Polymer Chemistry: Why it is important and how to be more green
Dr Matthew Jones and Mr Tom Forder
Centre for Sustainable Chemical Technologies, Bath University
Tom is about to complete a PhD on novel initiators for the production of copolymers.
What is Green Chemistry????
Paul Anastas’ so called “12 Principles of Green Chemistry”, which are summarized:
1 Prevent waste (try not to use cleaning agents).
2 Be atom efficient (use as many atoms from the raw materials as possible).
3 Avoid production of toxic/harmful by-products.
4 Products should be designed efficiently.
5 Avoid the use of solvents where possible.
6 Reduce energy requirements and environmental impact of any process.
7 Use renewable feedstocks.
8 Avoid unnecessary protection chemistry whenever possible.
9 Catalytic reagents are superior to stoichiometric reagents.
10 The final products are not harmful to the environment.
11 Develop real-time analytical methods to allow the detection of hazardous materials.
12 All materials should minimize the risk of explosion or harm to health/environment.
SECTION 1: Introductory concepts in polymer chemistry
Why study polymer chemistry?
Wide range of properties and applications
Wide range of chemistry – from organic synthesis to materials science
Good revision: kinetics, thermodynamics, spectroscopy, mechanisms, reactivity, stereochemistry
Huge commercial significance
Excellent examples of where control of synthesis controls molecular structure which controls the properties which determines the applications
Above left shows a polymer chain structure of one of the simplest and most common synthetic polymers: poly(ethene) Source: University of Florida.
Above right shows the formation of polythene (polyethylene). Many monomers are alkenes which react by addition to their unsaturated double bonds and in this case the monomer is ethene.
The above pictures show some of the uses of polythene however it does come with one major problem.
Without special treatment it is not readily biodegradable, and thus accumulates. It is a major cause for marine life destruction, Turtles cannot tell the difference between a jelly fish (its food) and a plastic bag.
The only thing we can do (at the moment) is send it to landfill.
However we are running out of space.
Of course polythene isn’t all bad. It can be used as a liner in an artificial hip.
There is need for greener and biodegradable polymers…….to prevent Pacific Ocean rubbish. This is what happens if we don’t recycle.
Also the origin of plastics (oil) is non-renewable.
So, what are polymers (or macromolecules)?
We are very familiar with polymer names such as polythene, polystyrene and nylon but we may not be that familiar with what they are.
Polymer chemistry or macromolecular chemistry is a multidisciplinary science that deals with the chemical synthesis and chemical properties of polymers or macromolecules.
IUPAC definition: A molecule of relatively high relative molecular mass, the structure of which essentially comprises the multiple repetitions of units derived, actually or conceptually, from molecules of low relative molecular mass.
Dr Jones’ definition: A polymer consists of large, chain like molecules which contain a number of repeating chemical structures covalently bonded together.
The repeat unit is the monomer, many of which combine together to form a polymer chain.
A catalyst increases the rate of reaction; it affects the rate of attainment of equilibrium. It is unchanged chemically at the end of the reaction
Initiator is like a catalyst in that it makes a chemical reaction happen more quickly BUT it is consumed during the process.
Inhibitor slows the reaction down, usually by reacting with the catalyst.
Propagation is polymer chain growth
Termination stops the catalysis and releases the polymer.
Polymer Characterisation – where chemistry meets materials science
In the solid state we need to consider how the chains interact to form a bulk polymer – rather than just isolated chains i.e. macroscopic properties.
Most chemistry concerns the properties and uses of individual molecules.
Material properties depend on how assemblies of molecules interact.
In the melt the polymer is disordered – the chains move around relative to each other – but there is no long range order.
If the polymer is suddenly frozen (quenched) then this disorder will be transferred to the solid:
If there is no long range order the material is amorphous and the chains adopt a random conformation.
Heating can change a polymer.
A thermoplastic, or thermosoftening plastic, is a polymer that becomes pliable or moldable above a specific temperature, and returns to a solid state upon cooling. Most thermoplastics have a high molecular weight. The polymer chains associate through intermolecular forces, which permit thermoplastics to be remoulded because the intermolecular interactions increase upon cooling and restore the bulk properties.
A thermosetting plastic, also known as a thermoset, is polymer material that irreversibly cures. The cure may be induced by heat, generally above 200 °C, through a chemical reaction, or suitable irradiation.
Polymer molecular weight
Polymer molecular weight distribution
Monomer connectivity aka. Macrostructure
Polymer tacticity “the relative stereochemistry of adjacent chiral centres in a macromolecule”
Thermal properties (melting point, glass transition)
Mechanical properties (shear, stress,
The tacticty is defined by the relative stereochemistry of the chiral centres in the chain:
When all the substituents occur on the same side of the backbone – “isotactic”.
If the substituents alternate between the sides of the chain – “syndiotactic”.
Where the groups are arranged randomly – “atactic”.
Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms that form the structure of molecules and their manipulation. An important branch of stereochemistry is the study of chiral molecules.
A chiral molecule is a type of molecule that has a non-superposable mirror image (an object that is not superposable on its mirror image). The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom.
Human hands are perhaps the most universally recognized example of chirality: the left hand is a non-superposable mirror image of the right hand; no matter how the two hands are oriented, it is impossible for all the major features of both hands to coincide.
Gel permeation chromatography (GPC) is a type of size exclusion chromatography (SEC) that separates analytes on the basis of size. The technique is often used for the analysis of polymers.
GPC Set Up
It is used to separate out different polymers due to their different lengths. The larger particle gets through quicker because the smaller particle can be slowed down because they can enter the porous bead.
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei. It determines the physical and chemical properties of atoms or the molecules in which they are contained. It relies on the phenomenon of nuclear magnetic resonance and can provide detailed information about the structure, dynamics, reaction state, and chemical environment of molecules.
Below left is a picture of a 900MHz NMR instrument with a 21.1 T magnet at HWB-NMR, Birmingham, UK. Below right is a picture of the NMR spectrum for the organic molecule seen with it.
MALDI-ToF mass spec
Soft ionisation technique that reduces the chance of fragmentation of macromolecules during ionisation.
Matrix-assisted laser desorption/ionization (MALDI) is a soft ionization technique used in mass spectrometry, allowing the analysis of biomolecules (biopolymers such as DNA, proteins, peptides and sugars etc) and large organic molecules (such as polymers, dendrimers and other macromolecules), which tend to be fragile and fragment when ionized by more conventional ionization methods. It is used for polymers to look for polydispersity, end group identification and side reaxtions.
Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature.
Types of polymerisations
Addition polymerisation: radical; coordination; cationic; anionic
An addition polymer is a polymer which is formed by an addition reaction, where many monomers bond together via rearrangement of bonds without the loss of any atom or molecule. This is in contrast to a condensation polymer which is formed by a condensation reaction where a molecule, usually water, is lost during the formation.
Free radical polymerization is a method of polymerization by which a polymer forms by the successive addition of free radical building blocks. Free radicals can be formed via a number of different mechanisms usually involving separate initiator molecules. Following its generation, the initiating free radical adds (nonradical) monomer units, thereby growing the polymer chain.
A coordination polymer is an inorganic or organometallic polymer structure containing metal cation centers linked by ligands. More formally a coordination polymer is a coordination compound with repeating coordination entities extending in 1, 2, or 3 dimensions.
Cationic polymerization is a type of chain growth polymerization in which a cationic initiator transfers charge to a monomer which becomes reactive. This reactive monomer goes on to react similarly with other monomers to form a polymer.
Anionic addition polymerization is a form of chain-growth polymerization or addition polymerization that involves the polymerization of vinyl monomers with strong electronegative groups.
Condensation polymers are any kind of polymers formed through a condensation reaction—where molecules join together–losing small molecules as by-products such as water or methanol, as opposed to addition polymers which involve the reaction of unsaturated monomers. Types of condensation polymers include polyamides, polyacetals and polyesters.
Ring opening polymerisation
In polymer chemistry, ring-opening polymerization is a form of chain-growth polymerization, in which the terminal end of a polymer acts as a reactive center, where further cyclic monomers join to form a larger polymer chain through ionic propagation.
Polycondensations – polyesters
The classic example is polyesterification. An example of this is the reaction between a carboxylic acid and an alcohol. An example of a product is Poly (ethylene terephthalate) PET.
A dimer is a chemical entity consisting of two structurally similar monomers joined by bonds that can be strong or weak, covalent or intermolecular.
The water is removed by an acid or a dimer.
Lactic Acid Polymerisation
This is a condensation polymer:
Advantages: Lactic acid is sustainable and the polymer will biodegrade
Disadvantages: You need a very a pure monomer and it is difficult to remove the water. You need to remove water to obtain a high Mn material but the process is difficult to control and you cannot get any stereoselectivity.
Number average molar mass or Mn.
The number average molecular mass is a way of determining the molecular mass of a polymer.
In chemistry, stereoselectivity is the property of a chemical reaction in which a single reactant forms an unequal mixture of stereoisomers during the non-stereospecific creation of a new stereocenter or during the non-stereospecific transformation of a pre-existing one. The selectivity arises from differences in steric effects and electronic effects in the mechanistic pathways leading to the different products. Stereoselectivity can vary in degree but it can never be total since the activation energy difference between the two pathways is finite. However, in favourable cases, the minor stereoisomer may not be detectable by the analytic methods used.
Ring Opening Polymerisation
This can involve metals.
The precise arrangement of substituents along the chain backbone can greatly affect the polymer properties.
The high reactivity of radicals mitigates against any stereochemical control.
The counter-ion can influence the mode of addition in ionic systems so that some control can be achieved.
However, the most precise control over stereochemistry can be achieved where the double bond coordinates about a metal centre.
Ziegler-Natta (alkene polymerisation)
A Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, is a catalyst used in the synthesis of polymers of 1-alkenes (α-olefins).
This is probably one of the most important contributions to science. The catalyst involves titanium and aluminium:
TiCl4 and Al(C2H5)3 are the precursors. These then react to form β-TiCl3 and Al(C2H5)3Cl and other compounds – it is a bit of a “witches brew”.
This forms a heterogeneous catalyst. It shows very high activity BUT can have issues with selectivity and molecular weight control as you can have more than one active site present in the catalyst.
The above left picture shows the Ziegler Natta catalyst support structure
The above right picture shows how the catalyst works.
Mechanism (insertion polymerisation)
1) Initiation: Formation of the catalytically active metal alkyl or metal hydride.
2) Propagation: Insertion of the alkene into the M-R or M-H bond
3) Termination; β-hydride elimination.
In this case it is possible to form isotactic, syndiotactic or atactic PP.
By changing the catalysts we can alter which one we form……..
Ring Opening Metathesis Polymerisation
Metatheses are exchanges and disproportionation reactions of carbon-carbon double bond in olefins and cyclic olefins
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon–carbon double bond.
We need an initiator – this is typically a metal carbine
Olefin metathesis is an organic reaction that entails the redistribution of fragments of alkenes (olefins) by the scission and regeneration of carbon-carbon double bonds. Catalysts for this reaction have evolved rapidly for the past few decades.
Ring Opening Metathesis Polymerisation
Ring-opening metathesis polymerization (ROMP) is a type of olefin metathesis chain-growth polymerization that produces industrially important products. The driving force of the reaction is relief of ring strain in cyclic olefins (e.g. norbornene or cyclopentene) and a wide variety of catalysts have been discovered. Research has shown that the addition of substituents to the monomer and the choice of solvent can alter the molecular weight of the polymer produced.
Grubb’s olefin metathesis polymerisation catalyst
Catalysts for this reaction have evolved rapidly for the past few decades. Because of the relative simplicity of olefin metathesis it often creates fewer undesired by-products and hazardous wastes than alternative organic reactions. Because of their elucidation of the reaction mechanism and their discovery of a variety of highly efficient and selective catalysts, Yves Chauvin, Robert H. Grubbs, and Richard R. Schrock were collectively awarded the 2005 Nobel Prize in Chemistry.
2nd generation (has a higher activity than the first)
Ring Opening Metathesis Polymerisation
Research in the Jones group
An anhydride formed by the removal of a water molecule from the hydroxyl and carboxyl radicals of hydroxy acids.
The above picture on the left is of osteoblasts.
Osteoblasts (from the Greek words for “bone” and “germ” or embryonic) are mononucleate cells that are responsible for bone formation; in essence, osteoblasts are specialized fibroblasts that in addition to fibroblastic products, express bone sialoprotein and osteocalcin.
1European Commission Joint Research Centre Report (2004) EUR 22103 EN
There is a large market for cyclic esters.
Non-wovens and Textiles;
Medical devices and Implants.
Total Polymer Production 230,000 kt p.a.
‘Biopolymer’ Production 222 kt p.a.
Ring Opening Polymerisation
In polymer chemistry, ring-opening polymerization is a form of chain-growth polymerization, in which the terminal end of a polymer acts as a reactive center, where further cyclic monomers join to form a larger polymer chain through ionic propagation. The treatment of some cyclic compounds with catalysts brings about cleavage of the ring followed by polymerization to yield high-molecular-weight polymers.
Current industrial production uses tin (II) 2-ethyl hexanoate
There is a drive to replace it with environmentally friendly alternatives
The key considerations for this are cost, activity and colour.
Possible Polymer Stereochemistry:
A stereoblock polymer is a polymer whose molecule is made of comparatively long sections of identical stereospecific structure, these sections being separated from one another by segments of different structure, as for example, blocks of an isotactic polymer interspersed with blocks of the same polymer with a syndiotactic structure.
In syndiotactic or syntactic macromolecules the substituents have alternate positions along the chain.
A heterotactic polymer has an irregular structural arrangement.
The properties of polymers are significantly influenced by their stereochemistry.
Microstructure Determination – 1H Homonuclear Decoupled NMR Spectroscopy
Nuclear magnetic resonance decoupling (NMR decoupling for short) is a special method used in nuclear magnetic resonance (NMR) spectroscopy where a sample to be analysed is irradiated at a certain frequency or frequency range to eliminate fully or partially the effect of coupling between certain nuclei. NMR coupling refers to the effect of nuclei on each other in atoms within a couple of bonds distance of each other in molecules. This effect causes NMR signals in a spectrum to be split into multiple peaks which are up to several hertz frequency from each other. Decoupling fully or partially eliminates splitting of the signal between the nuclei irradiated and other nuclei such as the nuclei being analysed in a certain spectrum. NMR spectroscopy and sometimes decoupling can help determine structures of chemical compounds.
Homonuclear decoupling is used to simplify complicated multiplet patterns.
The 1H nucleus is the most commonly observed nucleus in NMR spectroscopy. Hydrogen is found throughout most organic molecules and, fortunately for chemists, the proton has high intrinsic sensitivity as well as being almost 100% abundant in nature, all of which make it a favourable nucleus to observe. The proton spectrum contains a wealth of chemical shift and coupling information and is the starting point for most structure determinations.
Racemic relates to a chemical compound that contains equal quantities of dextrorotatory (rotates polarised light to the right) and levorotatory (rotates polarised light to the left) forms and therefore does not rotate the plane of incident polarized light.
New amine tris(phenolate)s
In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of a hydroxyl group (—OH) bonded directly to an aromatic hydrocarbon group. The simplest of the class is phenol, which is also called carbolic acid C6H5OH. Phenolic compounds are classified as simple phenols or polyphenols based on the number of phenol units in the molecule.
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group.
A series of group 4 amine tris(phenolate) complexes were prepared and characterised by single crystal X-ray diffraction and multinuclear NMR spectroscopy.
E.L. Whitelaw, M.D. Jones, M.F. Mahon Dalton Trans., 2009, 9020
Group 4 Salalen Complexes
E.L. Whitelaw, M.D. Jones, M.F. Mahon Inorg. Chem., 2010
In this paper, they report on the preparation and characterization of a series of unsymmetrical group 4 metal complexes based on ONNO salalen-type ligands. In these examples, the ligand is unsymmetrical with an amine and an imine nitrogen centre.
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex.
Kinetic measurements tell us how quickly a reaction is taking place.
The graphs below relate to the reactions discussed above and also give an indication how good the catalysts used are.
In polymer chemistry, polymerisation, as discussed previously, is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many forms of polymerisation and different systems exist to categorize them.
Polycarbonates (utilise CO2 as feedstock)
Polycarbonates (PC), known by the trademarked names Lexan, Makrolon, Makroclear and others, are a particular group of transparent thermoplastic polymers. They are easily worked, moulded, and thermoformed. Because of these properties, polycarbonates find many applications.
Relatively few examples of this controlled polymerisation. Limited to Co(II), Zn(II) and Cr(II) catalysts.
Regiochemistry/stereoselectivity of the polymer is important.
Industries use polycarbonate for making different products, from bulletproof windows to CDs and DVDs. The main advantages of polycarbonate are high strength and light weight.
Industries also use clear polycarbonate to make eyeglasses. This is because it has a very good transparency and durability. Lenses made from clear polycarbonate can be thinner than lenses made from ordinary glass.
Companies that create electronic equipment use polycarbonate to create the cover of cell phones, laptops and other products.
In chemistry, regioselectivity is the preference of one direction of chemical bond making or breaking over all other possible directions (chemical reaction in which the production of one structural isomer is favoured over all others).
In chemistry, stereoselectivity is the property of a chemical reaction in which a single reactant forms an unequal mixture of stereoisomers. This process can be manipulated to give one product in preference to another.
Isomers are any of two or more compounds with the same molecular formula but with different structure.
Investigate the potential to use heterogeneous catalysts to improve stereoselectivity.
In chemistry, heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. Phase here refers not only to solid, liquid, or gas, but also immiscible liquids, e.g. oil and water.
Use Co/Mn-AlPOs for the polymerisation
Timescale is 5-50 years. There really is only one cycle where the starting point consists of plants and carbon dioxide. This shown by the below right diagram
At the moment this is not currently economical. The main economical barrier is technology.
A biorefinery is a facility that integrates biomass conversion processes and equipment to produce fuels, power, heat, and value-added chemicals from biomass. The biorefinery concept is analogous to today’s petroleum refinery, which produce multiple fuels and products from petroleum.
Renewable monomers and polymers
Normally we would take fossil fuel oil and by a process called cracking make short hydrocarbon molecules. With the right process monomers can be produced, which in turn can be made into polymers or plastics.
Ideally we would use biomass to produce the monomers and polymers. This would remove the need for oil.
Of course renewable polymers are pointless if when the product is no longer needed it is sent to landfill or burnt.
Whichever method you use to produce the polymers and plastics you need to be able to recycle. Recycling of biomaterials enables more biofuels and biomaterials to be produced and if your original source is biomass then the carbon dioxide released on burning will be used by the plants which form the new biomass.
In petroleum geology and chemistry, cracking is the process whereby complex organic molecules such as kerogens or heavy hydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking of carbon-carbon bonds in the precursors.
A biofuel is a fuel that contains energy from geologically recent carbon fixation. These fuels are produced from living organisms. Examples of this carbon fixation occur in plants and microalgae. These fuels are made by a biomass conversion (biomass refers to recently living organisms, most often referring to plants or plant-derived materials). This biomass can be converted to convenient energy containing substances in three different ways: thermal conversion, chemical conversion, and biochemical conversion. This biomass conversion can result in fuel in solid, liquid, or gas form. This new biomass can be used for biofuels. Biofuels have increased in popularity because of rising oil prices and the need for energy security. However, according to the European Environment Agency, biofuels do not necessarily mitigate global warming.
Making plastics from biomass
Bioplastics are plastics derived from renewable biomass sources, such as vegetable fats and oils, corn starch, pea starch or microbiota.
Biopolyethylene (also known as renewable polyethylene) is polyethylene made out of ethanol, which becomes ethylene after a dehydration process. It can be made from various feedstocks including sugar cane, sugar beet, and wheat grain.
Ethylene glycol is produced from ethylene (ethene), via the intermediate ethylene oxide. So it can be sourced from biomass. It is an organic compound primarily used as a raw material in the manufacture of polyester fibres and fabric industry, and polyethylene terephthalate resins (PET) used in bottling.
Polyethylene terephthalate, commonly abbreviated PET, is a thermoplastic polymer resin of the polyester family and is used in synthetic fibres; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination with glass fibre Coca Cola (and many other fizzy drinks) bottles are made from it.
Polylactic acid or polylactide (PLA) is a thermoplastic aliphatic polyester derived from renewable resources, such as corn starch (in the United States), tapioca roots, chips or starch (mostly in Asia), or sugarcane (in the rest of the world). Events cups are made out of PLA.
Plastic recycling is the process of recovering scrap or waste plastic and reprocessing the material into useful products, sometimes completely different in form from their original state. For instance, this could mean melting down soft drink bottles and then casting them as plastic chairs and tables.
Both PET and PLA plastics can be recycled but not if they are mixed together. Before they are processed they need to be separated because the chemical makeup of PLA is distinct from PET. PLA is considered a contaminant of petroleum-derived plastic and can result in the part or the entire load of plastics to be discarded rather than recycled.
PLA products are labelled as biodegradable and compostable and break down into carbon dioxide and water in 30–45 days in a commercial compost facility, which can sustain a temperature of 140 degrees for several days. In a compost bin you may have in your backyard, one cup can take more than six months to break down. Therefore it is not a good idea to put it on your compost heap.
What is Lignin?
Lignin is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. It is one of the most abundant organic polymers on Earth, exceeded only by cellulose, employing 30% of non-fossil organic carbon, and constituting from a quarter to a third of the dry mass of wood.
Weng, J. K., X. Li, et al. (2008). Current Opinion in Biotechnology 19(2): 166-172.
Monolignols are phytochemicals acting as source materials for biosynthesis of both lignans and lignin.
Monolignols Hypothetical Structure
Lignin is largely ignored as a resource and is often simply burned for energy.
Currently it is a waste product of paper manufacturing.
Unfortunately there is the potential for it to degrade. This is particularly annoying in useful (expensive) products such antique books.
It is important to extract maximum value from the feedstock.
Cellulose is an organic compound with the formula (C6H10O5)n, a polysaccharide consisting of a linear chain of several hundred to over ten thousand β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall of green plants, many forms of algae and the oomycetes. Some species of bacteria secrete it to form biofilms. Cellulose is the most abundant organic polymer on Earth. The cellulose content of cotton fibre is 90%, that of wood is 40–50% and that of dried hemp is approximately 45%.
A hemicellulose (Also known as Polyose) is any of several heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all plant cell walls. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength. It is easily hydrolysed by dilute acid or base as well as myriad hemicellulase enzymes.
The graph below shows the relative abundance of the constituents of lignin.
Biobased product flow-chart for biomass feedstocks
From Top value added chemicals from biomass; Volume I – Results of screening for potential candidates from sugars and synthesis gas, PNNL, NREL, August 2004 (US Department of energy: Energy Efficiency and Renewable Energy)