2. The low IR detectability of any possible methyl end groups in the initial growing
polymer chains [17, 19–21] on the active sites also sheds great doubts on those
proposed active sites relating to metallacyclic species (4a, 9a, 12a) [16–18, 22, 23].
3. The models (2a, 3a, 6a) [24] involving a proposed Cossee–Arlman chain
propagation [25], with either Cr–C or Cr–H as active sites similar to conventional Ziegler–Natta catalysts, still hold the most popularity [2, 11], although the
origin of the first hydride scrambling is still obscure (e.g., for 2a). These models
are mainly speculated from the chain configuration of Phillips polyethylene
chains featuring one vinyl and one methyl group on each chain end. The vinyl
chain end is thought to be derived from chain transfer through β-hydride
elimination during a Cossee–Arlman chain propagation.
In the polyolefin industry, there exists a strong driving force for development
of new catalysts with better performance and improvements in the structures and
properties of PE products through successive catalyst innovations of the traditional Phillips catalyst [2–4, 11]. During the past 60 years, several modified
Phillips catalysts have been successfully developed and commercialized through
surface modification of the silica support and catalyst with Ti, F, Al, or B
compounds, more or less based on the progress in the academic field, although
innovation regarding this catalyst is very limited. Another important commercial
silica-supported Cr-based HDPE catalyst is Union Carbide’s silyl chromate S-2
catalyst, which is solely applied in the gas phase UNIPOL polymerization processes [26]. This catalyst is usually prepared by chemisorption of bis
(triphenylsilyl) chromate (BC) on partially dehydrated silica gel at around
600
C. Due to its similar structure and performance compared with the Phillips
catalyst, in our opinion it could be considered as a heterogeneous model of the
Phillips catalyst. Due to the presence of an electron-donating triphenylsilyl
ligand, a much longer induction period exists without using any organometallic
cocatalyst for ethylene polymerization. This catalyst combined with Al-alkyl
cocatalyst usually produces polyethylene with broader MWD at both ends of
the high and low molecular weight fractions than the Phillips catalyst without
using cocatalyst [26]. Almost no improvement of this silica-supported silyl
chromate S-2 catalyst has been reported, apart from a modified preparation
procedure through transformation from Phillips catalyst by addition of
triphenylsilanol (TPS) to avoid the use of highly toxic and expensive BC compound [27, 28]. Another Union Carbide Cr-based polymerization catalyst, formed
upon treating partially dehydrated silica with chromocene (Cp 2 Cr) and named S-9
catalyst, is not used industrially at present [29]. It is a supported metallocene
catalyst featuring very poor ability of α-olefin incorporation in copolymerization
with ethylene and produces polyethylenes with narrow MWD. It is very clear that
further catalyst innovations through modifications of the traditional Cr-based
industrial catalysts are still highly demanded [30].
During the last decade, increasing research efforts have been performed on
Phillips catalysts through various approaches including spectroscopic methods,
polymerization kinetics, heterogeneous model catalysts, homogeneous model
140
R. Cheng et al.
polymer chains [17, 19–21] on the active sites also sheds great doubts on those
proposed active sites relating to metallacyclic species (4a, 9a, 12a) [16–18, 22, 23].
3. The models (2a, 3a, 6a) [24] involving a proposed Cossee–Arlman chain
propagation [25], with either Cr–C or Cr–H as active sites similar to conventional Ziegler–Natta catalysts, still hold the most popularity [2, 11], although the
origin of the first hydride scrambling is still obscure (e.g., for 2a). These models
are mainly speculated from the chain configuration of Phillips polyethylene
chains featuring one vinyl and one methyl group on each chain end. The vinyl
chain end is thought to be derived from chain transfer through β-hydride
elimination during a Cossee–Arlman chain propagation.
In the polyolefin industry, there exists a strong driving force for development
of new catalysts with better performance and improvements in the structures and
properties of PE products through successive catalyst innovations of the traditional Phillips catalyst [2–4, 11]. During the past 60 years, several modified
Phillips catalysts have been successfully developed and commercialized through
surface modification of the silica support and catalyst with Ti, F, Al, or B
compounds, more or less based on the progress in the academic field, although
innovation regarding this catalyst is very limited. Another important commercial
silica-supported Cr-based HDPE catalyst is Union Carbide’s silyl chromate S-2
catalyst, which is solely applied in the gas phase UNIPOL polymerization processes [26]. This catalyst is usually prepared by chemisorption of bis
(triphenylsilyl) chromate (BC) on partially dehydrated silica gel at around
600
C. Due to its similar structure and performance compared with the Phillips
catalyst, in our opinion it could be considered as a heterogeneous model of the
Phillips catalyst. Due to the presence of an electron-donating triphenylsilyl
ligand, a much longer induction period exists without using any organometallic
cocatalyst for ethylene polymerization. This catalyst combined with Al-alkyl
cocatalyst usually produces polyethylene with broader MWD at both ends of
the high and low molecular weight fractions than the Phillips catalyst without
using cocatalyst [26]. Almost no improvement of this silica-supported silyl
chromate S-2 catalyst has been reported, apart from a modified preparation
procedure through transformation from Phillips catalyst by addition of
triphenylsilanol (TPS) to avoid the use of highly toxic and expensive BC compound [27, 28]. Another Union Carbide Cr-based polymerization catalyst, formed
upon treating partially dehydrated silica with chromocene (Cp 2 Cr) and named S-9
catalyst, is not used industrially at present [29]. It is a supported metallocene
catalyst featuring very poor ability of α-olefin incorporation in copolymerization
with ethylene and produces polyethylenes with narrow MWD. It is very clear that
further catalyst innovations through modifications of the traditional Cr-based
industrial catalysts are still highly demanded [30].
During the last decade, increasing research efforts have been performed on
Phillips catalysts through various approaches including spectroscopic methods,
polymerization kinetics, heterogeneous model catalysts, homogeneous model
140
R. Cheng et al.