Polymerisation Medium

Particularly for Ziegler-Natta type, low pressure polymerisations of olefins, the reaction is usually performed in an inert paraffinic solvent medium (from C4 to Cjg). In such cases, of course, traces of this solvent are found in the final polymer which persist for a long time and, indeed, are difficult to remove even by treatment of the polymer in vacuum. 

For polystyrene and polyvinylchloride (PVC), made by the suspension or emulsion processes, the reaction medium is an aqueous solution containing wetting agents, detergents, soaps and emulsifiers. Traces of all of these will occur in the final product. 

---

Studies have shown that, in marked contrast to carbanionic polymerisation, the reactivity of the free oxonium ion is of the same order of magnitude as that of its ion pair with the counterion (6). On the other hand, in the case of those counterions that can undergo an equiUbrium with the corresponding covalent ester species, the reactivity of the ionic species is so much greater than that of the ester that chain growth by external attack of monomer on covalent ester makes a negligible contribution to the polymerisation process. The relative concentration of the two species depends on the dielectric constant of the polymerisation medium, ie, on the choice of solvent. 

The type of solvent or diluent should be specified in reporting a Ziegler-Natta catalyst system. Alkene polymerisations are usually carried out in inert solvents, such as aliphatic or aromatic hydrocarbons (e.g. some gasoline fractions or toluene). The use of protic or aprotic polar solvents or diluents instead of the hydrocarbon polymerisation medium can drastically alter the reaction mechanism. This usually results in catalyst deactivation for alkene coordination polymerisation. Modern alkene polymerisation processes are carried out in a gas phase, using fluidised-bed catalysts, and in a liquid monomer as in the case of propylene polymerisation [28,37]. 

The obtained Ni(II) complex, containing an Ni-C (Ni-Ph) active bond, polymerises ethylene to high molecular weight high-density polyethylene, especially when precipitated with aliphatic hydrocarbon as the polymerisation medium. 

The use of borane-containing monomers clearly presents an effective and general approach in the functionalisation of polyolefins, which has the following advantages stability of the borane moiety to coordination catalysts, solubility of borane compounds in hydrocarbon solvents (such as hexane and toluene) used as the polymerisation medium, and versatility of borane groups, which can be transformed to a remarkable variety of functionalities as well as to free radicals for graft-form polymerisations. The functionalised polymers are very effective interfacial modifiers in improving the adhesion between polyolefin and substrates and the compatibility in polyolefin blends and composites [518],... 

The slurry process is the oldest and still widely used method for manufacturing polymers of ethylene, propylene and higher a-olefins. In this process, the monomer dissolves in the polymerisation medium (hydrocarbon diluent) and forms a solid polymer as a suspension containing ca 40 wt-% of the polymer the polymerisation occurs below the melting point of the polymer. In slurry polymerisation, the temperature ranges from 70 to 90 °C, with the ethylene pressure varying between 7 and 30 atm. The polymerisation time is 1-4 h and the polymer yield is 95-98 %. The polymer is obtained in the form of fine particles in the diluent and can be separated by filtration. Removal of the catalyst residues from the polymer can be achieved by the addition of alcohol (isopropanol, methanol), followed by recovery and extraction of the catalyst residues. The polymer is freed from diluent by centrifuging and then dried. In the case of polypropylene manufacture, the atactic fraction remains in the diluent [28,37]. 

Conjugated dienes have been polymerised using supported half-sandwich metallocene catalysts. For instance, catalysts derived by supporting CpTiCl3 on alumina-silica gels, containing—0-Ti(Cp)Cl2 species, displayed activity in isoprene polymerisation without the addition of any other activator. Depending on the alumina-silica gel composition, the kind of polymerisation medium and the temperature, these catalysts exhibited various activities and selectivities polyisoprenes with a predominant 3,4 structure and mixed 1,2/ trans-1,4 structure were obtained [118,119],... 

Various transition metal-based catalysts not containing preformed metal-carbon bonds have been developed for the polymerisation of conjugated dienes [27-35, 150-158]. These catalysts include monometallic precursors such as Rh, Co and Ni salts and bimetallic precursors such as C0CI2-AICI3. Some of them are soluble in a polymerisation medium, e.g. Rh(N03)3 in protic solvents (ROH, H2O) [27,150-154] and C0CI2—AICI3 in aprotic solvents [155-157], and some others are insoluble in a polymerisation diluent, e.g. TiCL—Ni(PCl3)4 [158]. 

A counterion (A ) is incorporated during synthesis to balance the charge on the polymer backbone. Common chemical oxidants are FeClj and (NH4)2S20g, which provide Cl and HS04 /S04, respectively, as the dopant anions. Electrochemical oxidation provides greater flexibility in terms of the anion that can be incorporated from the electrolyte (MA salt or HA acid) added to the polymerisation medium. 

In the case of copolymers, the composition is also a mean composition that generally reflects the composition of the different co-monomers used in the polymerisation medium after total conversion of the monomers into polymers. However, because the reactivity of monomers between each other can be quite different, the composition of the different molecules of copolymers in a single preparation can vary. Indeed, composition in monomer units of the copolymers formed at the beginning of the polymerisation reaction is not necessarily the same as composition of the copolymers formed at the end of the polymerisation reaction. This effect adds heterogeneity to chemically synthesised copolymers, and the only way to appreciate this effect is to analyse the composition of the polymers at low conversion degree during polymer synthesis. 

Other brominated FRs from Dead Sea Bromine are reactive types that should be compatible with any solvent system that is utilised as a polymerisation medium. So, aliphatic materials such as FR-513 (in alcohol) and FR-522 (diol) have good solubility in the polyol or trichloropropylphosphate (TCPP) systems used for the production of polyurethane foams. 

Radical polymerisation Medium Density Medium Density (MD)... 

This process exploits an imusual effect of the difference in solubility of acrylic acid monomer and polyacrylic acid in specific solvents. When products based on the process were first developed [4] and made commercially available, benzene was used as the polymerisation medium. The polymerisation reaction is initiated in a system containing a mixture of acrylic acid monomer and a cross-linking monomer (typically a multi-allyl ether derivative of sucrose or pentaeryrthritol) and, as the polymer network grows, the solubility in the solvent decreases until precipitation of the polymer network occurs in the form of a small particle size powder. The use of a cross-linking monomer results in a 3D network of... 

Practically, the polymerisation can be carried out in two different ways (Figure 5.3). The first method is to simply add the deactivator into the polymerisation medium containing monomer and radical initiator and then adapt the experimental conditions so that the trapped radical can be reactivated. The second method consists of using a pre-made dormant species of small size (where R-T is considered as a unimolecular initiator) to start the polymerisation without the need of a radical initiator. In both cases, initiation must be fast, so that all growing chains are created within a short time span. It should, however, be noted that the second method leads to a better control over molar mass as the initiator efficiency is usually close to 1, which might not always be the case when a classical radical initiator is used. 

In order to overcome the drawbacks related to metal contamination of polyesters prepared in the presence of tin and aluminium alkoxides, supercritical carbon dioxide is a promising polymerisation medium in order to prepare biomedical grade aliphatic polyesters, due to the possibiUty to extract the... 

---