Chem 109H Fall08

Biopolymer Natural Source What is it? Lactic Acid Beets, corn, potatoes, and others Produced through fermentation of sugar feedstocks, such as beets, and by converting starch in corn, potatoes, or other starch sources. It is polymerized to produce polylactic acid -- a polymer that is used to produce plastic. Triglycerides Vegetable oils These form a large part of the storage lipids found in plant and animal cells. Vegetable oils are one possible source of triglycerides that can be polymerized into plastics.

Using Fermentation to Produce Plastics Fermentation, used for hundreds of years by humans, is even more powerful when coupled with new biotechnology techniques.

Today, fermentation can be carried out with genetically engineered microorganisms, specially designed for the conditions under which fermentation takes place,

Fermentation, in fact, is the process by which bacteria can be used to create polyesters. Bacteria called Ralstonia eutropha are used to do this. The bacteria use the sugar of harvested plants, such as corn, to fuel their cellular processes. The by-product of these cellular processes is the polymer.

Lactic acid is fermented from sugar, much like the process used to directly manufacture polymers by bacteria. However, in this fermentation process, the final product of fermentation is lactic acid, rather than a polymer. After the lactic acid is produced, it is converted to polylactic acid using traditional polymerization processes.

Plants are becoming factories for the production of plastics. Researchers created a Arabidopis thaliana plant through genetic engineering. The plant contains the enzymes used by bacteria to create plastics. Bacteria create the plastic through the conversion of sunlight into energy. The researchers have transferred the gene that codes for this enzyme into the plant, as a result the plant produces plastic through its cellular processes. The plant is harvested and the plastic is extracted from it using a solvent. The liquid resulting from this process is distilled to separate the solvent from the plastic.

Currently, fossil fuel is still used as an energy source during the production process. This has raised questions by some regarding how much fossil fuel is actually saved by manufacturing bioplastics. Only a few processes have emerged that actually use less energy in the production process.

Energy use is not the only concern when it comes to biopolymers and bioplastics. There are also concerns about how to balance the need to grow plants for food, and the need to grow plants for use as raw materials. Agricultural space needs to be shared. Researchers are looking into creating a plant that can be used for food, but also as feedstock for plastic production.

Biopolymers and bioplastics are the main components in creating a sustainable plastics industry. These products reduce the dependence on non-renewable fossil fuels, and are easily biodegradable. Together, this greatly limits the environmental impacts of plastic use and manufacture. Also, characteristics such as being biodegradable make plastics more acceptable for long term use by society. It is likely that in the long term, these products will mean plastics will remain affordable, even as fossil fuel reserves diminish.

One such molecule is the ethylene monomer, the starting point for a variety of plastics. Ethylene is a small hydrocarbon consisting of four hydrogen atoms and two carbon atoms.

Polymerization is often started by combining the monomers through the use of a catalyst - a substance that aids a chemical reaction without undergoing any permanent chemical change itself. During the chemical reaction, hundreds or thousands of monomers combine to form a polymer chain, and millions of polymer chains are formed at the same time. The mass of polymers that results is known as a resin.

Polyethylene is made from just ethylene monomers - but it's also possible to create polymers from two or more different monomers. You can make hundreds of different polymers depending on which monomers and catalysts you use.

Cellulose, the basic component of plant cell walls is a polymer, and so are all the proteins produced in your body and the proteins you eat. Another famous example of a polymer is DNA - the long molecule in the nuclei of your cells that carries all the genetic information about you.

lastics are classified into two categories according to what happens to them when they're heated to high temperatures. Thermoplastics keep their plastic properties: They melt when heated, then harden again when cooled. Thermosets, on the other hand, are permanently "set" once they're initially formed and can't be melted. If they're exposed to enough heat, they'll crack or become charred.

Thermoplastics have long, linear polymer chains that are only weakly chemically bonded, or connected, to each other. When a thermoplastic object is heated, these bonds are easily broken, which makes the polymers able to glide past each other like strands of freshly cooked spaghetti. That's why thermoplastics can readily be remolded. The weak bonds between the polymers reform when the plastic object is cooled, which enable it to keep its new shape.

The linear chains are crosslinked - strongly chemically bonded. This prevents a thermoplastic object from being melted and reformed.

The most common method for making plastics is molding. To make a thermoplastic object, plastic granules known as resin are forced into a mold under high heat and pressure. When the material has cooled down, the mold is opened and the plastic object is complete. When making plastic fibers, the molten resin is sprayed through a strainer with tiny holes.

Thermosets are produced in two steps: 1. Linear polymers are formed. 2. The linear polymers are forced into a mold where "curing" takes place. This may involve heating, pressure, and the addition of catalysts. During this process, a cross-linked or networked structure forms, creating a permanently hard object that is no longer meltable or moldable.

For most applications, the ideal polymer is a long, straight chain with a highly regular molecular structure. Early synthetic polymers, however, often exhibited odd little branches and other irregularities. In the 1950s, German chemist Karl Ziegler (1898–1973) discovered that an entirely different type of catalyst - a combination of aluminum compounds with other metallic compounds - could solve some of these annoying problems and increase the length of a polymer chain, producing superior plastics.

olymers often have short side chains, which can occur on either side of the main chain. If side branches occur randomly to the left or right, the polymer has an irregular structure. Italian chemist Giulio Natta (1903–1979) discovered that some Ziegler catalysts led to a uniform structure in which all the side branches are on the same side.

Firstly, there is an environmental impact from plastics production; however the plastics industry has worked hard to reduce energy and water use, as well as waste generation during the manufacturing processes.

Secondly, during their lives, plastic products can save energy and reduce carbon dioxide emissions in a variety of ways. For example, they're lightweight, so transporting them is energy efficient. And plastic parts in cars and airplanes reduce the weight of those vehicles and therefore less energy is needed to operate them and lower emissions are created.

Take a piece of copper wiring," says Meddick. "It is encased in plastic - a kind of hydrocarbon material. We release all the hydrocarbons, which strips the casing off the wire." Not only does the process produce fuel in the form of oil and gas, it also makes it easier to extract the copper wire for recycling.

Autofluff is the stuff that is left over after a car has been shredded and the steel extracted. It contains plastics, rubber, wood, paper, fabrics, glass, sand, dirt, and various bits of metal. GRC says its Hawk-10 can extract enough oil and gas from the left-over fluff to run the Hawk-10 itself and a number of other machines used by Gershow.

Because it makes extracting reusable metal more efficient and evaporates water from autofluff, the Hawk-10 should also reduce the amount of end material that needs to be deposited in landfill sites.

Polysaccharides make ideal storage molecules for energy for a number of reasons; a) they are large, this makes them insoluble in water and therefore they exert no osmotic or chemical effect on the cell; b) they fold into compact shapes; c) they are easily converted into the required sugars when needed.

Glycogen is a branched polysaccharide found in nearly all animal cells and in certain protozoa and algae.

In humans and other vertebrates it is principally stored in the liver and muscles and is the main form of stored carbohydrate in the body, acting as a reservoir of glucose

Starch is similar to glycogen, however it is found in plant cells, protists and certain bacteria.

The starch granules are made up of two polysaccharides, amylose and amylopectin. Amylose is an unbranched molecule made up of several thousand glucose units, coiled helically into a more compact shape. Amylopectin is also compact but has a branched structure and is made up of twice as many glucose units as amylose.

For example, peptidoglycans, which are a combination of protein and polysaccharide and are found in the cell wall of certain bacteria. Glycolipids, a combination of polysaccharides and lipids are found in the cell membrane.

A five-carbon sugar (hence a pentose). Two kinds are found: Deoxyribose, which has a hydrogen atom attached to its #2 carbon atom (designated 2') Ribose, which has a hydroxyl group atom there

A nitrogen-containing ring structure called a base. The base is attached to the 1' carbon atom of the pentose. In DNA, four different bases are found:

The combination of a base and a pentose is called a nucleoside.

One (as shown in the first figure), two, or three phosphate groups. These are attached to the 5' carbon atom of the pentose.

The nucleic acids, both DNA and RNA, consist of polymers of nucleotides. The nucleotides are linked covalently between the 3' carbon atom of the pentose and the phosphate group attached to the 5' carbon of the adjacent pentose.

Most intact DNA molecules are made up of two strands of polymer, forming a "double helix". RNA molecules, while single-stranded, usually contain regions where two portions of the strand twist around each other to form helical regions.

They are colourless, odourless liquids produced by a simple chemical reaction, whereby molecules of water are eliminated from commercially produced petrochemical products.

The most commonly used plasticisers are phthalates.

Predominantly, phthalates are used in the plastics industry to soften the popular plastic PVC. This is used to make a diverse range of cost effective products with various levels of technical performance suited to a wide range of applications. Many of these PVC products we use everyday but tend to take for granted. They include everything from lifesaving medical devices such as medical tubing and blood bags, to footwear, electrical cables; packaging, stationery, and toys.

Table 7 shows that the composition varies from chains of only monomer A (homopolymer A) to chains containing only monomer B (homopolymer B).

In many of the structures no regularity can be detected, although there will be short sequences of one type of unit, and the copolymer can be regarded as completely random; such copolymers are usually said to be ideal copolymers. These possible copolymer structures are shown schematically in Table 7.

It can be shown that the rate of change of monomer concentration in any copolymerization is given by the equation where [M1] and [M2] are the concentrations of monomers 1 and 2 at any instant and r1 and r2., are reactivity ratios. The reactivity ratios represent the rate at which one type of growing chain end adds on to a monomer of the same structure relative to the rate at which it adds on to the alternative monomer. The copolymer equation can be used to predict chain structure in the three different ways, already mentioned.

An ideal copolymer will tend to form when each type of chain end shows an equal preference for adding on to either monomer. In this case, and the copolymer equation becomes Hence composition depends on the relative amounts of monomer present at any time and the relative reactivities of the two monomers.

Step growth copolymerizations produce ideal (random) copolymers since in this special case r 1 = r 2 = 1.

The main reason for copolymerizing different monomers is to adjust the physical properties of a given homopolymer to meet a specific demand. SBR elastomer, for example (Table 1), based on 24 wt% styrene monomer shows better mechanical properties and better resistance to degradation than polybutadiene alone

A second reason for copolymerization is to enhance the chemical reactivity of a polymer, particularly to aid crosslinking. Conventional vulcanization in rubbers is brought about by forming sulphur crosslinks at or near double bonds in the chain

To show the dramatic effect of copolymer structure on physical properties, consider the change from random SBR copolymer to a block copolymer of exactly the same chemical composition but where the styrene and butadiene parts are effectively homopolymer chains linked at two points: The material behaves like a vulcanized butadiene rubber without the need for chemical crosslinking since the styrene chains segregate together to form small islands or domains within the structure. Such so-called thermoplastic elastomers (TPEs) today form an important growth area for new polymers because of the process savings in manufacture that can be achieved with their use.

call Styrofoam. Polypropylene (PP) is used in dishwasher-safe containers

You'll often find polyethylene terephthalate (PET) in soda bottles, and it is sometimes recycled into fleece, upholstery fabrics, and other useful materials. And then of course there's polyvinyl chloride (PVC)

about 4 percent of the world's annual oil production of some 84.5 million barrels per day is used as feedstock for plastic, and another 4 percent or so provides the energy to transform the feedstock into handy plastic.

drilling. Recycling, however, does cut into energy use. According to the U.S. EPA, manufacturing new plastic from recycled plastic requires two-thirds of the energy used in virgin plastic manufacture. I have more numbers, too: one ton of recycled plastic saves 685 gallons of oil.

One way to produce polyethylene is called "free radical polymerization." As in other polymerizations, the process has three stages, known as initiation, propagation and termination. To begin, we need to add a catalyst to our supply of ethylene. A common catalyst is benzoyl peroxide, which when heated has the habit of splitting into two fragments, each with one unpaired electron, or free radical. These fragments are known as initiator fragments.

The unpaired electron naturally seeks another and finds a convenient target in the double bond between the carbon atoms in the ethylene molecule. Taking an electron from the carbon bond, the initiator fragment bonds itself to one of the monomer's carbon atoms.

The new radical also seeks a partner. And so ethylene monomers begin attaching themselves in a chain, creating new radicals each time and lengthening the chain. This stage is called propagation.

Eventually, free radical polymerization stops due to what are called termination reactions. For example, instead of stealing an electron from double-bonded carbons or a nearby propagating chain, the carbon atom with the free radical sometimes steals an entire hydrogen atom from another chain end. The polymer end--robbed of its hydrogen--easily forms a double bond with its adjacent carbon atom, and polymerization stops.

Because every part of the ethylene monomer is included in the finished polymer, the free radical polymerization of polyethylene is referred to as an addition polymerization

Polymerizations that use only portions of a monomer, however, are known as condensation polymerizations. The monomers that condense with each other must contain at least two reactive groups in order to form a chain.

For example, poly(ethyleneterepthalate), a polyester known as PET that is commonly found in soda bottles, forms from a reaction of two monomers: ethylene glycol and terephthoyl chloride. At the reaction's end, an atom of hydrogen and an atom of chlorine are left out of each PET molecular junction, resulting in a by-product of hydrogen chloride (HCl) gas.

a source of energy

building blocks for polysaccharides (giant carbohydrates

components of other molecules eg DNA, RNA, glycolipids, glycoproteins, ATP

Monosaccharides are the simplest carbohydrates and are often called single sugars.

Monosaccharides have the general molecular formula (CH2O)n, where n can be 3, 5 or 6.

n = 3 trioses, e.g. glyceraldehyde n = 5 pentoses, e.g. ribose and deoxyribose ('pent' indicates 5) n = 6 hexoses, e.g. fructose, glucose and galactose ('hex' indicates 6)

Molecules that have the same molecular formula but different structural formulae are called structural isomers.

Monosaccharides containing the aldehyde group are classified as aldoses, and those with a ketone group are classified as ketoses. Aldoses are reducing sugars; ketoses are non-reducing sugars.

in water pentoses and hexoses exist mainly in the cyclic form, and it is in this form that they combine to form larger saccharide molecules.

There are two forms of the cyclic glucose molecule: α-glucose and β-glucose.

Two glucose molecules react to form the dissacharide maltose. Starch and cellulose are polysaccharides made up of glucose units.

Galactose molecules look very similar to glucose molecules. They can also exist in α and β forms. Galactose reacts with glucose to make the dissacharide lactose.

However, glucose and galactose cannot be easily converted into one another. Galactose cannot play the same part in respiration as glucose.

Fructose reacts with glucose to make the dissacharide sucrose.

Ribose and deoxyribose are pentoses. The ribose unit forms part of a nucleotide of RNA. The deoxyribose unit forms part of the nucleotide of DNA.

Monosaccharides are rare in nature. Most sugars found in nature are disaccharides. These form when two monosaccharides react.

The three most important disaccharides are sucrose, lactose and maltose.

Disaccharides are soluble in water, but they are too big to pass through the cell membrane by diffusion.

This is a hydrolysis reaction and is the reverse of a condensation reaction. It releases energy.

Monosaccharides are converted into disaccharides in the cell by condensation reactions. Further condensation reactions result in the formation of polysaccharides. These are giant molecules which, importantly, are too big to escape from the cell. These are broken down by hydrolysis into monosaccharides when energy is needed by the cell.

Monosaccharides can undergo a series of condensation reactions, adding one unit after another to the chain until very large molecules (polysaccharides) are formed. This is called condensation polymerisation, and the building blocks are called monomers. The properties of a polysaccharide molecule depend on: its length (though they are usually very long) the extent of any branching (addition of units to the side of the chain rather than one of its ends) any folding which results in a more compact molecule whether the chain is 'straight' or 'coiled'

Starch is often produced in plants as a way of storing energy. It exists in two forms: amylose and amylopectin

Amylose is an unbranched polymer of α-glucose. The molecules coil into a helical structure. It forms a colloidal suspension in hot water. Amylopectin is a branched polymer of α-glucose. It is completely insoluble in water.

Glycogen is amylopectin with very short distances between the branching side-chains.

Inside the cell, glucose can be polymerised to make glycogen which acts as a carbohydrate energy store.

Cellulose is a third polymer made from glucose. But this time it's made from β-glucose molecules and the polymer molecules are 'straight'.

Cellulose serves a very different purpose in nature to starch and glycogen. It makes up the cell walls in plant cells. These are much tougher than cell membranes. This toughness is due to the arrangement of glucose units in the polymer chain and the hydrogen-bonding between neighbouring chains.

Cellulose is not hydrolysed easily and, therefore, cannot be digested so it is not a source of energy for humans.

For sugars with more than one chiral center, the D or L designation refers to the asymmetric carbon farthest from the aldehyde or keto group. Most naturally occurring sugars are D isomers.

Natural rubber is from the monomer isoprene (2-methyl-1,3-butadiene). Since isoprene has two double bonds, it still retains one of them after the polymerization reaction. Natural rubber has the cis configuration for the methyl groups.

Charles Goodyear accidentally discovered that by mixing sulfur and rubber, the properties of the rubber improved in being tougher, resistant to heat and cold, and increased in elasticity. This process was later called vulcanization

Vulcanization causes shorter chains to cross link through the sulfur to longer chains.

Some of the most commercially important addition polymers are the copolymers. These are polymers made by polymerizing amixture of two or more monomers. An example is styrene-butadiene rubber (SBR) - which is a copolymer of 1,3-butadiene and styrene which is mixed in a 3 to 1 ratio, respectively.

More than 40% of the synthetic rubber production is SBR and is used in tire production. A tiny amount is used for bubble-gum in the unvulcanized form.

. At the nipple end of the balloon, there is lots of rubber and therefore many, many polymer chains - still loosely coiled. These chains can be pierced without popping the balloon because the the chains can still be stretched. This is because they allow the skewer in between the chains without breaking the chains or the bonds that connect them. But on the sides of the balloon, these chains are stretched almost to their limit and very far apart. The piercing is too much for the stretched chains and they break apart., and the balloon pops.

Advantages of emulsion polymerization include:[1] High molecular weight polymers can be made at fast polymerization rates. By contrast, in bulk and solution free radical polymerization, there is a tradeoff between molecular weight and polymerization rate. The continuous water phase is an excellent conductor of heat and allows the heat to be removed from the system, allowing many reaction methods to increase their rate. Since polymer molecules are contained within the particles, viscosity remains close to that of water and is not dependent on molecular weight. The final product can be used as is and does not generally need to be altered or processed. Disadvantages of emulsion polymerization include: Surfactants and other polymerization adjuvants remain in the polymer or are difficult to remove For dry (isolated) polymers, water removal is an energy-intensive process Emulsion polymerizations are usually designed to operate at high conversion of monomer to polymer. This can result in significant chain transfer to polymer.

The first "true" emulsion polymerizations, which used a surface-active agent and polymerization initiator, were conducted in the 1920s to polymerize isoprene.[6][7]

The Smith-Ewart-Harkins theory for the mechanism of free-radical emulsion polymerization is summarized by the following steps: A monomer is dispersed or emulsified in a solution of surfactant and water forming relatively large droplets of monomer in water. Excess surfactant creates micelles in the water. Small amounts of monomer diffuse through the water to the micelle. A water-soluble initiator is introduced into the water phase where it reacts with monomer in the micelles. (This characteristic differs from suspension polymerization where an oil-soluble initiator dissolves in the monomer, followed by polymer formation in the monomer droplets themselves.) This is considered Smith-Ewart Interval 1. The total surface area of the micelles is much greater than the total surface area of the fewer, larger monomer droplets; therefore the initiator typically reacts in the micelle and not the monomer droplet. Monomer in the micelle quickly polymerizes and the growing chain terminates. At this point the monomer-swollen micelle has turned into a polymer particle. When both monomer droplets and polymer particles are present in the system, this is considered Smith-Ewart Interval 2. More monomer from the droplets diffuses to the growing particle, where more initiators will eventually react. Eventually the free monomer droplets disappear and all remaining monomer is located in the particles. This is considered Smith-Ewart Interval 3. Depending on the particular product and monomer, additional monomer and initiator may be continuously and slowly added to maintain their levels in the system as the particles grow. The final product is a dispersion of polymer particles in water. It can also be known as a polymer colloid, a latex, or commonly and inaccurately as an 'emulsion'.

Both thermal and redox generation of free radicals have been used in emulsion polymerization. Persulfate salts are commonly used in both initiation modes. The persulfate ion readily breaks up into sulfate radical ions above about 50°C, providing a thermal source of initiation.

Emulsion polymerizations have been used in batch, semi-batch, and continuous processes. The choice depends on the properties desired in the final polymer or dispersion and on the economics of the product. Modern process control schemes have enabled the development of complex reaction processes, with ingredients such as initiator, monomer, and surfactant added at the beginning, during, or at the end of the reaction.

Colloidal stability is a factor in design of an emulsion polymerization process. For dry or isolated products, the polymer dispersion must be isolated, or converted into solid form. This can be accomplished by simple heating of the dispersion until all water evaporates. More commonly, the dispersion is destabilized (sometimes called "broken") by addition of a multivalent cation. Alternatively, acidification will destabilize a dispersion with a carboxylic acid surfactant. These techniques may be employed in combination with application of shear to increase the rate of destabilization. After isolation of the polymer, it is usually washed, dried, and packaged.

Ethylene and other simple olefins must be polymerized at very high pressures (up to 800 bar).

Copolymerization is common in emulsion polymerization. The same rules and comonomer pairs that exist in radical polymerization operate in emulsion polymerization.

Monomers with greater aqueous solubility will tend to partition in the aqueous phase and not in the polymer particle. They will not get incorporated as readily in the polymer chain as monomers with lower aqueous solubility.

Ethylene and other olefins are used as minor comonomers in emulsion polymerization, notably in vinyl acetate copolymers.

Redox initiation takes place when an oxidant such as a persulfate salt, a reducing agent such as glucose, Rongalite, or sulfite, and a redox catalyst such as an iron compound are all included in the polymerization recipe. Redox recipes are not limited by temperature and are used for polymerizations that take place below 50°C.

Selection of the correct surfactant is critical to the development of any emulsion polymerization process. The surfactant must enable a fast rate of polymerization, minimize coagulum or fouling in the reactor and other process equipment, prevent an unacceptably high viscosity during polymerization (which leads to poor heat transfer), and maintain or even improve properties in the final product such as tensile strength, gloss, and water absorption

Anionic, nonionic, and cationic surfactants have been used, although anionic surfactants are by far most prevalent.

Examples of surfactants commonly used in emulsion polymerization include fatty acids, sodium lauryl sulfate, and alpha olefin sulfonate.

Some grades of poly(vinyl alcohol) and other water soluble polymers can promote emulsion polymerization even though they do not typically form micelles and do not act as surfactants (for example, they do not lower surface tension). It is believed that these polymers graft onto growing polymer particles and stabilize them.[12]

Other ingredients found in emulsion polymerization include chain transfer agents, buffering agents, and inert salts. Preservatives are added to products sold as liquid dispersions to retard bacterial growth. These are usually added after polymerization, however.

Polymers produced by emulsion polymerization can be divided into three rough categories. Synthetic rubber Some grades of styrene-butadiene (SBR) Some grades of Polybutadiene Polychloroprene (Neoprene) Nitrile rubber Acrylic rubber Fluoroelastomer (FKM) Plastics Some grades of PVC Some grades of polystyrene Some grades of PMMA Acrylonitrile-butadiene-styrene terpolymer (ABS) Polyvinylidene fluoride PTFE Dispersions (i.e. polymers sold as aqueous dispersions) polyvinyl acetate polyvinyl acetate copolymers latexacrylic paint Styrene-butadiene VAE (vinyl acetate - ethylene copolymers)

their products have variable physical properties. To this day, the systems are little understood, but the monomers (polymer starting materials) react through a number of reaction sites on the catalyst. Unfortunately, this means the polymer can grow from many sites and at different rates, leading to a very wide distribution in the molecular weight, based on the polymer chain length.

Modern life has demanded more of the humble polymer. We want polymers that are stronger than steel, lighter than aluminium, and can be dyed any colour imaginable

Metallocenes are positively charged metal ions, most commonly Titanium or Zirconium, sandwiched between two negatively charged cyclopentadienyl rings (see fig). Their big advantage over the Ziegler-Natta systems is that they catalyse the reaction of olefins through only one reactive site. Due to this “single site” reaction, the polymerization continues in a far more controllable fashion, leading to polymers with narrow ranges of molecular weight and, more importantly, predictable and desirable properties.

it has been found that changing the ligands (functional groups attached to the metal) upon the metallocene molecule can controllably affect the properties of the polymer.

The drawback of metallocene catalysts is that they are unable to polymerize polar molecules, such as common acrylics or vinyl chloride. This is due to the metallocenes’ oxophilicity – their propensity for binding to oxygen.

Catalysts using late transition metals – those metals from groups 6 and higher in the Periodic Table – have become increasingly utilized. These compounds have good polymerization activity, although slightly less than metallocenes. However, crucially they can polymerize reactions with polar monomers.

The most commercially advanced of this type of catalysts are the Brookhart catalysts [6], which are diimine complexes of palladium or nickel (see fig).