1.1 The Birth and Development of POE
Polyolefin Elastomer (POE) is fundamentally a random copolymer of ethylene and α-olefin monomers, typically containing no less than 20% α-olefin as the second monomer. Due to its unique two-phase microstructure, POE possesses both the high elasticity of rubber and good thermoplastic processability. The birth and development of POE are inextricably linked to the application of metallocene catalysts and advancements in solution polymerization processes.
POE is an ethylene/α-olefin copolymer derived from LLDPE. Before the industrial application of metallocene catalysts, random ethylene/α-olefin copolymers were primarily Linear Low-Density Polyethylene (LLDPE), with densities between 0.915-0.940 g/cm³. These were produced using multi-site Ziegler-Natta catalysts, mostly via gas-phase or slurry polymerization processes, with solution polymerization rarely used. Limited by catalyst performance, the insertion level of the α-olefin comonomer in these copolymers was low (generally <10%), and the products had issues like broad molecular weight distribution. Although Z-N catalysts were continuously improved, expanding the range of α-olefins copolymerized with LLDPE to include 1-butene, 1-hexene, and 1-octene, it remained impossible to produce ethylene/α-olefin copolymer products with high comonomer content using Z-N catalysts. Particularly in traditional gas-phase or slurry polymerization processes, as the comonomer content increased, the product became severely sticky, making the polymerization process uncontrollable.
Metallocene catalysts and solution polymerization processes were the two primary enablers for the birth of POE. In the mid-1980s, Dow Chemical utilized solution polymerization to expand its LLDPE product range to include Polyolefin Plastomers (POPs), reducing polymer density to 0.890-0.915 g/cm³. The industrial application of metallocene catalysts in the early 1990s was pivotal. Unlike Z-N catalysts, metallocene catalysts are single-site, allowing precise control over polymer chain structure in ethylene solution polymerization, resulting in ethylene/α-olefin copolymers with very narrow molecular weight distribution and uniform composition distribution.
More importantly, metallocene catalysts have a stronger copolymerization capability for α-olefins compared to Z-N catalysts, enabling the production of ethylene/α-olefin random copolymers with higher comonomer content. Against this backdrop, Polyolefin Elastomers (POE) were developed. POE typically has a density less than 0.890 g/cm³, generally between 0.860~0.890 g/cm³, with significantly higher added value than LLDPE. In the development of POE, Dow Chemical's introduction of the Constrained Geometry Catalyst (CGC) in 1993, coupled with high-temperature solution polymerization technology, played a crucial role.
A high proportion of α-olefin is a necessary condition for POE's excellent properties. Generally, the higher the α-olefin content, the better the mechanical properties of POE, and the longer the comonomer branch chain, the more effective it is at reducing product density. In industrial applications, ethylene/1-octene elastomers offer the best overall performance and are currently the mainstream POE product type. Ethylene/1-butene elastomers are also supplied by several companies due to the relative abundance of raw material sources. Ethylene/1-hexene products are relatively less common on the market. Typically, POE products have a 1-octene content between 15% and 45% (generally >20%) and a crystallinity of less than 25%. Metallocene catalysis during polymerization, along with the high-temperature solution polymerization process compatible with its catalytic mechanism, are the necessary technical conditions for achieving high comonomer incorporation of α-olefins.
Several multinational corporations have successfully developed POE products, while Chinese companies have not yet achieved a breakthrough. Following Dow Chemical's successful production of the Engage™ series POE elastomers in 1993, new grades for molding and extrusion were added to this series in 2003, primarily used for modifying polyolefins in non-automotive applications. In 2004, Dow successfully produced two Affinity™ grades using its Insite™ process for the hot melt adhesive market. In 2005, ExxonMobil in the US also developed POE copolymers using metallocene catalysts and a high-pressure ion process, marketed under the trade name Exact™, primarily used as impact modifiers in automotive polyolefin compounds. In 2008, SCG-Dow, the joint venture between Dow Chemical and Thailand's Siam Cement Group, built a production facility for the newest plastomers and elastomers at its Map Ta Phut production base in Thailand.
Mitsui Chemicals of Japan built and commissioned a POE plant in 2005, marketed under the trade name Tafmer™. LG Chem in Korea combined its proprietary metallocene catalyst with solution polymerization to produce ethylene-based polyolefin elastomers, branded as Lusene™, applied in automotive parts, footwear, wires and cables, sheets, and films. In 2015, the SK and SABIC joint venture successfully commissioned a solution polymerization plant in Ulsan, South Korea, producing mLLDPE, POP, and POE. So far, no domestic Chinese company has achieved the industrial production of POE.
