Chemistry Assessment

PolymersA Polymer is “a substance having

large molecules consisting of repeated units” (Dainteth, 2008, p428). Polymers are used in almost everything from ski boots to modern telecommunications which has caused 20th century to be dubbed by some the “plastic age” (Reusch, 2010). Polymers have existed naturally throughout geological history but it wasn’t until the 1930’s that humans truly understood them (O-Ring Catalog, n.d.).

Polymers are mainly made up of long carbon chains. Some polymers are made up of chains of other elements such as Silicone which is Silicon based, but these represent a small minority of the polymers. (Lenntech, n.d). Carbon is the main element that makes up polymers because of its unique properties, its ability to form single, double and triple bonds and its strong catenation ability. Carbon’s ability to form multiple bonds with itself allows polymers to be created with less energy as the pi bonds that form the second and third bonds in alkenes and alkynes are easier to break than the sigma bonds that make up alkanes (Alkenes Reactivity, n.d., Chempaths, 2009, Encyclopædia Britannica, 2011). This causes the alkenes and alkynes to require less energy to break bonds to allow them to join with other monomers, which means that they are more reactive. These extra bonds also allow other elements usually halogens to bond to the polymers. Carbons strong sigma bonds are the main reason it has strong catenation ability but also its low number of shells allow carbon to form long stable chains of itself (Encyclopædia Britannica, 2011).

Polyhydroxybutyrate Polyhydroxybutyrate (PHB) is a “high molecular weight polyester” (Poirier, Dennis, Klomparens, Somerville,1992) and is also a biopolymer, which is a type of polymer that occurs in nature and is also biodegradable. PHB belongs to the polyhydroxyalkanoate (PHA) group of polymers. (Chen, 2010) It has the chemical formula of - {O-CH(CH3)-CH2-(C=O)}-n and is currently produced through bacterial fermentation (Abdel Ghaffar, 2002).

Properties of Polyhydroxybutyrate PHB is biodegradable (Abdel Ghaffar, 2002). It has a melting point of 175oC. Has a tensile strength close to polypropylene (Belgacem &

Gandini, 2008 p460). PHB is insoluble in water and also sinks in water (which

allows it to biodegrade in sediments). It is a nontoxic polymer (ChemEurope, n.d.). It has good shelf stability (it can be stored at room

temperature for a long period of time without decomposing). Soluble in chloroform and other chlorinated hydrocarbons

(Abdel Ghaffar, 2002). It is a very brittle material (elongates only 5% before

breaking compared with polypropylene which elongates 400% before breaking) (Belgacem & Gandini, 2008 p460).

History of PolyhydroxybutyratePHB was first discovered in 1925 by Maurice Lemoigne when he was researching whether bacteria could produce polymers. However PHB was overlooked and forgotten and it wasn’t until 1957 when it was rediscovered in Britian and the USA that it’s applications began to be realized. (ChemEurope, n.d., Belgacem & Gandini, 2008 p453). It was researched during the 1960’s and the

particular enzymes used in the biosynthesis of PHB were identified by Schegel in 1973. In the “1970’s the interest in PHA’s rose” (Belgacem & Gandini, 2008 p453) and brought about an understanding of the biosynthesis of PHB which allowed copolymers of PHB to be produced. This solved PHB’s serious brittleness problem. In 1982 PHB was part of a thermoplastic called “Biopol” created by Imperial Chemical Industries which was a copolyester with “randomly arranged units” of HB (PHB’s monomer) and HV and was completely biodegradable (Belgacem & Gandini, 2008 p451, 453, 454). In 1992 a team from the DoE plant research lab at Michigan State University placed genes from the bacteria Alcaligenes eutrophus that produced acetoacetyl-CoA reductase and PHB synthase and placed them into the “Arabidopsis thaliana through Ti plasmid-mediated transformation” (Poirier et al.,1992). This experiment was designed to see if PHB could be produced by a plant that had been genetically modified. This experiment proved effective and now PHB can be created by plants (Poirier et al.,1992).

Biosynthesis of PolyhydroxybutyratePolyhydroxybutyrate is biosynthesized in several main stages. The first stage is the processing of carbohydrates through photosynthesis (6CO2+6H2O=C6H12O6+6O2) in plants. This glucose (C6H12O6) (other sugars are also sometimes used such as fructose) is then fed to bacteria such as Alcaligenes eutrophus which have multiplied so that there are a high number of bacteria and therefore a higher polymer yield. These bacteria are then deprived of an essential nutrient but given a surplus of carbon (Abdel Ghaffar, 2002). Without this deprivation of an essential nutrient the amount of Coenzyme A (C21H36N7O16P3S) produced is relatively high so PHB production is limited (Belgacem & Gandini, 2008 p454). The bacteria in optimum conditions produce a mass of up to 80% of there own body weight of PHB (Abdel Ghaffar, 2002).

(Abdel Ghaffar, 2002)

In the bacteria Alcaligenes eutrophus this nutrient deficiency and excess supply of carbon causes the enzyme 3-ketothiolase (HO-(C=O)-CH(NH2)-CH(CH3)-CH2-CH3) to catalyze two acetyl-CoA ((C2H3O)-S-CoA) to form acetoacetyl-CoA ((C4H5O2)-S-CoA) in a reversible condensation reaction where H-CoA is the simple molecule eliminated (Poirier et al.,1992, Dainteth, 2008, p138, Belgacem & Gandini, 2008, p 454, 455).

Chemical Equation of Condensation Polymerization reaction of acetoacetyl2((C2H3O)-S-CoA) HS-CoA+( C4H5O2)-S-CoA

3-Ketothiolase

The acetoacetyl-CoA is then catalyzed by 3-hydroxybutyryl-CoA dehydrogenase (C21H26N7O17P3)(NADPH) the reductase in a reversible reduction reaction. The acetoacetyl-CoA gains one hydrogen atom from the reductase and a further ionized hydrogen

atom (Poirier et al.,1992). This reaction breaks one of the pi bonds with one of the oxygen in the acetoacetyl-CoA which allows these two hydrogen to join the acetoacetyl-CoA. This gain of 2 hydrogen changes the acetoacetyl-CoA to (R)-3-Hydroxybutyryl-CoA ((C4H7O2)-S-CoA) (R is any member of the alkyl group) (Poirier et al.,1992, Belgacem & Gandini, 2008, p 454, 455).

Chemical Equation of Reduction reaction of acetoacetyl-CoA and 3-hydroxybutyryl-CoA dehydrogenase.

(C4H5O2)-S-CoA+ C21H26N7O17P3 +H+ ( C4H7O2)-S-CoA+ C21H25N7O17P3

+

Chemical Structure of (R)-3-Hydroxybutyryl-CoA (Bond Watts, Bellerose, Chang, 2011)

The monomer (R)-3-Hydroxybutyryl-CoA then polymerizes through insertion reactions (Poirier et al., 1992, Belgacem & Gandini, 2008, p 454, 455). This means however that only monomers that all have the same (R) configuration can polymerize together which creates an isotactic polymer. This polymerization occurs by “bonding another monomer to the free thiol group of the active site” (Biosynthesis of Polyhydroxybutyrate) which means that the C4H7O2 loses a hydrogen atom and inserts itself into the bond between the sulfur and carbon. This oxyester bond has a higher energy level than the sulfur and carbon bond and is the reason why the polymerization occurs spontaneously (Belgacem & Gandini, 2008, p 454, 455).

Chemical Equation of Insertion polymerization reaction of Polyhydroxybutyrate(C4H7O2)-S-CoA+-(O-CH(CH3)-CH2-(C=O))-n -(O-CH(CH3)-CH2-(C=O))-n+1+HS-CoA

The Alcaligenes eutrophus then have their cell wall mechanically destroyed and then the polymer is placed in a solvent such as chloroform and is extracted from the mixture through centrifuge and filtration. The PHB is dried and then created into the required product (Abdel Ghaffar, 2002).

Applications There is currently no large production of PHB as its production cost is too high (it takes 2.8kg of sugar per 1kg of PHB) (Abdel Ghaffar, 2002), however with the increasing scarcity of fossil fuels which are the basis for most modern polymers this high cost is slowly being offset along with new research (such as using plants for production). However PHB is currently being used in pilot programs and small businesses. Some of the possible future applications of PHB are in medicine It has potential to be used for implants as the human body can reabsorb PHB due to its solubility in chlorinated hydrocarbons. This means that a secondary operation to remove the plastic is avoided and decreases the risk of a secondary infection (Abdel Ghaffar, 2002). PHB is also non-

toxic meaning that there are no side effects of PHB being used in the operations. This application of being used in medicine has recently been tested by a group from the Department of Biological Science at the Middle East Technical University where they were testing “treatment of osteomyelitis by providing a high local dose of antibiotic at the infected site” using “biotechnologically produced biodegradable polymers” (PHB and its co-polymers) (Turesin, Gumusyazici, Kok, Gursel, Alaaddinoglu, Hasirci, 2000).

If its production cost drops PHB has the ability to, become an important polymer for packaging and one use products such as bottles and fishnets. The main reason why PHB could become a critical polymer in packaging is that is completely renewable, has good shelf stability and is biodegradable. It also has a possible use in the sporting, garden and building sectors by being used as water

proof plastic such as for garden pots. PHB’s moisture resistance and shelf stability are the main factors that make it suitable for these industries (SpecialChem, n.d). While PHB is currently not produced on a large scale, the continued research that is lowering its production costs, the dwindling ability to produce petroleum based polymers and the fact that it is a renewable and biodegradable resource means that large scale production of PHB is highly likely to occur in the future.

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