Polymerisation a-olefin

The catalyst derived from the reaction according to scheme (18) and related catalysts appeared capable of polymerising a-olefins surprisingly, the structure of the poly(a-olefin)s formed is consistent with a 2, co-coupling [191] but not with the usual 1,2-coupling of the monomers ... 

If 1-butene or 1-hexene is chosen instead of propylene as the monomer polymerising with the Me2C(MeCp)(Flu)ZrCl2-based catalyst, the polymers obtained become enriched in m diads. This has been suggested to testify to the preference of site isomerisation prior to the coordination of the next monomer molecule with increasing size of the polymerising a-olefin [121]. 

As already discussed, most Ziegler-Natta and related coordination catalysts polymerise a-olefins by a 1,2-insertion mechanism, yielding highly regio- and stereoregular polymers. When some nickel-based coordination catalysts are used, however, isomerisation of the active species may accompany the propagation, and polymers containing 2, co-coupled monomeric units are formed [183,191],... 

Readily polymerisable a-olefins consumed by the polymerisation are replenished by the isomerisation of /i-olefins [447], Kinetic studies of /1-olefm isomerisation-polymerisation indicate that the polymerisation, and not the isomerisation, is the rate-determining step under the conditions mentioned. The advantage of the /1-olefin polymerisation procedure, apart from the properties of the polymers obtained, is the one-pot synthesis of a-olefin polymers using / -olefin monomers [444],... 

In coordination polymerisation, a-olefins of the formula CH2=CH(CH2)aH may be inserted into the growing polymer chain in a 1,2 or 2,1 manner. Normally, these insertion steps lead to 1,2-enchainment or 2,1-enchainment of the monomer. Both of these steps form a -(CH2)aH branch. However, with some catalysts herein, some of the initial product of 1,2-insertion can be rearranged by migration of the coordinated metal atom to the end of the last inserted monomer before insertion of additional monomer, similarly to that presented in schemes (69) and (70). This results in m,2-enchainment and the formation of a methyl branch ... 

Explain why only coordination catalysts are useful for polymerising a-olefins. 

The inactivity of pure anhydrous Lewis acid haUdes in Friedel-Crafts polymerisation of olefins was first demonstrated in 1936 (203) it was found that pure, dry aluminum chloride does not react with ethylene. Subsequentiy it was shown (204) that boron ttifluoride alone does not catalyse the polymerisation of isobutylene when kept absolutely dry in a vacuum system. However, polymers form upon admission of traces of water. The active catalyst is boron ttifluoride hydrate, BF H20, ie, a conjugate protic acid H" (BF20H) . 

High density polyethylene (HDPE) is defined by ASTM D1248-84 as a product of ethylene polymerisation with a density of 0.940 g/cm or higher. This range includes both homopolymers of ethylene and its copolymers with small amounts of a-olefins. The first commercial processes for HDPE manufacture were developed in the early 1950s and utilised a variety of transition-metal polymerisation catalysts based on molybdenum (1), chromium (2,3), and titanium (4). Commercial production of HDPE was started in 1956 in the United States by Phillips Petroleum Company and in Europe by Hoechst (5). HDPE is one of the largest volume commodity plastics produced in the world, with a worldwide capacity in 1994 of over 14 x 10 t/yr and a 32% share of the total polyethylene production. 

The chemical iadustry manufactures a large variety of semicrystalline ethylene copolymers containing small amounts of a-olefins. These copolymers are produced ia catalytic polymerisation reactions and have densities lower than those of ethylene homopolymers known as high density polyethylene (HDPE). Ethylene copolymers produced ia catalytic polymerisation reactions are usually described as linear ethylene polymers, to distiaguish them from ethylene polymers containing long branches which are produced ia radical polymerisation reactions at high pressures (see Olefin POLYMERS, LOWDENSITY polyethylene). 

Tetraneopentyltitanium [36945-13-8] Np Ti, forms from the reaction of TiCl and neopentyllithium ia hexane at —80° C ia modest yield only because of extensive reduction of Ti(IV). Tetranorbomyltitanium [36333-76-3] can be prepared similarly. When exposed to oxygen, (NpO)4Ti forms. If it is boiled ia ben2ene, it decomposes to neopentane. When dissolved ia monomers, eg, a-olefins or dienes, styrene, or methyl methacrylate, it initiates a slow polymerisation (211,212). Results from copolymerisation studies iadicate a radical mechanism (212). Ultraviolet light iacreases the rate of dissociation to... 

Another important use of BCl is as a Ftiedel-Crafts catalyst ia various polymerisation, alkylation, and acylation reactions, and ia other organic syntheses (see Friedel-Crafts reaction). Examples include conversion of cyclophosphasenes to polymers (81,82) polymerisation of olefins such as ethylene (75,83—88) graft polymerisation of vinyl chloride and isobutylene (89) stereospecific polymerisation of propylene (90) copolymerisation of isobutylene and styrene (91,92), and other unsaturated aromatics with maleic anhydride (93) polymerisation of norhornene (94), butadiene (95) preparation of electrically conducting epoxy resins (96), and polymers containing B and N (97) and selective demethylation of methoxy groups ortho to OH groups (98). 

Abstract Over the past decade significant advances have been made in the fields of polymerisation, oligomerisation and telomerisation with metal-NHC catalysts. Complexes from across the transition series, as well as lanthanide examples, have been employed as catalysts for these reactions. Recent developments in the use of metal-NHC complexes in a-olefin polymerisation and oligomerisation, CO/olefm copolymerisation, atom-transfer radical polymerisation (ATRP) and diene telomerisation are discnssed in subsequent sections. 

For more general overviews of post-metallocene a-olefin polymerisation catalysts, the reader is referred to a series of reviews [8, 9, 10, 11, 12], while recent reviews pertaining to the importance of 2,6-bis(imino)pyridines and to iron and cobalt systems per se have also been documented [13, 14],... 

The cationic polymerisation of olefins by metal halides has been interpreted in two ways. The first theory, proposed by Hunter and Yohe [1], ascribed the catalysis to the formation of a polarised complex between the metal halide and the olefin ... 

Approaches Towards a Comprehensive Theory of the Cationic Polymerisation of Olefins (1974)... 

Neither for olefins nor for heterocyclic monomers do we yet have a sufficiently extensive body of activation energies of the kp-s to make a detailed discussion profitable. It is worth noting, however, that for the cationic (as opposed to the pseudo-cationic) polymerisation of olefins in solvents of DC greater than about 10, it is likely that a reduction of the temperature does not affect the rate except through its effect on k p, since these reactions are mainly carried by free ions only. 

For cationic polymerisation of olefins in solvents of DC appreciably less than ca. 10 and for those of heterocyclic monomers in all solvents of DC up to perhaps 15-20, this is not so. For such systems the polymerisations are probably at least dieidic (free ions and ion-pairs) and a lowering of the temperature will increase the DC of the ion pairs. Thus in such systems the change of temperature affects not only k p and k"p, but also the relative abundance of the different types of chain-carriers therefore the proper interpretation of the apparent activation energies is difficult and by no means obvious. 

The polymerisations of olefinic and cyclic compounds catalysed by conventional acids and by syncatalytic systems comprising a metal halide and a co-catalyst have been reviewed from several points of view [1-6]. These reactions are unusual in that the ideas used in their interpretation have changed frequently and the theoretical position is still somewhat confused. I shall examine here some of the reasons for this situation and attempt to clarify the picture by an historical approach. I will also give an account of some recent work leading to a revival of the old ester theory and indicate some of its implications and connections. 

Table 3. Relative polymerisation rate3) of (S) and (R) antipodes of some racemic a-olefins in the presence of (+ )-bis-[(S)-2-methyl-butyl]sinc and TiClt or TiCl3 ARA"... 

J. P. Kennedy, Cationic Polymerisation of Olefins A Critical Inventory,JohnWHey Sons, Inc., New York, 1975, pp. 164—166. 

The detail of the structure of the polymerisation centre present in suppported Ziegler-Natta catalysts for a-olefin polymerisation has been the subject of much research effort (e.g./-/2) The catalyst consists of a solid catalyst MgC /TiC /electron donor and a co-catalyst, an aluminium alkyl complexed with an electron donor. Proposed mechanisms for the polymerisation involve a titanium species attached to magnesium chloride with the olefin coordinated to titanium. The detail of the site at which the titanium species is attached is an important area of study in understanding the mechanism of catalysis and several recent papers 10-12) have investigated the surface structure of magnesium chloride and the attachment of TiCl4, in particular the interaction of titanium species with the 100 and 110 planes of a and (3- magnesium chloride. 

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