system, it is possible to use a single reactor with multi-site catalysts, produced by
blending together single-site catalyst on the same support. For instance, Kurek and
Mu ¨lhaupt reported on the formation of polyethylene with tailored bi- and trimodal
ultrabroad molar mass distributions [191]. Ethylene was polymerized on silicasupported post-metallocenes and half sandwich multi-site catalysts consisting of
2,6-bis-[1-(2,6-dimethylphenylimino) ethyl]pyridine chromium(III) chloride
(Cr-1), 2,6-bis-[1-(2,6-dimethylphenylimino)ethyl]pyridine iron(II) chloride
(Fe-2), and η
5 -[3,4,5-trimethyl-1-(8-quinolyl)-2-trimethyl-silylcyclopentadienyl]
chromium(III) chloride (Cr-3). Whereas UHMWPE is formed on Cr-3, Cr-1
produces wax-like HDPE with molar mass around 1,000 g/mol and Fe-2 produces
medium range molar mass around 10
5 g/mol. In contrast to early catalyst
generations, the Cr-3 catalyst is very robust and the UHMWPE molar mass
produced by Cr-3 is independent of cocatalyst and molar ratio of Cr-1/Fe-2/Cr-3.
As shown in Fig. 13, the mixing ratio of Cr-1/Fe-2/Cr-3 affords precise control of
the reactor blend composition and molar mass distributions. At constant Cr-1/Fe2 molar ratio, the UHMWPE content is exclusively controlled by increasing the
Cr-3 content without affecting the average molar mass of UHMWPE. The resulting
intimate, molecular blend of HDPE with in situ formed nanometer-scaled
UHMWPE enables injection molding and blow molding of reactor blends
containing more than 10 wt% UHMWPE [192]. According to the TEM image of
the reactor blend, displayed in Fig. 14, shish–kebab UHMWPE nanofibers are
formed during melt processing. Most likely, the UHMPE slowly crystallizes and
forms UHMWPE extended chain crystals, which are highly effective nucleating
sites for the lower molecular weight polyethylene with much higher crystallization
speed. In the absence of micrometer-scaled UHMWPE, no large UHMWPE
particles are found in the HDPE matrix. In comparison to the conventional
talcum-filled polyethylene composites and the above mentioned HDPE
nanocomposites containing boehmite and graphene, self-reinforced HDPE/
UHMWPE reactor blends afford superior strength and stiffness but are composed
exclusively of polyethylene.
In HDPE/UHMWPE reactor blend technology and processing of UHMWPE, the
entanglement of polymer chains plays an important role. It is well known that the
entanglement of (ultra)high molecular weight polyolefins accounts for high melt
viscosity reflected by very low melt flow, slow chain mobility, and slow crystallization rates. As a result, UHMW polyolefins are difficult to process by conventional
melt processing like extrusion and injection molding. Typically, UHMWPE is
processed by compression molding, sintering, and solid state extrusion to produce
sheets, plates, and rods, which require subsequent machining. Most of these
UHMWPE materials still contain defects and grain boundaries resulting from
incomplete melting and bonding of the micron-sized UHMWPE particles. To
enable conventional processing and improve the properties of UHMWPE it is
imperative to achieve disentanglement of polyethylene chains, enabling extended
chain formation during crystallization. Special processing technology such as gel
spinning was developed to produce ultrastrong disentangled extended-chain polyethylene fibers, exhibiting a tensile modulus equivalent to the theoretical
value [193].
296
M. Stu ¨rzel et al.
blending together single-site catalyst on the same support. For instance, Kurek and
Mu ¨lhaupt reported on the formation of polyethylene with tailored bi- and trimodal
ultrabroad molar mass distributions [191]. Ethylene was polymerized on silicasupported post-metallocenes and half sandwich multi-site catalysts consisting of
2,6-bis-[1-(2,6-dimethylphenylimino) ethyl]pyridine chromium(III) chloride
(Cr-1), 2,6-bis-[1-(2,6-dimethylphenylimino)ethyl]pyridine iron(II) chloride
(Fe-2), and η
5 -[3,4,5-trimethyl-1-(8-quinolyl)-2-trimethyl-silylcyclopentadienyl]
chromium(III) chloride (Cr-3). Whereas UHMWPE is formed on Cr-3, Cr-1
produces wax-like HDPE with molar mass around 1,000 g/mol and Fe-2 produces
medium range molar mass around 10
5 g/mol. In contrast to early catalyst
generations, the Cr-3 catalyst is very robust and the UHMWPE molar mass
produced by Cr-3 is independent of cocatalyst and molar ratio of Cr-1/Fe-2/Cr-3.
As shown in Fig. 13, the mixing ratio of Cr-1/Fe-2/Cr-3 affords precise control of
the reactor blend composition and molar mass distributions. At constant Cr-1/Fe2 molar ratio, the UHMWPE content is exclusively controlled by increasing the
Cr-3 content without affecting the average molar mass of UHMWPE. The resulting
intimate, molecular blend of HDPE with in situ formed nanometer-scaled
UHMWPE enables injection molding and blow molding of reactor blends
containing more than 10 wt% UHMWPE [192]. According to the TEM image of
the reactor blend, displayed in Fig. 14, shish–kebab UHMWPE nanofibers are
formed during melt processing. Most likely, the UHMPE slowly crystallizes and
forms UHMWPE extended chain crystals, which are highly effective nucleating
sites for the lower molecular weight polyethylene with much higher crystallization
speed. In the absence of micrometer-scaled UHMWPE, no large UHMWPE
particles are found in the HDPE matrix. In comparison to the conventional
talcum-filled polyethylene composites and the above mentioned HDPE
nanocomposites containing boehmite and graphene, self-reinforced HDPE/
UHMWPE reactor blends afford superior strength and stiffness but are composed
exclusively of polyethylene.
In HDPE/UHMWPE reactor blend technology and processing of UHMWPE, the
entanglement of polymer chains plays an important role. It is well known that the
entanglement of (ultra)high molecular weight polyolefins accounts for high melt
viscosity reflected by very low melt flow, slow chain mobility, and slow crystallization rates. As a result, UHMW polyolefins are difficult to process by conventional
melt processing like extrusion and injection molding. Typically, UHMWPE is
processed by compression molding, sintering, and solid state extrusion to produce
sheets, plates, and rods, which require subsequent machining. Most of these
UHMWPE materials still contain defects and grain boundaries resulting from
incomplete melting and bonding of the micron-sized UHMWPE particles. To
enable conventional processing and improve the properties of UHMWPE it is
imperative to achieve disentanglement of polyethylene chains, enabling extended
chain formation during crystallization. Special processing technology such as gel
spinning was developed to produce ultrastrong disentangled extended-chain polyethylene fibers, exhibiting a tensile modulus equivalent to the theoretical
value [193].
296
M. Stu ¨rzel et al.