1. Field of Invention
The present invention relates to branched crystalline polymers, preferably polypropylene (BCPP) compositions and supercritical polymerization processes for the preparation of such compositions.
2. Description of Related Art
Various processes have been proposed for making polypropylene compositions. Such different processes will typically have different variables and parameters, including different monomer compositions, solvents, additives, reaction conditions, catalyst systems, etc. The properties and characteristics of the final product have a great deal to do with the process variables and parameters that are selected, and it has been recognized that small modifications in such variables and parameters can create significant differences in not only the final product, e.g., polymer properties, but also in the effectiveness of the overall process, e.g., catalyst productivity, presence or absence of gel.
An ongoing need exists for processes that provide polypropylene with good processability and high melt strength, which is desirable for applications such as thermoforming, foaming, and blow molding. Poor melt strength of polypropylenes shows up as excess sag in sheet extrusion, rapid thinning of walls in parts thermoformed in the melt phase, low draw-down ratios in extrusion coating, poor bubble formation in extrusion foam materials, and relative weakness in large-part blow molding. Thus, it would be highly desirable to produce a polypropylene having enhanced melt strength as well as commercially valuable processability.
Furthermore, a need exists for a relatively straightforward method of preparing polypropylene having desirable properties, particularly a method for making a branched polymer that does not require the use of a cross-linking agent, e.g., post-polymerization treatments, or the use of comonomers that have been found to result in undesirable gel formation, such as certain types of diene comonomers. Finally, there is a need for a process demonstrating high catalyst productivity in forming the polymers described herein.
Additional references of interest include: U.S. Pat. No. 6,084,041, U.S. Pat. No. 5,969,062, U.S. Pat. No. 5,408,017, WO 93/11171, U.S. Pat. No. 6,355,741, WO 92/14766, U.S. Pat. No. 5,326,835, German Application DE 4,130,299 (equivalent to CA 2,118,711), Olefin Polymerization Using Highly Congested ansa-Metallocenes under High Pressure: Formation of Superhigh Molecular Weight Polyolefins, Suzuki, et al., Macromolecules, 2000, 33, 754-759, EP 1 123 226, WO 00 12572, WO 00 37514, EP 1 195 391, U.S. Pat. No. 6,355,741, and Ethylene Bis(Indenyl)Zirconocenes . . . , Schaverien, C. J. et al., Organometallics, ACS, Columbus Ohio, vol 20, no. 16, August 2001, pg. 3436-3452, WO 96/34023, WO 97/11098, U.S. Pat. No. 5,084,534, U.S. Pat. No. 2,852,501, WO 93/05082, EP 129 368 B1, WO 97/45434, JP 96-208535 199660807, U.S. Pat. No. 5,096,867, WO 96/12744, U.S. Pat. No. 5,408,017, U.S. Pat. No. 5,084,534, U.S. Pat. No. 6,225,432, WO 02/090399, EP 1 195 391, WO 02/50145, US 2002 013440, WO 01/46273, JP 10110003, and EP 1 008 607; JP 101 1003; and JP 3421202.
Another item of interest is an abstract obtained from the Borealis website that stated:                Barbo Loefgren, E. Kokko, L. Huhtanen, M Lahelin, Petri Lehmus, Udo Stehling “Metallocene-PP produced under supercritical conditions.” 1st Bluesky Conference on Catalytic Olefin Polymerization, 17.-20.6.2002, Sorrrento, Italy., ( ), 2002. “mPP produced in bulk conditions (100% propylene), especially at elevated temperature and under supercritical conditions, shows rheological behaviour indicative for small amounts of LCB in the polymer. This is a feature that can be utilized to produce mPP with enhanced melt strength under industrially meaningful conditions.”        