Long-chain branching residence time

Radical chain polymerization of ethylene to polyethylene is carried out at high pressures of 120-300 MPa (17,000-43,000 psi) and at temperatures above the Tm of polyethylene (Fig. 3-18) [Doak, 1986]. Batch processes are not useful since the long residence time gives relatively poor control of product properties. Long-chain branching due to intermolecular chain transfer becomes excessive with deleterious effects on the physical properties. Continuous processes allow better control of the polymerization. 

The main advantages for the high-pressure process compared to other PE processes are short residence time and the ability to switch from homopolymers to copolymers incorporating polar comonomers in the same reactor. The high-pressure process produces long-chain, branched products from ethylene without expensive comonomers that are required by other processes to reduce product density. Also, the high-pressure process allows fast and efficient transition for a broad range of polymers. 

In the reaction of branched olefins, contrary to straight chain olefins, the amount of acid (mole to mole) can be lowered and the concentration of water in the acid catalyst may be higher [603-605]. If long residence times of carbon monoxide, cone, acid and starting material are applied before the reaction is stopped by addition of water or alcohol, considerable amounts of ketones may be formed, especially starting from mixtures of olefins and paraffins [606]. 

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