Chemistry in Action (Polymers)Chemistry in Action (Polymers)Chemistry in Action (Polymers)Chemistry in Action (Polymers)Chemistry in Action (Polymers)Chemistry in Action (Polymers)Chemistry in Action (Polymers)Chemistry in Action (Polymers)Chemistry in Action (Polymers)Chemistry in Action (Polymers)
Chemistry in Action (Polymers) Polymers Polymers are macromolecules formed by repeated joining of many small molecules Polymerisation is the process of joining together many small molecules repeatedly to form very large molecules like polymers Monomers are compounds that join togeth er repeatedly to form a polymer in the process of polymerisation Polymers can be natural or synthetic The natural polymers covered include proteins, polysaccharides and nucleic acids The most important naturally occurring polymers are: Proteins Polysaccharides (e.g cellulose, starch) Nucleic acids (e.g DNA, RNA) Rubber Synthetic polymers are produced commercially on a very large scale They have a wide range of properties and uses Plastics are all synthetic polymers Synthetic polymers can be made from monomers by two basic polymerisation processes: (a) (b) addition polymerisation which produces addition polymers condensation polymerisation which produces condensation polymers Well-known examples of synthetic polymers are: Polyethene (PE) Polystyrene (PS) Polyvinyl chloride (PVC) Nylon Urea-methanal Page Chemistry in Action (Polymers) Natural Polymers Amino acids and protein Introduction Amino acid are bifunctional compounds containing both the amino ( -NH2) and carboxylic (-COOH) groups NH2 COOH Classification of amino acids Neutral amino acids: Number of amino groups = number of carboxylic groups E.g Glysine NH2CH2COOH Basic amino acids Number of amino groups > number of carboxylic groups E.g Lysin H2N NH2 COOH Acidic amino acids Number of amino groups < nu mber of carboxylic groups E.g Aspartic acid HOOC COOH H2N Stereochemistry of Amino Acids All amino acids except aminoethanoic acid contain an asymmetric atom and exhibit optical isomerism Example: Alanine They are optical isomers, but optical inactive, since they are racemic mixture Page Chemistry in Action (Polymers) Laboratory synthesized amino acids are ONLY optically inactive because of the formation of race mixture Physical properties of Amino Acids The dipole moments of the amino acids are very large For example, + NH3CH2COO- CH3CH2COOH CH3(CH2)2CH2NH2 Dipole Moment 14D 1.7D 1.4D Ionic Compound Acid Base In fact, in the solid state and in solution, amino cids exist as internal ionic salts, called Zwitterions So, Amino acids are high melting point solids e.g Glycine melts at 235 ղ They are very soluble in water, but they only dissolove slightly in organic solvents They have a very large dipole moment Chemical properties of Amino Acids Amphoteric nature of amino acids At some intermediate pH value, a dipolar ( zwitterions) form is produced The hydrogen ion from the carboxyl group is trasnsferred to the basic amino group within the molecule + H3N ± CH ± COOH | R OH H+ + H3N ± CH ± | R Page COO OH H+ H2N ± CH ± COO | R Chemistry in Action (Polymers) The existence of the zwitterion ic form can be explained in terms of acid -base theory: ±NH2 is a stronger base than ±COO2 ±COOH is a stronger acid than ±NH3+ Further evidence for zwitterions formation is electrophoresis Methods to separate a mixture of amino acids Paper chromatography will be used to separate amino acids There is a thin film of water on the chromatography paper The amino acids distribute themselves between the stationary phase (water on the paper) and the moving phas e (the solvent/eluent) To make the amino acid spots visible to naked eyes, spray chromatography paper with ninhydrin solution which reacts with amino acids to give purple coloured compounds (also accept using UV radiation/ iodine vapour to detect the amino acid spots.) Reactions of Amino acids Two main types reaction of the Amino Acids reaction of the carboxyl group reactions of the amino group Page Chemistry in Action (Polymers) Dil NaOH(aq) H NH2 CCOO- Na+ R SOCl2 or PCl5 HO NH2 C-C-Cl Showing acidic Properties R H Fusing with soda lime H NH2-C-H R HO CH3 OH/H+ + Dil HCl - NH2 CCOOH R Showing basic Properties NH3 C-C-OCH3 R H Cl NH3 CCOOH + R CH3COCl OHH CH3-C-N-C-COOH R Peptides, Polypeptides and protein Dipeptide The (-NH2) group of one amino acid can react with the ( -COOH) group of another to form an amide The resultion molecule is a dimmer containing two amino acid units which is describes as a dipeptide In the process, the two amino acid molecules are joined by the condensation reaction A water molecule is eliminated Page Chemistry in Action (Polymers) Amino acids are linked together by peptide linkage Polypeptide Amino acids undergo condensation polymerization to form long -chain polyamide molecules dipeptide Further reaction of each end Polypeptide/protein Page Chemistry in Action (Polymers) (1) If n< about 50, the product is a polypeptide (2) If n> about 50, the product is a protein Structure of proteins Protein structure is describe at l evels: 1־, 2 ־, 3 & ־quaternary Being polyamides, both proteins and nylon can be hydrolysed and are thus broken down to their constituent amino acids For example, Polypeptide Dipeptide Amino acids Page Chemistry in Action (Polymers) The peptide linkages in a protein molecule can be broken by hydrolysis using mineral acids or some enzymes On complete hydrolysis, the protein is broken down into amino acids By analyzing the resulting amino acids, the composition of the protein molecule may be deduced Carbohydrates Monosaccharide, disaccharide and polysaccharide Sugar, starch and cellulose are carbohydrates Carbohydrates are important in the diet as a source of energy They are compounds containing carbon, hydrogen and oxygen with the general formula C xHyOz Carbohydrates may be divided into three groups, Monosaccharides Disaccharides Polysaccharides The simplest carbohydrates are the sugars (glucose, fructose and ribose) Monosaccharides The monosaccharides consist of a single polyhydroxyaldehyde or polyhydroxyketone Monosaccharides are a group of sweet, soluble crystalline molecules with relatively low molecular masses They cannot be hydrolyzed into simpler compounds The monosaccharides commonly found in food have the general formula C6H12O6 Two most important examples are glucose and fructose They are found in many fruits and in honey Glucose is also found in the blood of animals (including humans) Each monosaccharides molecule contains one carbonyl group All the other carbon atoms are bonded to hydroxyl groups There are aldose and ketose, for which the carbonyl group is and is NOT terminal respectively Page Chemistry in Action (Polymers) aldehyde ketone Open chain and ring structures of glucose and fructose Glucose can exist in acyclic and cyclic forms: 0.02% 36% 64% Page Chemistry in Action (Polymers) Glucose contains an aldehyde group in its acyclic form Glucose is an aldohexose Most of the reactions of glucose in aqueous solutions are due to presence of the free aldehyde group of the acyclic form These reactions include its reducing action Fructose can exist as acyclic form, as well as cyclic forms of -membered rings and 5-membered rings Fructose contains a keto group in its acyclic form fructose is an ketohexose Most of the reactions of fructose in aqueous solutions are due to: presence of the free keto group of the acyclic form Disaccharides Page 10 Chemistry in Action (Polymers) Condensation Polymers Formation and Uses of Condensation Polymer Condensation polymerization is a chemical process in which monomer molecules are joined together to form a polymer with elimination of small molecules such as water, ammonia and hydrogen chloride Ethanoic acid Ethanol Ethyl ethanoate (An ester) Each monomer molecule must have at least two functional groups Polyamide Nylon Nylon 6,6 faster Page 26 Chemistry in Action (Polymers) When a solution of hexane -1,6-dioyl dichloride in hexane is poured gently onto a solution of 1,6 -diaminohexane in water, a white film of nylon is formed at the interface between the two layers The film can be pulled up as a string and wound onto a stirring rod Used for making carpets, thread, cords and various kinds of clothing from stockings to jackets Advantages: drips dry easily not easily attacked by insects resists creasing There are interchain hydrogen bond so Nylon6 , 10 is expected to have a lower tensile strength than nylon6,6 There is decrease in the number of hydrogen bond per unit length, as a result of the longer carbon chain in nylon 6,10 Kevlar Aramid is a synthetic poly amide Aliphatic hydrocarbon unit within the polmer chain has been replaced by an aromatic unit in Aramid Ց Kevlar is an aromatic polyamide Page 27 Chemistry in Action (Polymers) Ց The structure of Kevlar is similar to nylon -6,6 Ց The two monomers of Kevlar are benzene -1,4-dicarboxylic acid and 1,4-diaminobenzene Both monomers are bifunctional In Kevlar, the starting material was modif ied to create straighter chains in the polymer A polyamide was produced with the heat resistance of asbestos Strength was much greater than steel In Kevlar the aliphatic hydrocarbon chain parts of the poly amide are replaced by benzene rings These parts of the polymer chain make the chains inflexible due to delocalized bonding Some of this delocalization extends beyond the benzene rings and onto part of the amide link resulting in long, rigid molecules that not easily flex or twist This extended delocalization also leads to enhanced intermolecular hydrogen bonging between the adjacent Kevlay polymer chains This hydrogen bonding network causes the chains to interlock each other, forming a sheet structure Page 28 Chemistry in Action (Polymers) All the C = O and ± N ± H groups in the polymer chains are on opposite sides This makes the chains highly symmetrical The regular structure of the polymer chains allows them to interlock with each other Applications: (a) Kevlar is an unusual polymer with fire resistant properties and also great strength It is found in the crash helmets of Formula I racing drivers as well as in the suits of racing motorcyclists (b) The hull of this offshore racing craft is also reinforced with Kevlar (c) Kevlar is used in making bullet proof vests A more recent innovation is to use carbonanotubes to make fibres for these bullet proof vests These new bullet proof vests can be made 30 % lighter, but 1.5 times more bullet resistant than conventional Kevlar vests Page 29 Chemistry in Action (Polymers) (b) (a) (c) Dacron Formed by repeated condensation reactions of benzene -1,4-dicarboxylic acid (also called terephthalic acid) and ethane -1,2-diol (also called ethylene glycol) in the presence of a catalyst and at a low pressure and moderate temperature (about 250 tC) The two monomers of Dacron are: The polymerization begins with the formation of an ester A water molecule is eliminated Page 30 Chemistry in Action (Polymers) Due to polarization of the carbonyl groups C=O, Dacron chains are crosslinked by strong dipole-dipole attractions Properties of Dacron: High tensile strength High resistance to stretching Low absorption of moisture Garments made of Dacron: are tough can resist wrinkling can be washed and dried easily and quickly Excellent for making trousers and skirts, sheets and boat sails Can be used alone or blended with cotton to make it absorb sweat better Urea-methanal Produced by the condensation polymerization of urea and methanal under heat and pressure When an urea molecule joins up with a methanal molecule , water molecule is eliminated In the presence of excess methanal, further condensation reactions between the polymer chains and methanal occur.Cross-linkages between the polymer chains are formed A rigid structure of urea-methanal is produced Page 31 Chemistry in Action (Polymers) Ց Urea-methanal is a thermosetting plastic and cannot be softened or melted again by heating once they have been set hard Ց Excellent electrical insulator Resistant to chemical attack Ց Effect of Structure on Properties of Polymers Ց Ց Polymers are long-chain giant molecules The final form and the properties of the polymers depend on how these long polymer chains are packed togethe r Ց If the polymer chains not have a specific arrangement but are loosely packed together the polymer is said to be amorphous Ց Amorphous polymers are generally transparent, flexible and less dense Ց When the polymer chains are regularly packed together , the polymer is said to be crystalline Ց Polymers with a high degree of crystallinity are translucent or opaque, harder and denser Ց The attractive forces holding polymer chains together also affect the properties of polymers Page 32 Chemistry in Action (Polymers) Ց Polymer chains containing carbon and hy drogen atoms only are held together by weak van der Waals Ո force Ց Ց Ց slow melting points low mechanical strength If polymer chains are held together by stronger van der Waals Ո forces or hydrogen bonds, the mechanical strength of the polymers would be stronger Ց If cross-linkages are present between polymer chains , the polymers would be mechanically stronger, more elastic or more rigid ,depending on the extent of cross- linkages in the polymer Low Density Polyethene and High Density Polyethene High Density Polyethene Ց When Ziegler-Natta catalysts are used, the polymer chains produced are long molecules with very little branching The polymer chains can pack closely together into a largely crystalline structure Thus, the polymer has a higher density Ց Compared with LDPE, HDPE is harder and stiffer has a higher melting point has greater tensile strength has strong resistance to chemical attack has low permeability to gases blow-molded objects: bottles for milk, soft drinks, shampoos, bleaches and so on Page 33 Chemistry in Action (Polymers) Low Density Polyethene Ց When ethene is polymerized at 200 tC and 1000 atm using peroxide as the catalyst, low density polyethene (LDPE) is made Ց Under these reaction conditions, highly branched polymer chains are formed Ց The branches prevent the polymer chains from getting close to each other The polymer chains not pack together well and creates a significant proportion of amorphous regions in the structure Ց Thus, the polyethene made has a low density Low density polyethene is a Waxy Semirigid Translucent material Low melting point Nylon and Kevlar Ց Nylon is a group of polyamides Page 34 Chemistry in Action (Polymers) Ց It contains a relatively large number of crystalline regions arranged in a random manner Ց When nylon is spun into fibres and is dr awn Ց Ց the crystalline regions are aligned leads to an increase in the tensile strength Non-aligned crystalline region Aligned crystalline region In the stretched or drawn nylon , the polymer chains line up and are parallel to each other The amide groups on adjacent chains form strong hy drogen bonds with each other These hydrogen bonds hold the adjacent chains together making nylon thread strong The structure of Kevlar is basically the same as nylon -6,6 When molten Kevlar is spun into fibres , the polymer has a crystalline arrangement and the polymer chains oriented parallel to each other Page 35 Chemistry in Action (Polymers) Ց Kevlar is much stronger than nylon Ց The difference in their strength is due to the orientation of the amide groups along the polymer chains Ց In nylon, between the amide groups are the carbon chains the C = O and N H groups can be on opposite sides or on the same side Ց When the C = O and N H groups are on the same side, the polymer chain would not be straight and the number of hydrogen bonds formed between adjacent chains would be less Ց Kevlar has a regular structure the polymer chains interlock with each other Kevlar fibres are very strong used for making reinforced rubbers and bullet -proof vests Vulcanization of Polymers Natural rubber is a polymer of the monomer 2-methylbuta-1,3-diene (isoprene) 2-Methylbuta-1,3-diene Ց Poly(2-methylbuta-1,3-diene) or polyisoprene can exist in two isomeric forms Ց Natural rubber is the cis-form Page 36 Chemistry in Action (Polymers) Part of a polymer chain of natural rubber Natural rubber is not a useful polymer because it is too soft and too chemically reactive The long chain molecules can be coiled twisted and interwined with one another Vulcanization of natural rubber is the chemical process that confers cross-linkage among the polymer chains of rubb er, turning natural rubber into a flexible elastic material Ց In the process of vulcanization,1Ղ3 % by mass of sulphur is added to natural rubber and the mixture is heated Ց Short chains of sulphur atoms (i.e cross -linkages) are formed between the polymer chains Ց The sulphur changes rubber into a thermosetting polymer by cross linking the polymer chains through reaction at some of the double bonds as shown: This makes the rubber harder and reduces its susceptibility to oxidation or other chemical attrack Ց When vulcanized rubber gets hot, the polymer chains cannot slip across one another since they are still held together by short chains of sulphur atoms Ց That is why vulcanized rubber does not melt when heated and does not Page 37 Chemistry in Action (Polymers) become brittle when cooled The extent of the cross-linkages formed between the polymer chains affects the properties of vulcanized rubber Ց If the rubber has few cross-linkages, the rubber is softer, more flexible and more elastic Ց If the rubber has many cross-linkages, it is stiffer, less flexible and less elastic Ց Car tyres are made of vulcanized rubber Because of the presence of cross-linkages among the polymer chains, the rubber does not melt when it gets hot Ց Ց That is the reason why car tyres not melt when driv ers drive really fast Car tyres are made of vulcanized rubber Degradable Plastics Ց Natural polymers (e.g wood and paper) are biodegradable Micro-organisms in water and in the soil use them as food Ց Synthetic polymers (e.g plastics) are non-biodegradable can remain in the environment for a very long time Ց Nowadays, plastic waste constitutes about % of household waste Ց In Hong Kong, plastic waste is buried in landfill sites it remains unchanged for decades more and more landfill sites have to be found Uses of plastics in Hong Kong (an approximation) (a) Hong Kong is a highly densely populated city (b) The volume of domestic waste generated daily is very great (c) Plastic waste contributes to the main bulk of our domestic waste (d) Few sites are left to be used for landfill (e) The building / operation cost of incineration plants is high and recycling Page 38 Chemistry in Action (Polymers) of plastics also involves very tedious procedures In order to tackle the pollution problems caused by the disposal of plastic waste, degradable plastics have been invented Several types of degradable plastics: Ց Ց biopolymers photodegradable plastics Ց synthetic biodegradable plastics Biopolymers Ց Polymers made by living micro-organisms (e.g paracoccus, bacillus and spirullum) Ց e.g The biopolymer poly(3 -hydroxybutanoic acid) (PHB) is made by certain bacteria from glucose Ց When PHB is disposed, the micro-organisms found in the soil natural water sources are able to break it down within months Ց However, PHB is 15 times more expensive than polyethene Paracoccus (a) (b) Bacillus (c) and Spirullum Photodegradable Plastics Ց Photodegradable plastics have light -sensitive functional groups (e.g carbonyl groups) incorporated into their polymer chains Ց These groups will absorb sunlight use the energy to break the chemical Page 39 Chemistry in Action (Polymers) bonds in the polymer to form sm all fragments Synthetic Biodegradable Plastics Ց Made by incorporating starch or cellulose into the polymers during production Micro-organisms consume starch or cellulose and the plastics are broken down into small pieces Ց The very small pieces left have a large surface area greatly speeds up their biodegradation Ց Drawbacks of this method: Ց the products of biodegradation may cause water pollution Ց the rate of biodegradation is still too low for the l arge quantity of plastic waste generated Ց They are much more expensive than ordinary materials When buried in landfill, they will not be exposed to sunlight light and may therefore remain unchanged for many years Ց Ց Their long term effects on the environment are unknown of any residues Ց They may encourage a µthrowaway is OK¶ culture Ց They interfere with the present recycle program END Page 40 ... reaction mechanism consists of three stages: chain initiation chain propagation chain termination Chain initiation diacyl peroxide molecule as a initiator Page 18 Chemistry in Action (Polymers) ... joined by condensation reactions, two identical double Page 15 Chemistry in Action (Polymers) helix DNA molecules result Page 16 Chemistry in Action (Polymers) Synthetic Polymers Addition Polymers. . .Chemistry in Action (Polymers) Natural Polymers Amino acids and protein Introduction Amino acid are bifunctional compounds containing both the amino ( -NH2) and carboxylic