Molecular weight distribution polymerisation processes

The polymerisation of butadiene results in a polymer with a narrow molecular weight distribution which can be difficult to process. Indeed, commercially available grades present a compromise between processibility and performance. Most polybutadiene rubbers are inherently difficult to break down during mixing and milling, have low inherent tack, and the inherent elasticity of the polymer gives poor extrudability. Peptisers can be used to facilitate breakdown and hence aid processing. 

To produce a saleable polymer, the polymer produced should have a required Molecular weight, Molecular weight distribution and degree of branching. To obtain such a product various factors have to be taken into consideration. Factors like the nature of the monomer, the type of polymerisation mechanism chosen, the required physical form of the polymer and the viability of the process for industrial production dictate the physical conditions under which polymerisation is to be carried out. 

A foamable isotactic polypropylene homopolymer is obtained by metallocene catalysis and has a molecular weight distribution and density, which fall within broad ranges. It may be prepared in a multiple stage polymerisation process nsing the same metallocene component in at least two stages. 

When products of low molecular weight are obtained from a chemical reaction process, it is often possible to separate these products after they have left the reactor. Thus, the choice of reactor conditions can be taken from a wide range of options. With polymerisation processes, the results of reaction selectivity (i.e. the molecular weight distribution of polymer molecules) cannot be changed easily once the material has left the reactor. Since polymer properties depend on the molecular weight distribution, the relative yields of polymers with particular sizes must be matched to a required specification. Therefore, the choice of reactor type is very important. 

In some processes the polymerisation is carried out in a series of cascade reactors to allow variation in the hydrogen concentration through the operating steps in order to control the molecular weight distribution in the polymers formed. 

The increase of product yield and polymer quality in the case of a polymerisation process (molecular weight grows and molecular weight distribution shrinks). In the case of a faster reaction mixture passing through a reactor, compaction of the reaction zone is observed. 

The early work [1-6] established that fluorine substituted monomers could be polymerised by classical initiator systems derived from transition metal chlorides, however, the process was poorly characterised and gave largely atactic products with broad molecular weight distributions. 

Polymerisation is carried out via a semi-batch (semi-continuous) process that allows reactants to be added during the polymerisation. It is usual for at least one of the redox initiators to be added as a metered feed over the course of the reaction. Other reactants can be added in order to control desired properties such as molecular weight distribution. Temperature control over the course of the polymerisation is also possible, particularly as the oil phase helps to dissipate the heat of polymerisation. Overall the semi-batch polymerisation technique is more versatile than the batch process and, as a consequence, it is possible to better manipulate polymer properties via this route. 

Molecular weight distribution (MWD) is most important factor to determine the mechanical property and processability of polyethylene (PE). The control of MWD is one of the main targets for development of new generation polymerisation catalyst. In this work, a novel Si02-supported Ziegler-Natta catalyst designed in a molecular level on the silica gel surface modified with linear long alkene chains was developed for ethylene polymerisation. The coordinative nature of alkyl-Al co-catalyst should be expected to vary with its structure. The catalysts modified by various alkyl-Al co-catalysts were studied by Al MAS method, and the relationship between the coordinative states of A1 species and the MW and MWD of PE produced was clarified. ... 

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