During in situ production of alkylaluminoxanes, water molecules present on the
surface of pristine clay react with alkylaluminum compounds to produce MAO
oligomers on the clay surface. The modified clay can then be used directly as a
polymerization cocatalyst, or impregnated with a catalyst solution prior to polymerization. In this case, a high temperature thermal treatment to remove surfacebonded water molecules is not required.
Novokshonova et al. [109] reported a procedure in which the Al:H 2 O ratio was
lower than unity and alkylaluminum was added drop-wise to the clay solution until
the evolution of volatiles (e.g., CH 4 in the case of TMA) stopped. Alternatively, in a
second procedure, they used a Al:H 2 O ratio equal to 1 and the alkylaluminum was
added in a single step to the clay suspension. It was observed that the second
procedure resulted in higher polymerization activities and no extra MAO was
required during the polymerization. The authors suggested that higher degrees of
alkylaluminum hydrolysis were obtained in the first procedure and that fewer alkyl
groups remained available for alkylation reactions of the metallocene. On the other
hand, higher Al:H 2 O ratios resulted in partial hydrolysis of the alkylaluminum
molecules and, consequently, more alkylaluminum molecules were available for
metallocene alkylation. It was also found that the MAO formed on the clay surface
had similar structure to that of commercial MAO.
Despite the fact that the presence of organic modifications on the clay surfaces
enhances the dispersion of the clay nanolayers in nonpolar polymer matrices, the
nanolayers tend to degrade rapidly under the high temperature required for polymer
extrusion, leading to clay agglomeration and poor nanocomposite mechanical
properties [110, 111]. Scott et al. [86] proposed a technique to overcome this
problem by avoiding the use of clay organic modifiers. By comparing different
clay treatments for catalyst support and in situ olefin polymerization, they showed
that MMT treatment with mineral acid extensively disturbs its layered structure and
increases its Lewis acidity. They proposed that the nickel complex can be activated
for ethylene polymerization by binding a Lewis acid such as B(C 6 F 5 ) 3 at the
carbonyl group on the ligand backbone, generating a Zwitterionic adduct
(Fig. 14). This catalyst was particularly effective at promoting polymerization
solely on the clay surface, leading to effective clay dispersion in the polymer
matrix, while not needing any cocatalyst or scavenger.
Unfortunately, this procedure was reported to be inadequate for metallocenes
such as Cp 2 ZrMe 2 . The authors speculated that the low activity of the claysupported metallocene was due to severe decomposition of the catalyst in contact
with the strong Brønsted acidic surface of the clay. Scott et al. [86] also showed that
the clay dispersion in the polyolefin matrix is stable during annealing at 170
C for
30 min and related this behavior to the high molecular weight and high viscosity at
the test temperature (170
C).
In order to reduce the deactivating interaction between the coordination catalyst
and the clay surface, Huang and coworkers [112] proposed an indirect supporting
method in which a common support, such as MgCl 2 or SiO 2 , is deposited onto the
clay surface to increase the hydroxyl population on the clay surface where the loading
of active catalyst occurs. It is well known that MgCl 2 dissolves in alcohols to form
Polyolefin/Layered Silicate Nanocomposites Prepared by In Situ Polymerization
329
surface of pristine clay react with alkylaluminum compounds to produce MAO
oligomers on the clay surface. The modified clay can then be used directly as a
polymerization cocatalyst, or impregnated with a catalyst solution prior to polymerization. In this case, a high temperature thermal treatment to remove surfacebonded water molecules is not required.
Novokshonova et al. [109] reported a procedure in which the Al:H 2 O ratio was
lower than unity and alkylaluminum was added drop-wise to the clay solution until
the evolution of volatiles (e.g., CH 4 in the case of TMA) stopped. Alternatively, in a
second procedure, they used a Al:H 2 O ratio equal to 1 and the alkylaluminum was
added in a single step to the clay suspension. It was observed that the second
procedure resulted in higher polymerization activities and no extra MAO was
required during the polymerization. The authors suggested that higher degrees of
alkylaluminum hydrolysis were obtained in the first procedure and that fewer alkyl
groups remained available for alkylation reactions of the metallocene. On the other
hand, higher Al:H 2 O ratios resulted in partial hydrolysis of the alkylaluminum
molecules and, consequently, more alkylaluminum molecules were available for
metallocene alkylation. It was also found that the MAO formed on the clay surface
had similar structure to that of commercial MAO.
Despite the fact that the presence of organic modifications on the clay surfaces
enhances the dispersion of the clay nanolayers in nonpolar polymer matrices, the
nanolayers tend to degrade rapidly under the high temperature required for polymer
extrusion, leading to clay agglomeration and poor nanocomposite mechanical
properties [110, 111]. Scott et al. [86] proposed a technique to overcome this
problem by avoiding the use of clay organic modifiers. By comparing different
clay treatments for catalyst support and in situ olefin polymerization, they showed
that MMT treatment with mineral acid extensively disturbs its layered structure and
increases its Lewis acidity. They proposed that the nickel complex can be activated
for ethylene polymerization by binding a Lewis acid such as B(C 6 F 5 ) 3 at the
carbonyl group on the ligand backbone, generating a Zwitterionic adduct
(Fig. 14). This catalyst was particularly effective at promoting polymerization
solely on the clay surface, leading to effective clay dispersion in the polymer
matrix, while not needing any cocatalyst or scavenger.
Unfortunately, this procedure was reported to be inadequate for metallocenes
such as Cp 2 ZrMe 2 . The authors speculated that the low activity of the claysupported metallocene was due to severe decomposition of the catalyst in contact
with the strong Brønsted acidic surface of the clay. Scott et al. [86] also showed that
the clay dispersion in the polyolefin matrix is stable during annealing at 170
C for
30 min and related this behavior to the high molecular weight and high viscosity at
the test temperature (170
C).
In order to reduce the deactivating interaction between the coordination catalyst
and the clay surface, Huang and coworkers [112] proposed an indirect supporting
method in which a common support, such as MgCl 2 or SiO 2 , is deposited onto the
clay surface to increase the hydroxyl population on the clay surface where the loading
of active catalyst occurs. It is well known that MgCl 2 dissolves in alcohols to form
Polyolefin/Layered Silicate Nanocomposites Prepared by In Situ Polymerization
329