Polymerisation Catalysts and

Cycloolefins, unlike acyclic internal olefins, undergo coordination polymerisation owing to the presence of ring strain. Loss of ring strain is an important contribution to the driving force of this polymerisation. 

The most strained cycloolefins, which are substituted cyclopropenes, e.g. 3,3-dimethylcyclopropene or 3-methyl-3-ethylcyclopropene, appeared to be polymerised readily to respective substituted poly(l,2-cyclopropene)s in the presence of Pd-based catalysts containing very bulky non-labile ligands. Such catalysts are characterised by reduced activity in order to prevent ring opening of the cyclopropene monomer [23], 

In particular, C2- and Cs-symmetrical zirconocenes, activated by methylalu-minoxane, exhibit very high activities in the polymerisation [18], Polymers of norbornene obtained with these catalysts are characterised by predominant erythro-disotactic and m7/ o-disyndiotactic structure respectively [15]. Active species formed in the rac.-Me2Si(Ind)2ZrCl2-[Al(Me)0]A. catalyst possess homotopic coordination sites for the incoming monomer, and hence pure 

In contrast to metallocene catalysts, Pd-based catalysts produce atactic polymers of norbornene [8], 

The polymerisation of norbornene showed features of a living process when [Pd(RCN)4][BF4]2 complexes were used as catalysts. Poly(2,3-bicyclo[2.2.1]-hept-2-ene)s of high molecular weight (in the approximate range 10 x 103—100 x 103) with small polydispersities (Mw/Mn 1.10), characterised by a relatively high glass transition temperature (Tg = 300 °Q, could be obtained when a solvent mixture of chlorobenzene and nitrobenzene was used at a reaction temperature of 0 °C [10], 

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This brief review attempts to summarise some recent advances in the mechanistic understanding of metallocene polymerisation catalysts and the role of NMR spectroscopy in these endeavors. For further information the reader is referred to a series of excellent recent reviews covering various aspects of the chemistry of metallocene polymerisation catalysts, for example Refs. [20-28]. 

Melamine formaldehyde can be crosslinked at elevated temperatures with both hydroxyl and carboxyl functional groups. (See thermosetting acrylics chapter for reaction mechanisms.) The temperature required is at least 120°C, at which point the hydroxyl group will react, but the carboxyl group needs a slightly higher heat input, approximately 150°C. Systems are unlikely to require an acid catalyst because of the catalytic effects of the polymerisation catalysts and surfactants in the acrylic latices. If required, p-toluene sulphonic acid is the most suitable (typically at levels of 0.2 - 0.4%). Alternatively, the melamine resin could be incorporated in an unneutralised, acidic emulsion, which reduces the cure temperature, but will sacrifice stability. 

In each case the configuration around the boron changes from trigonal planar to tetrahedral on adduct formation. Because of this ability to form additional compounds, boron trifluoride is an important catalyst and is used in many organic reactions, notably polymerisation, esterification, and Friedel-Crafts acylation and alkylations. 

The full ab-initio molecular dynamics simulation revealed the insertion of ethylene into the Zr-C bond, leading to propyl formation. The dynamics simulations showed that this first step in ethylene polymerisation is extremely fast. Figure 2 shows the distance between the carbon atoms in ethylene and between an ethylene carbon and the methyl carbon, from which it follows that the insertion time is only about 170 fs. This observation suggests the absence of any significant barrier of activation at this stage of the polymerisation process, and for this catalyst. The absence or very small value of a barrier for insertion of ethylene into a bis-cyclopentadienyl titanocene or zirconocene has also been confirmed by static quantum simulations reported independently... 

Polymer is separated from the polymerisation slurry and slurried with acetic anhydride and sodium acetate catalyst. Acetylation of polymer end groups is carried out in a series of stirred tank reactors at temperatures up to 140°C. End-capped polymer is separated by filtration and washed at least twice, once with acetone and then with water. Polymer is made ready for extmsion compounding and other finishing steps by drying in a steam-tube drier. 

Sodium trichloroacetate [650-51-17, C2Cl202Na, is used as a herbicide for various grasses and cattails (2). The free acid has been used as an astringent, antiseptic, and polymerisation catalyst. The esters have antimicrobial activity. The oral toxicity of sodium trichloroacetate is quite low (LD q rats, 5.0 g/kg). Although very corrosive to skin, trichloroacetic acid does not have the skin absorption toxicity found with chloroacetic acid (28). 

Current Polymerisation Catalysts—Peroxide, and Other Initiators, muIti-cHent study. Catalyst Consultants, Inc., Spring House, Pa., Nov. 1991. 

Histotically, the classification of PE lesias has developed ia conjunction with the discovery of new catalysts for ethylene polymerisation as well as new polymerisation processes and appHcations. The classification (given ia Table 1) is based on two parameters that could be easily measured ia the 1950s ia a commercial environment with minimum iastmmentation the resia density and its melt iadex. In its present state, this classification provides a simple means for a basic differentiation of PE resias, even though it cannot easily describe some important distinctions between the stmctures and properties of various resia brands. 

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. 

J. Boor, Jr., Ziegler-Natta Catalysts and Polymerisations, Academic Press, Inc., New York, 1979. 

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). 

Today the term anionic polymerisation is used to embrace a variety of mechanisms initiated by anionic catalysts and it is now common to use it for all polymerisations initiated by organometallic compounds (other than those that also involve transition metal compounds). Anionic polymerisation does not necessarily imply the presence of a free anion on the growing polymer chain. 

The Ticona materials are prepared by continuous polymerisation in solution using metallocene catalysts and a co-catalyst. The ethylene is dissolved in a solvent which may be the comonomer 2-norbomene itself or another hydrocarbon solvent. The comonomer ratio in the reactor is kept constant by continuous feeding of both monomers. After polymerisation the catalyst is deactivated and separated to give polymers of a low residual ash content and the filtration is followed by several degassing steps with monomers and solvents being recycled. 

The development of rubbers with a more closely controlled molecular structure. Such materials are made using anionic or Ziegler-Natta catalysts and are polymerised in solution (solution SBRs). 

More frequently either methyl ethyl ketone peroxide or cyclohexanone peroxide is used for room temperature curing in conjunction with a cobalt compound such as a naphthenate, octoate or other organic solvent-soluble soap. The peroxides (strictly speaking polymerisation initiators) are referred to as catalysts and the cobalt compound as an accelerator . Other curing systems have been devised but are seldom used. 

One variation in polyester intermediates that has roused some interest are those prepared by a ring-opening polymerisation of e-caprolactone and methyl-e-caprolactones with titanium catalysts and diol and triol initiators Figure 27.6). 

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. 

A number of highly active ethylene polymerisation catalysts have resulted from the combination of functionalised NHC ligands with Ti, the first of these was the bis(phenolate)carbene ligated complex 3 [8], Upon activation with modified MAO (MMAO), this species gave an activity of 290 kg-mol bar h in the one test reported, making it one of the most active carbene-based olefin polymerisation catalysts known. In later work the same complex was evaluated with straight MAO activation, and activities of up to ca. 100 kg mol -bar" -h" were reported for linear polyethylene production [9],... 

There has also been some interest in NHC-lanthanide complexes as polymerisation catalysts. Indenyl and fluorenyl functionalised NHC complexes of structures 14 and 15 (Fig. 4.5) were evaluated for isoprene polymerisation following activation... 

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