Homogenous systems, polymerisation

Interest has been generated by the advances made in the homogeneously cataly2ed polymerisation of propylene using metallocenes (10). Polymers produced with these catalyst systems have an unusual balance of properties that may become commercially attractive. 

Polymerisation in homogeneous systems has been found to occur very rapidly the time of chain growth is approximately 10—3—10 2 s. With a degree of... 

Chain transfer reactions in homogeneous olefin polymerisation systems with metallocene catalysts, which terminate individual polymer chains, in some instances can also terminate the polymerisation kinetic chain. For example, chain transfer with the monomer in propylene oligomerisation or polymerisation, which involves a bond metathesis reactions between the Mt-C species of the growing polymer chain and the C H species of methyl [scheme (45)] or vinyl [scheme (46)] groups in the monomer, gives rise to temporally inactive metal allyl or metal-vinyl species respectively [177, 241, 264] ... 

In the case of chlorine-free catalysts such as Mg(OH)2/Ti(OBu)4—[A1 (Me)0]x and Si02/Ti(0Bu)4—[Al(Me)0]x, heterogeneous species are assumed to promote the syndiospecific polymerisation of styrene [67,68]. In a polymerisation system with the latter catalyst, best results were achieved when treating the carrier with [AI(Mc)0]x prior to supporting Ti(OBu)4 (no further activation with methylaluminoxane was needed). The polymerisation rate reaches a maximum at an Al/Ti molar ratio of 20 this is much lower than the value of the Al/Ti molar ratio required to reach the maximum polymerisation rate in the respective homogeneous system, i.e. the system without a carrier [54]. 

The mechanism of the initiation of the radical polymerisation of polar monomers in systems comprising organometallic compounds Homogeneous anionic polymerisation of unsaturated monomers Metallopolymers... 

In addition, we can consider the differential stream reactor (DSR) as a generalisation of PFR model, in which the inlet and outlet streams enter or leave at different side points. In practice DSR can be met as cold-shot reactors for methanol and ammonia synthesis, or as multi-feed tubular reactor for gas-phase ethylene polymerisation. However, the approach is not restricted to homogeneous systems. In fact, the most incentive is the field of heterogeneous reactor when several phases (gas, liquid, solid) are involved. 

Solution polymerisation. A solvent is used to solubilise both the monomers and the forming polymer, obtaining a homogeneous system. Solubilisation allows an easy control of the polymerisation heat and also decreases viscosity, allowing bulk stirring. On the other hand, it decreases the concentrations of the reactive species, lowering polymerisation rate. Solvent should be carefully selected with inert properties respect to the polymerisation reaction. Moreover, at the end of the reaction, the solvent should be completely removed. 

The above mechanism for the preparation of polymer particles is usually applied for preparation of non-aqueous dispersions (latex particles dispersed in a non-aqueous medium). As mentioned above, the two main criteria for this type of polymerisation are the insolubility of the formed polymer in the continuous phase and the solubility of the monomer and initiator in the dispersion medium. Initially, polymerisation starts as a homogeneous system, but after polymerisation proceeds to some extent, the insolubility of the formed polymer chains causes their precipi-... 

The term IPN was first used in 1960 to describe the apparently homogeneous product obtained from styrene crosslinked with divinylbenzene. IPNs were prepared from this system by taking a crosslinked poly(styrene) network and allowing it to absorb a controlled amount of styrene and a 50% divinylbenzene-toluene solution containing initiator. Polymerisation of this latter component led to the formation of an IPN, the density of which was... 

HOMPOLY Homogeneous Free-Radical Polymerisation System... 

Aldol condensations were originally carried out in the liquid phase and catalysed homogeneously by acids or bases this way of operation is still predominant. Solid-catalysed aldol reactions can also be performed in the liquid phase (in trickle or submerged beds of catalyst), but in many cases vapour phase systems are preferred the factors determining the choice are the boiling points and the stability of the reactants at elevated temperatures. At higher temperatures, the formation of a, j3-unsaturated aldehydes or ketones [reactions (B) and (C)] is preferred to aldol (ketol) formation [reaction (A)]. A side reaction, which may become important in some cases, is the self-condensation of the more reactive carbonyl compound if a mixed condensation of two different aldehydes or ketones is occurring. The Cannizzaro reaction of some aldehydes or polymerisation to polyols or other resin-like products can also accompany the main reaction. 

The alkene metathesis reaction arose serendipitously from the exploration of transition-metal-catalysed alkene polymerisation. Due to the complexity of the polymeric products, the metathetic nature of the reaction seems to have been overlooked in early reports. However, in 1964, Banks and Bailey reported on what was described as the olefin disproportionation of acyclic alkenes where exchange was evident due to the monomeric nature of the products [8]. The reaction was actually a combination of isomerisation and metathesis, leading to complex mixtures, but by 1966 Calderon and co-workers had reported on the preparation of a homogeneous W/Al-based catalyst system that effected extraordinarily rapid alkylidene... 

Nevertheless, many vanadium-based catalysts and polymerisation systems comprising them have received much academic attention in the hope that they might provide models for heterogeneous catalysts and polymerisation systems, since the problems connected with surface properties and particle size were believed to have been overcome. It must be noted, however, that homogeneous vanadium-based catalysts appeared to be more complex than was thought. There is no decisive evidence on the structure of catalytic sites formed by reaction between the procatalyst and activator. 

Many supported highly active catalysts show behaviour similar to case B in Figure 3.13 the polymerisation rate may also start at a maximum value and then decrease more or less rapidly with time. Such kinetic behaviour is also characteristic of some homogeneous catalysts. Other polymerisation systems show no acceleration period but have a polymerisation rate that remains almost constant with time this is a rare case and relates, for instance, to 4-methyl-l-pentene polymerisation with MgCl2-supported catalysts containing phthalate esters as well as to ethylene polymerisation with the Cp2TiCl2—[Al(Mc)0]x catalyst (apart from a short settling period in the latter case) [240],... 

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