Low pressure or linear polyethylene is produced commercially using either Ziegler-Natta or supported chromium catalysts.  These catalysts have high activities, and produce a variety of homopolymers and copolymers of ethylene and alpha olefins.  When making copolymers, these catalysts typically produce resins of moderately broad to very broad molecular weight distribution. 
Ziegler-Natta and supported chromium catalysts produce copolymers of ethylene and alpha olefins of non-uniform branching distribution.  The alpha olefins are preferentially incorporated into the lower molecular weight portions of the copolymer.  This non-uniform incorporation affects polymer properties.  At a given polymer density, higher comonomer percent incorporation is required and a higher polymer melting point is seen.  For example, ethylene/1-hexene copolymers of 1.0 I.sub.2 and 0.918 gm/cc density produced by a typical Ziegler-Natta catalyst will contain 3.0 to 3.5 mole percent 1-hexene and have melting points of 126.degree.  to 127.degree.  C.
Recently, a new type of olefin polymerization catalyst has been described.  These catalysts are metallocene derivatives of transition metals, typically group IV transition metals such as zirconium, of the empirical formula CP.sub.m MA.sub.n B.sub.p.  These compounds are activated with methylaluminoxane (MAO) and produce olefin polymers and copolymers, such as ethylene and propylene homopolymers, and ethylene/butene and ethylene/hexene copolymers.  These are described in Kaminsky et al, U.S. Pat.  No.  4,542,199 and Sinn et al, U.S. Pat.  No.  4,404,344; the entire contents of both are incorporated herein by reference. 
Compared to earlier Ziegler-Natta catalysts, zirconocene/MAO catalysts produce polyethylene resins of relatively narrow molecular weight distribution and a highly homogeneous branching distribution.  Ethylene/1-hexene copolymers of 1.0 I.sub.2 and 0.918 gm/cc density produced by these catalysts usually contain 2.5 mole percent 1-hexene and have melting points of 114.degree.  to 115.degree.  C. These resins can be used to make films of significantly higher impact strength and better clarity than those of resins prepared with standard Ziegler-Natta catalysts. 
A new series of reactions have been described in which dialkylzirconocenes (Cp.sub.2 ZrRR' where R and R' are straight chain hydrocarbon groups) are activated without aluminoxane to produce a catalytically active transition metal cation.  Jordan et al, J. Amer Chem Soc.  1987, 109, 4111 has reacted Cp.sub.2 ZrMe.sub.2 with (Cp.sub.2 Fe).sub.+ B(C.sub.6 H.sub.5).sub.4 - in CH.sub.3 CN to produce Cp.sub.2 ZrMe(CH.sub.3 CN)+B(C.sub.6 H.sub.5).sub.4 -.  This ionic complex has rather poor activity for olefin polymerization due to the coordinated solvent molecule. 
Common anions, such as B(C.sub.6 H.sub.5).sub.4, react with the zirconocene cation in the absence of a coordinating solvent.  These reactions produce products that are not effective olefin polymerization catalysts. 
Stable, solvent-free, zirconocene cations have been produced by Chien et al, J. Amer.  Chem.  Soc.  1991, 113, 8570.  Reacting Cp.sub.2 ZrMe.sub.2 with Ph.sub.3 C.sup.+ B(C.sub.6 F.sub.5).sub.4.sup.- in a non-coordinating solvent produces Cp.sub.2 ZrMe.sup.+ B(C.sub.6 F.sub.5).sub.4.  Likewise, Marks et al, J. Amer.  Chem.  Soc.  1991, 113, 3623, react Cp*.sub.2 ThMe.sub.2 with B(C.sub.6 F.sub.5).sub.3 in a non-coordinating solvent to produce CP.sub.2 ThM.sup.+ MeB(C.sub.6 F.sub.5).sub.3.  These ionic complexes are highly active olefin polymerization catalysts.