Polymer supports physical characteristics

Ivanchev et al. compared, under the same ethylene polymerization conditions, the performances of some supported systems with those of traditional ones. The authors not only found that the activity of supported systems is determined by the chemical-physical characteristics of the carrier, which influence the number and the propagation constant of the active centres, but also, in agreement with other authors that changes take place, in the elementary polymerization processes, such as to modify the polymer molecular structure. Indeed, as one can observe (Table 6), it is... 

In this polymerization, the biofunctional component (enzyme) can be concentrated in an interfacial area between the frozen ice crystal and the supercooled monomer phase, and immobilized by molecular entanglement between the enzyme and polymer molecules. This is a different procedure for fixation from the usual entrapping method with a crosslinked structure in a gel. Therefore, we may call this procedure the adhesion-method to distinguish it from the usual entrapping. This term was extended to cover the use of the usual synthetic polymers including hydrophobic polymers as the supports. One of the characteristic properties of products obtained in this way was that there is a maximum activity at a certain monomer concentration. The maximum activity is observed when the increased inner surface area is balanced by the increased leakage of enzyme and these occur with a decrease of monomer concentration. Immobilization by physical entrapping was also studied by Rosiak [26], Carenza [27] and Ha [28]. 

Candida antarctica Lipase B (CALB) is atfracting increasing attention as a biocatalyst for the synthesis of low molar mass and polymeric molecules. Almost all publications on immobilized CALB use the commercially available catalyst Novozym 435, which consists of CALB physically adsorbed onto a macroporous acrylic polymer resin (Lewatit VP OC 1600, Bayer). Primarily, commercial uses of CALB are limited to production of high-priced specialty chemicals because of the high cost of commercially available CALB preparations Novozym 435 (Novozymes A/S) and Chirazyme (Roche Molecular Biochemicals). Studies to better correlate enzyme activity to support parameters will lead to improved catalysts that have acceptable price-performance characteristics for an expanded range of industrial processes. 

Other types of supports have other physical and mechanical properties and therefore different fragmentation behavior and kinetic characteristics. MgCl2 as a support fragments much earlier and extensively even at low polymer yields because it consists of loose agglomerations of many small crystalline subparticles [52-54], Hence, here the polymerization rate shows no initial period of low activity, but immediately rises steeply, passes through a maximum, and decelerates slowly in a diffusion-controlled manner. A similar kinetic behavior is observed when reversibly aggregated polymer latex nanoparticles are used as support. In ethylene slurry polymerization [55], the monomer at once has access to the primary latex particles so that polymerization and macroparticle growth start immediately and rise steeply. 

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