Polyethylene formation

Dow in Midland, MI, USA, performed metallocene-catalyzed polymerization of ethylene using a home-built tube reactor setup with advanced, microflow tailored plant peripherals for heating, temperature monitoring, pressure control, and dosing 

Ethylene is handled at 60 °C, well above the critical temperature [26]. Various combinations of precatalysts and activators were sampled and loaded by an autoinjector. 

Temperature profiles versus time were taken for different positions at the reactor tube [26]. The maximum rise in temperature was about 23 °C. Improved pressure control was exerted by using advanced pressure control electronics [26]. In the regions of large temperature increase, pressure was slightly fluctuating this effect diminished downstream. By deliberately changing pressure (in a loop), the temperature response followed immediately [26]. This proved that control of pressure is crucial for obtaining stable temperature baselines. 

Catalyst plug-induced microchannel ethylene polymerization allows one to process about 10 runs per hour [26]. This is considerably more than achievable with conventional equipment (Parr reactors) processing only four to six runs per day. 

---

Drivers for Performing Polyethylene Formation in Micro Reactors... 

Beneficial Micro Reactor Properties for Polyethylene Formation... 

Polyethylene Formation Investigated in Micro Reactors Organic synthesis 62 [OS 62] Radical polymerization of ethylene... 

Addition of traces of chloride in the form of bis(cyclopentadienyl)-titanium dichloride lowered the yield of polyethylene and initiated the known reduction reaction (129). Finally, it was found that polyethylene formation was caused by traces of water ( 10-8 mol%). Consequently, the yield increased to 500,000 g polyethylene per gram of titanium when two equivalents of trimethyl- or triethylaluminum previously treated with one equivalent of water was added to dimethylbis(cyclopentadienyl)ti-tanium (Table VII). 

Discussion Point DPS The previously unexpected observation of side-chain branches in diimine-nickel catalyzed polyethylene formation is explained by the reaction scheme represented in Figure 15. Propose related chain migration schemes which explain i) the chain straightening , i.e. the incorporation of propylene methyl substituents into the backbone of polypropylene chains produced by these catalysts, ii) the 2,co-concatenation of higher a-olefins by some Ni-based catalysts, and Hi) the introduction of stereoerrors in isotactic polypropylene by chain-migration of chiral ansa-zirconocene catalysts. 

In polyethylene formation of the products described in the following scheme ( Ranby B., RabekJ.R, 1978)... 

The general consensus on the mechanistic details of transition metal-catalyzed polyethylene formation is that the active site comprises a metal with an alkyl group as active chain end and a free coordination site, with the metal incorporated in a ligand or in a salt crystal [32]. Ethylene is inserted in a syn fashion into the metal-carbon bond. Iron bis(iminoaryl)pyridyl dichloride (BI P FeCl2, where R denotes the ortho substituents on the aryl entity Fig. 1) in combination with MAO or (tri)alkyl aluminum compounds (AIR3) yields active ethylene polymerization systems [23]. Both the free coordination site and the alkyl group of the iron center thus originate from the interaction with the aluminum compounds. 

Overall rates of polyethylene formation are directly related to the feed rate of ethylene to a semi-batch reactor operated at constant pressure and temperature. However, to compare the activity of various catalysts it is necessary to normalize the rates with respect to monomer and catalyst concentrations. This normalization is usually done by means of a simple rate expression which is first order in monomer and catalyst concentrations, t.e. 

An increase in the reaction pressure increases the ethylene conversion and the amount of linear long-chain oligomers. To avoid polyethylene formation, sulfur or nitrogen compounds are added to the reaction mixture. 

During photo-oxidative degradation of polymers, e.g. polyethylene, formation of unsaturated groups (—CH=CH—) has been observed. The presence of these groups can be detected by UV absorption spectroscopy, e.g. tetracene (310 nm), trienyl (325 nm), pentacene (342 nm), tetraenyl (360 nm) and penta-enyl (394nm) [711]. 

Aubriet F, Muller JF, Poleunis C, et al Activation processes and polyethylene formation on a Phillips model catalyst studied by laser ablation, laser desorption, and static secondary ion mass spectrometry, J Am Soc Mass Spectrom 17(3) 406-414, 2006. 

---