ethylene polymerization. Recently, a novel hybrid catalyst presented the merits of
two important chromium-based catalysts, namely inorganic Phillips and organic
S-2 catalysts. It was successfully prepared, and named Cat-C, as shown in
Scheme 20 [165]. This method utilized the surface residual hydroxyl groups on a
Phillips catalyst and anchored the BC complex under mild conditions. Thus, the
surface residual hydroxyl group population could greatly influence the degree of
supported BC, as well as the polymerization activity and the structure of the
polymers. For a series of catalysts with total 0.5 wt% Cr loading prepared with
increasing Cr BC loading of 20% (Cat-C1), 50% (Cat-C2), and 80% (Cat-C3), the
thermogravimetric peaks from the catalysts after the reaction of HMDS and the
residual hydroxyl groups became more and more apparent, suggesting that the
residual surface hydroxyl group population increases with the increasing relative
addition amount of Cr BC . The ethylene polymerization kinetic curves over the
Cat-C catalysts activated by Al-alkyl cocatalyst during polymerization were insensitive to the type of cocatalyst (TEA, TiBA, and MAO) and similar to the kinetic
type shown in Fig. 10a, implying the existence of two kinds of active sites. As can
be seen from Table 7, Cat-C2 catalyst with 50 wt% Cr BC relative loading showed
well-balanced properties of ethylene homopolymerization and ethylene/1-hexene
copolymerization in terms of activities and MW of the polymers. Its copolymers
had a higher average MW and broader MWD than those obtained from the Phillips
catalyst, as well as higher 1-hexene incorporation than those obtained from Phillips
and S-2 catalysts.
In order to investigate the SCB distribution of the ethylene/1-hexene copolymers
made by Cat-C, TREF combined with SSA was applied according to a method
established by us previously [166, 167]. The SCBs contents for each PE fraction
from TREF were qualitatively obtained from the lamella thickness measured by
SSA. The lamella thickness distribution of the fractions obtained by TREF for each
copolymer is shown in Fig. 29. Comparison of the lamella thickness distribution of
the copolymers in the highest temperature fraction (124
C fraction, corresponding
to the highest MW part of the copolymer) suggested that the lamella thickness of
copolymers obtained from Phillips and Cat-C2 catalysts were similar and slightly
thinner than those obtained from S-2 catalyst. This result indicated that the
corresponding relative SCB content of copolymers in the highest MW part obtained
from Phillips and Cat-C2 catalysts were slightly higher than that obtained from
S-2 catalyst. In the lowest temperature fraction (40
C fraction, corresponding to the
lowest MW part), the copolymers obtained from the Phillips catalyst showed much
thinner lamella thickness (corresponding to much higher relative SCB content) than
those obtained from S-2 and Cat-C2 catalysts. The copolymer obtained from
Cat-C2 catalyst showed thinner lamella than that obtained from S-2 catalyst.
Hence, it should have the thickest lamella thickness (corresponding to the least
relative SCB content) in the copolymers in the lowest MW part. Simultaneously,
considering the relative SCB contents in both the lowest and highest temperature
fractions of the copolymers, it was suggested that the SCB distribution of
copolymers obtained from Cat-C2 catalyst was the best: the copolymer had similar
relative SCB contents in the highest MW part to those obtained from the Phillips
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R. Cheng et al.
two important chromium-based catalysts, namely inorganic Phillips and organic
S-2 catalysts. It was successfully prepared, and named Cat-C, as shown in
Scheme 20 [165]. This method utilized the surface residual hydroxyl groups on a
Phillips catalyst and anchored the BC complex under mild conditions. Thus, the
surface residual hydroxyl group population could greatly influence the degree of
supported BC, as well as the polymerization activity and the structure of the
polymers. For a series of catalysts with total 0.5 wt% Cr loading prepared with
increasing Cr BC loading of 20% (Cat-C1), 50% (Cat-C2), and 80% (Cat-C3), the
thermogravimetric peaks from the catalysts after the reaction of HMDS and the
residual hydroxyl groups became more and more apparent, suggesting that the
residual surface hydroxyl group population increases with the increasing relative
addition amount of Cr BC . The ethylene polymerization kinetic curves over the
Cat-C catalysts activated by Al-alkyl cocatalyst during polymerization were insensitive to the type of cocatalyst (TEA, TiBA, and MAO) and similar to the kinetic
type shown in Fig. 10a, implying the existence of two kinds of active sites. As can
be seen from Table 7, Cat-C2 catalyst with 50 wt% Cr BC relative loading showed
well-balanced properties of ethylene homopolymerization and ethylene/1-hexene
copolymerization in terms of activities and MW of the polymers. Its copolymers
had a higher average MW and broader MWD than those obtained from the Phillips
catalyst, as well as higher 1-hexene incorporation than those obtained from Phillips
and S-2 catalysts.
In order to investigate the SCB distribution of the ethylene/1-hexene copolymers
made by Cat-C, TREF combined with SSA was applied according to a method
established by us previously [166, 167]. The SCBs contents for each PE fraction
from TREF were qualitatively obtained from the lamella thickness measured by
SSA. The lamella thickness distribution of the fractions obtained by TREF for each
copolymer is shown in Fig. 29. Comparison of the lamella thickness distribution of
the copolymers in the highest temperature fraction (124
C fraction, corresponding
to the highest MW part of the copolymer) suggested that the lamella thickness of
copolymers obtained from Phillips and Cat-C2 catalysts were similar and slightly
thinner than those obtained from S-2 catalyst. This result indicated that the
corresponding relative SCB content of copolymers in the highest MW part obtained
from Phillips and Cat-C2 catalysts were slightly higher than that obtained from
S-2 catalyst. In the lowest temperature fraction (40
C fraction, corresponding to the
lowest MW part), the copolymers obtained from the Phillips catalyst showed much
thinner lamella thickness (corresponding to much higher relative SCB content) than
those obtained from S-2 and Cat-C2 catalysts. The copolymer obtained from
Cat-C2 catalyst showed thinner lamella than that obtained from S-2 catalyst.
Hence, it should have the thickest lamella thickness (corresponding to the least
relative SCB content) in the copolymers in the lowest MW part. Simultaneously,
considering the relative SCB contents in both the lowest and highest temperature
fractions of the copolymers, it was suggested that the SCB distribution of
copolymers obtained from Cat-C2 catalyst was the best: the copolymer had similar
relative SCB contents in the highest MW part to those obtained from the Phillips
194
R. Cheng et al.