Ethylene copolymers are a well-known class of olefin copolymers from which various plastic products are produced. Such products include films, fibers, coatings and then no molded articles such as containers and consumer goods. The polymers used to prepare these articles are prepared from ethylene, optionally with one or more additional copolymerizable monomers. Low density polyethylene ("LDPE") as produced by free radical polymerization consists of highly branched polymers where the branches occur randomly throughout the polymer, that is on any number of formed segments or branches. This structure exhibits easy processing, that is polymers with it can be melt processed in high volumes at low energy input. Machinery for conducting this melt processing, for example extruders and film dies of various configurations, was designed into product finishing manufacturing processes with optimal design features based on the processing characteristics of the LDPE.
However, with the advent of effective coordination catalysis of ethylene copolymers, the degree of branching was significantly decreased, both for the now traditional Ziegler-Natta ethylene copolymers and the newer metallocene catalyzed ethylene copolymers. Both, particularly the metallocene copolymers, are essentially linear polymers, which are more difficult to melt process when the molecular weight distribution (PDI=M.sub.w /M.sub.n, where M.sub.w is weight-average molecular weight and M.sub.n is number-average molecular weight) is narrower than about 3.5. Thus broad PDI copolymers are more easily processed but can lack desirable solid state attributes otherwise available from the metallocene copolymers. Thus it has become desirable to develop effective and efficient methods of improving the melt processing of olefin copolymers while retaining desirable melt properties and end use characteristics.
The introduction of long chain branches into substantially linear olefin copolymers has been observed to improve processing characteristics of the polymers. Such has been done using metallocene-catalyzed polymers where significant numbers of olefinically unsaturated chain ends are produced during the polymerization reaction. See, e.g., U.S. Pat. No. 5,324,800. The olefinically unsaturated polymer chains can become "macromonomers" or "macromers" and, apparently, can be re-inserted with other copolymerizable monomers to form the branched copolymers. International publication WO 94/07930 addresses advantages of including long chain branches in polyethylene from incorporating vinyl-terminated macromers into polyethylene chains where the macromers have critical molecular weights greater than 3,800, or, in other words contain 250 or more carbon atoms. This document describes a large class of both monocyclopentadienyl and biscyclopentadienyl metallocenes as suitable in accordance with the invention when activated by either alumoxanes or ionizing compounds providing stabilizing, noncoordinating anions.
U.S. Pat. Nos. 5,272,236 and 5,278,272 describe "substantially linear" ethylene polymers which are said to have up to about 3 long chain branches per 1000 carbon atoms. These polymers are described as being prepared with monocyclopentadienyl transition metal olefin polymerization catalysts, such as those described in U.S. Pat. No. 5,026,798. The copolymer is said to be useful for a variety of fabricated articles and as a component in blends with other polymers. EP-A-0 659 773 A1 describes a gas phase process using metallocene catalysts said to be suitable for producing polyethylene with up to 3 long chain branches per 1000 carbon atoms in the main chain, the branches having greater than 18 carbon atoms.
Reduced melt viscosity polymers are addressed in U.S. Pat. Nos. 5,206,303 and 5,294,678. "Brush" polymer architecture is described where the branched copolymers have side chains that are of molecular weights that inhibit entanglement of the backbone chain. These branch weight-average molecular weights are described to be from 0.02-2.0 M.sub.e.sup.B, where M.sub.e.sup.B is the entanglement molecular weight of the side branches. Though the polymers illustrated are isobutylene-styrene copolymers, calculated entanglement molecular weights for ethylene polymers and ethylene-propylene copolymers of 1,250 and 1,660 are provided. Comb-like polymers of ethylene and longer alpha-olefins, having from 10 to 100 carbon atoms, are described in U.S. Pat. No. 5,475,075. The polymers are prepared by copolymerizing ethylene and the longer alpha-olefins which form the side branches. Improvements in end-use properties, such as for films and adhesive compositions are taught.
A limitation with the polyethylene compositions of the prior art is that though the processability, ease of melt processing or increase in shear-thinning properties, can be improved with the introduction of branching in the polymers, the molecular weight distribution as measured by the polydispersity index (PDI) tends to increase with increased branching even though the melt strength remains well below that exhibited by traditional LDPE. Typically the improved processing was achieved by blending different molecular weight polyethylene copolymer components or introducing various levels of branching into polyethylene copolymers. Accordingly, it has been generally thought that the advantages of the narrow PDI made possible by metallocene catalysis needed to be sacrificed, at least in part, if improved melt strength polyethylene copolymer compositions were sought.