9.2.B - Biological Polymers


Fossil fuels are non-renewable resources, available in fixed amounts. Human activity has the potential to completely exhaust reserves of fossil fuel resources. Biomass, organic matter produced by photosynthesis in plants, is a renewable resource. Biomass consists mostly of cellulose and can be used and then formed again from its products by the input of solar energy during photosynthesis. If the matter involved is recycled, biomass could be a source of raw materials for as long as the sun supplies solar energy.

Petrochemicals are chemicals made from compounds in petroleum or natural gas. Currently Australia has petroleum reserves that will last about ten years and natural gas reserves that will last about one hundred years. Fossil fuels have taken hundreds of millions of years to accumulate. Over 95% of fossil fuel is burnt as a source of energy and once burnt, fossil fuels are no longer available. Less than 5% of fossil fuel is used to make plastics and only a small percentage of that plastic is recycled. If energy and material needs are to be met in the future, alternative sources will be needed as fossil fuel sources are used up.

In this topic students:

  • Identify and describe examples of condensation reactions,
  • Describe the structure, formation and properties of cellulose,
  • Assess potential of cellulose as a replacement for compounds currently obtained from petrochemicals, and
  • Describe the production, properties and use of a named biolpolymer.


Condensation Polymerisation

A condensation polymerisation reaction occurs when a polymer is formed from a reaction that leaves behind a small molecule, often water. The formation of peptide bonds in proteins is an example of a condensation polymerisation. In this case, an amine reacts with a carboxylic acid to form an amide bond. Glycine is the simplest amino acid: the reaction below shows the reaction between two glycine molecules to form a glycine dimer. Note that you do not have to learn this reaction. It is presented here by way of example.

The atoms in the red box drop away to form the water molecule on the right. This is the reaction that is used for all amino acids in the formation of proteins and is fundamental to life as we know it.

A common industrial condensation polymerisation is the formation of Dacron, a polyester. In this case an alcohol (in green) reacts with a carboxylic acid (in red) to form an ester bond (in blue) and a water molecule drops away. Again, this reaction is provided here by way of example only.

During the polymerisation process, the monomers tend to form dimers (two linked monomers) and trimers (three linked monomers) first. Then, these very short chains react with each other and with monomers. The overall result is that, at the beginning of polymerisation, there are many relatively short chains. It is only near the end of polymerisation that very long chains are formed.

 

Cellulose - A Condensation Polymer

Cellulose is a form of carbohydrate in which some 1500 glucose rings are joined together in a chain as a condensation polymer of glucose. It is the chief constituent of cell walls in living organisms. Wood is mostly cellulose, making cellulose the most abundant type of organic compound on the Earth.



Drs Valder and Honeydew discuss the origin of starch and cellulose in
plants.

Cellulose molecules tend to be straight chains, and the fibres which result from collections of cellulose molecules have the strength to form the supporting structures of plants. This is because of extensive hydrogen bonding that forms between the hydroxyl groups on the polymer chains. Even though human digestion cannot break down cellulose for use as a food, animals such as cattle and termites rely on the energy content of cellulose. They have protozoa and bacteria with the necessary enzymes in their digestive systems. Cellulose in the human diet is needed for fibre.The existence of cellulose as the common material of plant cell walls was first recognized by Anselm Payen in 1838. It occurs in almost pure form in cotton fibre and in combination with other materials, such as lignin and hemicelluloses, in wood, plant leaves and stalks, etc. Because cellulose is the major structural component of plants, we say that it is the major constituent of biomass.



The structure of cellulose made up of chains of repeating glucose rings joined to each other through a glycosidic
bond. This bond forms through a condensation polymerisation reaction with the hydroxyl groups on each glucose molecule. Hydrogen bonds holds the chains together to form sheets and more hydrogen bonding keeps each sheet firmly bonded to each adjacent sheet. This extensive hydrogen bonding is repsonsible for the rigidity of cellulose.


Chemically, cellulose is a long chain polymer, made up of repeating monomer units of glucose which is a simple sugar. The structure of glucose is shown in the diagram above. The presences of the hydroxyl groups (-OH) provides the basis for the reaction that turns glucose the biological polymer, cellulose. When the monomer units join together, a hydrogen from one monomer and a hydroxyl group from another monomer are eliminated in the reaction and a glycosidic bond is formed (see diagram below). The elimination of hydrogen and a hydroxyl group forms a water molecule and this reaction is, therefore, a condensation polymerisation reaction.

The equation for the formation of cellulose can be written as follows:

n(C6H12O6) ⇒ (C6H10O5)n + (n-1)H20

The glycosidic bond the is the name given to the bond that links glucose molecules together. There is more than one way that these molecules can be joined. Starch and cellulose are both polymers of glucose but they have very different properties because they have different glycosidic bonds and these change the structure of the polymer.

The difference in the structure of starch and cellulose are shown in the diagrams on the right. Notice the orientation of the glucose rings. In starch (b) they are all aligned the same way with the glycosidic bond pointing downwards. In cellulose (c), the glucose rings alternate up and down as does the glycosidic bond.

Starch is the polymer used by plants to store glucose until it is needed for respiration while cellulose is the structural material. Another polymer of glucose not shown here is glycogen and this is used by animals to store glucose again until it is needed for respiration.

 

Cellulose - Potential as a Replacement for Petrochemicals

The chemistry involved in making cellulose into an
alternative source of petrochemicals.

Petrochemicals are chemicals derived from petroleum. As we have already seen, ethene is a very common petrochemical and can be used to make many other chemicals such as the addition polymers, polyethylene, PVC and polyvinyl chloride.

Cellulose can be used to produce substances such as ethene that is currently made from petrochemicals through a conversion process first to glucose (hydrolysis), then to ethanol (fermentation) and finally to ethene (dehydration). Because cellulose is a component of biomass, it is renewable and abundant. As such it has potential as a replacement for non-renewable petroleum as a source of petrochemicals, however, the cost of making ethene this way currently prohibits the potential of cellulose as a viable alternative to petrochemcials.

The conversion of cellulose to ethene involves three main chemical processes or steps. The first involves the conversion of cellulose to glucose. Celulose can be hydrolysed to glucose in the presence of water. A family of enzymes (biological catalysts) known as cellulases are able to do this and are found in the stomachs of animals.

Once the glucose has been made, it can be converted to ethanol through the process of fermentation (see more on this in the next section). This process is performed in the absence of oxygen by the zymase enzyme secreted by yeast. Finally, ethanol can be dehydrated to produce ethene in the presence of concentrated sulfuric acid.

While it is possible to produce ethene from cellulose, the process is not well developed industrially and mass production of ethene this way is expensive and currently not feasible. Further research and development is required to reduce costs before cellulose can present itself as a truly viable alternative source of petrochemicals such as ethene.

 



Biofuels: The Cellulose Barrier
In congested, polluted central London they are researching the possibilities of making biofuels from trees. They hope that soon, the poplar will become a reliable, renewable source of biofuel.

 

Biological Polymers

Plastics that are being produced from the petrochemical industries are non-biodegradable. They remain in rubbish tips for centuries and do not decompose. Research is being conducted to find ways of producing synthetic biopolymers that will degrade in a short time in rubbish tips. Biopolymers made using microorganisms such as bacteria will help reduce our reliance on petrochemicals for plastics. Summaries of the production and properties of two biopolymers made using such microorganisms are outlined below.

Polyhydroxyalkanoates

Polyhydroxyalkanoates (PHAs) are often called bacterial plastics because they are made using genetically modified bacteria. One widely manfuactured and used polyhydroxyalkanoate was once known commercially as Biopol. It is a copolymer of hydroxybutyric acid (a type of butanoic acid) and valeric acid (pentanoic acid) and is known chemically as PHBV.

The structure of the copolymer PHBV and its two monomers.

Originally Biopol was polyhydroxybutanoate (PHB) - a simple polymer of hydroxybutyric acid produced when a culture of the bacteria Alcaligenes eutrophus is placed in a growth medium containing glucose and other nutrients at about 30°C. This causes the bacteria to multiply rapidly and then when nitrogen is restricted from the nutrient supply, PHB is produced by the bacteria at levels up to 80% of the bacterium's dry body mass. The organism is then harvested and the polymer separated. The PHB is then milled to a power or a pellet that can be used in moulds. PHB is a polyester with thermoplastic properties, meaning that it either melts or is softened when heated and hardens when cooled. This process can be repeated any number of times. Pure PHB is a very stiff and brittle material with melting points, molecular weights and tensile strength similar to that of polypropylene. The brittle nature of PHB limited its use and further research found that altering the medium on which the bacteria grow could produce a copolymer that had different chemical and physical properties. If the bacteria is grown on a medium containing valeric acid, a copolymer of PHB and poly(valerate) was formed (PHBV). This copolymer is stronger and more flexible than pure PHB and has a much wider range of uses as a result.

The biodegradability of PHVB makes it a potential replacement for the widely used polymers such as polypropylene and polyethylene that are presently manufactured from petrochemicals. These polymers are an environmental hazard because they do not decompose and can cause injury to plants and animals. They are also manufactured from petrochemicals which are non-renewable resources. PHBV could be used as a replacement in disposable nappies, bottles, bags and various packaging materials without fear of pollution because it can be completely broken down by bacteria into only carbon dioxide and water. The biodegradability of PHB also facilitates its use in medical applications whereby plates and sutures made from PHVB and left in place to heal fractured bones or wounds. Once healed, the PHB can be slowly broken down by the body without any adverse effects.

Despite its appeal over petrochemical plastics, higher production costs mean that manufacturing PHVB-based plastics is still several times more expensive than that of those plastics made from petrochemicals. Intensive research into production methods is endeavouring to overcome these current economic disadvantages and produce financially viable plastics that are environmentally friendly. Since PHBV have similar properties to existing polymers manufactured from petrochemicals, they have significant potential for replacing them. The biodegradability of biopolymers also opens up new medical applications for their use and also makes them a better environmental alternative to existing petrochemical plastics. Despite their economic disadvantages, the properties of such biopolymers make them a superior choice for the uses outlined above.

History of Development of PHVB

ICI was the first to develop the material to pilot plant stage in the 1980s, but interest faded when it became clear that the cost of material was too high, and its properties could not match those of polypropylene. In 1996 Monsanto (who sold PHB as a copolymer with PHV under the trade name Biopol) bought all patents for making the polymer from ICI/Zeneca. However, Monsanto's rights to Biopol were sold to the American company Metabolix in 2001 and Monsanto's fermenters producing PHB from bacteria were closed down at the start of 2004. Monsanto began to focus on producing PHB from plants instead of bacteria. But now with so much media attention on GM crops, there has been little news of Monsanto's plans for PHB. In June 2005, a US company, Metabolix, received the Presidential Green Chemistry Challenge Award (small business category) for their development and commercialisation of a cost-effective method for manufacturing PHAs in general, including PHB. Another group of researchers at Micromidas Inc. have begun to produce PHB from the bacteria in municipal waste water. This approach shows promise for the future of human waste disposal and biodegradable plastic production.

Polylactic acid

Starch waste from potatoes, corn and cheese whey can be converted to simple sugars such as lactose, which can then be polymerised to produce lactic acid. Lactic acid monomers are polymerised by condensation polymerisation to form polylactic acid (PLA). This polymer is an example of polyester, as the condensation process involves the alcohol and carboxylic acid functional groups of the monomer. PLA is very strong and has high optical clarity. It burns like paper and is biodegradable. Disposable plates and tableware, food packaging, carpets and composting bags are some of the uses of this synthetic biopolymer.