a mixture of paraffins and
vinylic compounds. The ratio is dependent on the ratio of aluminum alkyls to
ethylene pressure. The higher the concentration of aluminum (or zinc [45]) alkyls,
the shorter is the chain length of the polyethylene and the lower the percentage of
olefinic chain ends. This is in a first approximation resulting from competition
between the chain transfer process and propagation and β-hydrogen elimination.
A reversible chain transfer is responsible for the Poisson distribution of the
paraffins [45]. In the case of aluminum alkyls as activation agents, the distribution
has an appreciably higher polydispersity index (PDI), probably because polymeryl
trialkyl alumina is less effective in reaching the necessary coordination to iron for
transmetallation and thus the transfer is not (completely) reversible. In addition, a
molecular weight over the solubility limit brings a diffusion barrier to the reaction
between the iron and the aluminum polymeryl. The solubility limit in toluene, a
frequently used solvent, lies in the range of 500–1,000 Da. It has been argued on the
basis of a resolution of GPC traces into Flory components [44] that several catalytic
species need to be present to account for this behavior [46]. Such an analysis has
typically been carried out for traditional MgCl 2 -supported Ziegler catalysts,
showing that several types of catalytic center are operative. The analysis in the
case of the BIP FeCl 2 precatalysts arrives at the conclusion that two groups of
catalytic reactions occur, one leading to low molecular weight products and one to
high molecular weight products. Each of the groups is subdivided into several
fractions, with a PDI of 2. The subfractions are thought to be generated from
several types of closely related catalytic species. The two groups of products are
accounted for by assuming that initially formed species decompose to further
catalytically active species that generate the high molecular mass fraction. This
indeed would account for the observed time dependence of the products formed
during the course of the polymerization experiment. Higher molecular weights are
predominantly formed at longer reaction times. This does not, however, contradict
the explanation by Gibson that the formation of high molecular weight products
results from the depleting concentration of chain transfer agents with reaction time
[47]. Further work will be necessary to complete the mechanistic picture and to
explain the kinetic behavior, and probably needs to encompass the precipitation of
products with or without active catalyst and, for example, the matter of dormant
species [48] (see chapter 2.3).
The concentration of the alkylating species certainly has a marked influence on
the product constitution [47]. A typical distribution is shown in Fig. 2. The high
Iron Catalyst in the Preparation of Polyolefin Composites
347
The polyethylene products from (many) bis(imino)pyridyl iron dichloride/aluminum alkyl catalyst systems are bimodal and broadly distributed [35, 44]. This
results from at least two different chain termination (transfer) reactions (Scheme 1).
Next to the common β-hydrogen elimination of basically all polymerization
catalysts, alkyl exchange between aluminum and iron terminates chain growth by
a formal chain transfer reaction [27, 28]. The ease of such a reaction is easily
envisioned in the bimetallic complexes of (1) and (2). The polyethylene obtained by
bis(imino)pyridine iron catalysts after hydrolysis is thus a mixture of paraffins and
vinylic compounds. The ratio is dependent on the ratio of aluminum alkyls to
ethylene pressure. The higher the concentration of aluminum (or zinc [45]) alkyls,
the shorter is the chain length of the polyethylene and the lower the percentage of
olefinic chain ends. This is in a first approximation resulting from competition
between the chain transfer process and propagation and β-hydrogen elimination.
A reversible chain transfer is responsible for the Poisson distribution of the
paraffins [45]. In the case of aluminum alkyls as activation agents, the distribution
has an appreciably higher polydispersity index (PDI), probably because polymeryl
trialkyl alumina is less effective in reaching the necessary coordination to iron for
transmetallation and thus the transfer is not (completely) reversible. In addition, a
molecular weight over the solubility limit brings a diffusion barrier to the reaction
between the iron and the aluminum polymeryl. The solubility limit in toluene, a
frequently used solvent, lies in the range of 500–1,000 Da. It has been argued on the
basis of a resolution of GPC traces into Flory components [44] that several catalytic
species need to be present to account for this behavior [46]. Such an analysis has
typically been carried out for traditional MgCl 2 -supported Ziegler catalysts,
showing that several types of catalytic center are operative. The analysis in the
case of the BIP FeCl 2 precatalysts arrives at the conclusion that two groups of
catalytic reactions occur, one leading to low molecular weight products and one to
high molecular weight products. Each of the groups is subdivided into several
fractions, with a PDI of 2. The subfractions are thought to be generated from
several types of closely related catalytic species. The two groups of products are
accounted for by assuming that initially formed species decompose to further
catalytically active species that generate the high molecular mass fraction. This
indeed would account for the observed time dependence of the products formed
during the course of the polymerization experiment. Higher molecular weights are
predominantly formed at longer reaction times. This does not, however, contradict
the explanation by Gibson that the formation of high molecular weight products
results from the depleting concentration of chain transfer agents with reaction time
[47]. Further work will be necessary to complete the mechanistic picture and to
explain the kinetic behavior, and probably needs to encompass the precipitation of
products with or without active catalyst and, for example, the matter of dormant
species [48] (see chapter 2.3).
The concentration of the alkylating species certainly has a marked influence on
the product constitution [47]. A typical distribution is shown in Fig. 2. The high
Iron Catalyst in the Preparation of Polyolefin Composites
347