Metallocene Catalysts for Broad Molecular Weight Distribution Polymers
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Solution Overview
Problem
Metallocene-based catalyst systems produce polyolefins with narrow molecular weight distribution, which can lead to flow instabilities and reduced production rates in polymer processing, and existing methods to broaden the distribution using chromium or Ziegler-type catalysts are hindered by hydrogen's narrowing effect.
Innovation Solution
A process using a catalyst composition of metallocene compounds and activators in the presence of hydrogen, producing ethylene polymers with a targeted molecular weight ratio of Mw/Mn from 3 to 6, characterized by specific properties such as melt index, reverse comonomer distribution, and low long chain branches, to achieve broader molecular weight distribution without hydrogen-induced narrowing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a metallocene-based catalyst system is used to produce polyolefins, then the polymerization process is efficient and controlled, but the resulting polymer has a narrow molecular weight distribution which causes flow instabilities and reduced production rates
Solution Approach 1:
The patent changes the chemical parameters of the catalyst system by combining metallocene catalysts with specific organometallic compounds (such as borates, aluminoxanes, or carboranes) to modify the polymerization kinetics. This parameter change enables the system to produce polymers with broader molecular weight distributions (Mw/Mn ratio of 3-6) while maintaining efficient polymerization control, thereby resolving the contradiction between process control and production rate
2Manufacturing precision
If hydrogen is added during olefin polymerization to control molecular weight, then the average molecular weight is reduced, but the molecular weight distribution becomes narrower which worsens processing stability
Solution Approach 1:
The patent employs a dual-catalyst or multi-catalyst system where different catalyst components create multiple polymerization pathways simultaneously. Each catalyst pathway produces polymer chains with different characteristics, and the combination of these pathways results in a broader overall molecular weight distribution even in the presence of hydrogen, thus copying the beneficial effects of multiple catalyst systems while maintaining molecular weight control
Solution Approach 2:
The invention creates a composite catalyst system combining metallocene compounds with organometallic activators (borates, aluminoxanes, carboranes). This composite catalyst system exhibits synergistic effects where the combination produces a broader molecular weight distribution than either catalyst alone, while still allowing hydrogen to effectively control average molecular weight, thereby resolving the contradiction between molecular weight control and distribution breadth
3Stability of the object's composition
If chromium or Ziegler-type catalyst systems are used to produce broader molecular weight distribution polymers, then the molecular weight distribution broadens, but hydrogen still causes narrowing of the distribution
Solution Approach 1:
The patent employs readily available organometallic compounds (borates, aluminoxanes, carboranes) as activators that can be easily combined with metallocene catalysts. These activators are cost-effective and enable the catalyst system to maintain broad molecular weight distribution in the presence of hydrogen, providing an economically viable alternative to chromium or Ziegler-type catalysts while achieving the desired molecular weight characteristics
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The process effectively produces ethylene polymers with a broader molecular weight distribution, enhancing processing stability and production rates while maintaining desired polymer properties, thus overcoming the limitations of narrow distribution polymers.
Implementation Method 1
contacting a catalyst composition with an ethylene monomer and at least one olefin comonomer under polymerization conditions to produce an ethylene polymer; wherein: the catalyst composition comprises a contact product of at least one metallocene compound and at least one activator
Implementation Method 2
the polymerization process is conducted in the presence of hydrogen
Data Source
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AI summary
The present invention provides an ethylene polymer having a melt index from about 0.1 to about 100 g/10 min; a ratio of Mw/Mn from 3 to 6; a reverse comonomer distribution; less than about 0.05 long chain branches (LCB) per 1000 total carbon atoms; and less than 5% by weight of the polymer eluted below a temperature of 40°C in an ATREF test.