Download 1-16 polymers

SCH 4UK
Organic
Polymers. The word comes from the Greek, poly meaning many and mer meaning parts. Many polymers
are synthetic, i.e made by humans, but other polymers are natural. Examples are starch, amber, tortoise
shell, cotton, silk, natural rubber, wool etc.
The focus of this unit is on polymers that are plastics. Although most chemists would classify plastics into
the categories of thermoset and thermoplastic polymers. The former cannot be remelted, the latter can. We
will classify polymers by the reactions that produce them rather than their properties.
ADDITION POLYMERS. Made from alkenes, a.k.a. olefins.
Addition polymers are made by linking many molecules together. All atoms present in the monomer are
retained in the polymer.
H
H
C C
e.g. polyethylene. Recall that ethylene is the trivial name for ethene. The monomer is ethene,
H
H
H
H
C C
H
H
+
H
ethene
H
H
C C
H
ethene
C C
+
H
H
H
+
H
ethene
...
C
C C
C
C C
or
C
C
n
polyethene (polyethylene)
The reaction can be initiated by free radicals, and may continue until thousands of ethene molecules have
joined the chain. Ultrahigh molecular weight polyethylene can have M of 3 000 000 g/mol. That would
correspond to more than 100 000 monomers linked.) Although poythene is fairly inert, biodegradability can
be built in by increasing the number of double bonds ( add some butadiene), which gives microbes
somewhere to chew.
Uses of polyethylene: many hundreds of grades of PE are available which differ in their properties in one
way or another. The properties mostly relate to M and degree of branching. Branches prevent the polymer
chains from packing together regularly and closely and have a predominant effect on the density of PE. The
highest density for this family of polymers is about 0.98 g/cm3 Stiffness, hardness, tear strength, melting
point etc. are of interest to the person using the PE. But the general non-reactivity of all polyethylenes
relates to the strong C-C bonding. They are unreactive in the same way that alkanes are. The forces between
the long chains are weak, which allow the chains to slide over one another. This allows polyethylene to be
flexible. An increase in temperature will also increase flexibility. Above about 110°C polyethylene has
softened so much, that it limits its applications.
Polyethylene has been produced commercially since 1943. It is used in manufacturing flexible bottles, films,
sheets, and insulation for electric wires.
Low density polyethylene: Density is about 0.915 to 0.935 g/mol. Made at 150 to 350 MPa’s (1500 to 3000
times atmospheric pressure) and between 80ºC and 500 C. The chains formed here have branches that give
the polymer low density. The branches prevent the main chains from lying close to one another, decreasing
density and inter-chain attraction. The main use of LDPE is in the manufacture of the tough transparent film
that is used in packaging and grocery bags. (5 billion kg of LDPE are made /a in the US alone)
HDPE is made from non-branching Polyethene. Major use is for tougher containers which do not require
heat resistance, e.g. plastic juice bottles, shampoo bottles…
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Other Addition Polymers: Any alkene can undergo addition, and most will polymerize.
Polypropene, a.k.a. polypropylene: Used in ropes and carpet. It is stiffer and harder than high density
polyethylene and has a higher melting point too.
Propene polymerizes the same way that ethene does, and can be thought of as methylethene.
H
H
C C
H
H
+
CH3
C C
H
H
H
C C
+
CH3
H
H
+
...
CH3
CH3
CH3
C C C C C
CH3
CH3
CH3
Polypropylene is actually denser and harder that Polyethene because the methyl groups can be made to occur
on the same side of the chain (isotactic) and then the chains pack together quite closely making the polymer
more crystalline and therefore stronger.
If the ethene has a chlorine substituted instead of a methyl, the product of polymerization will be polyvinyl
chloride, or PVC. PVC is used for electrical wire insulation, a fabric coating and for raincoat and upholstery
materials, sewage pipes and medical equipment, as it is non-flammable. Unfortunately the monomer is a
carcinogen.
Styrene is ethene with a benzene ring attached, vinyl benzene. Its polymer is polystyrene. Pure polystyrene
is quite brittle. You may know it as the plastic often used in disposable drinking glasses.
If the substituted group is OH, i.e. the monomer is ethenol then the polymer is polyvinyl alcohol, a
compound used in hair spray and styling gels as well as contact lens wetting solutions. In actual practice,
the monomer is ethenoate which is polymerized and then hydrolyzed to form the alcohol.
If the substituted groups are all F, so the monomer is tetrafluoroethane, then the polymer is Teflon. Teflon
derives its properties from the fact that the F-C bond is even stronger than the H-C bond. Teflon is even less
reactive than polyethylene. The chains are able to pack closer together, so the plastic is denser and also has a
higher softening temperature.
Addition Polymerization has three main steps, initiation (starting) propagation (letting it go) and
termination (stopping)
Initiation starts often with a free radical, a molecule with an unpaired electron, forming a bond to one of the
carbon atoms in the double bond of a molecule. The electrons shift in the newly bonded molecule, which
now has an unpaired electron at the other end of it original double bond. This unpaired electron is now
available to form another covalent bond with another atom or group. This repeats and the chain
“propagates”. When any two unpaired electrons ends combine and form a covalent bond that will link two
growing chain ends together, the growth “terminates”.
Demo: 6 volunteers
smile=sigma bond. Arms above and below the smile are the two halves of the pi bond.
bond breaks open. One arm (electron) to the bond to the right, the other to the bond to the left.
Propagate the r’n
Plastics: Not all polymers are plastics. What is meant by the term? A plastic is a synthetic substance that
can be moulded (often under heat and pressure) and that then retains its given shape. The forces of
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attraction between the long polymer molecules are largely London’s forces with some electrostatic
attractions (dipole-dipole) due to substituted groups. The chains may be thousands of atoms long.
Cross linking is a reaction that can occur e.g. when dienes polymerize. This means the monomer can grab
with three hands, not just two. When a chain links with another chain in its middle, this is called cross
linking. It has the effect of making the polymer stronger. (See text pg 104/105)
Homework: pg 107 # 1-6
As well as Addition polymers, there are Condensation polymers. We have seen condensation reactions
before, when two alcohols combined to form an ether, when an alcohol and acid formed an ester, and when
an acid and an amine combined to form an amide.
If a diol and a diacid react, then there are two ‘business’ ends to each molecule, and the reaction takes place
at both ends, and can propagate.
H
HO C
H
H
O
R' C OH
O
C R C
+
HO
H
H
+
HO C
OH
HO C
H
R' C OH
H
O
H
H
H
H
O
C R C
R' C O
H
H
O C
H
R' C OH
H
+ H2O
H
The formation of a polyester.
Another condensation polymer involves the reaction between a diacid and a diamine.
O
O
H2N R' NH2
+
C R C
O
C R C
HO
O
+
OH
H2N R' NH2
H2N
R' N
H
N R' NH2 + H2O
H
The product is a polyamide.
Nylon is probably the most famous example of this type of polymer. It was invented to replace silk during
the 30’s when US relations with Japan were becoming difficult. The name is a contraction of New York and
London.
Another well known polyamide is Kevlar, invented in 1962, and first marketed in 1971. It is used because it
is stronger than steel, heat resistant and lightweight enough to wear. Kevlar® has dozens of important
applications, including radial tires and brake pads (a replacement for asbestos), racing sails, fiberoptic cable,
water-, air- and spacecraft shells, and mooring and suspension bridge cables. It is now used to make skis,
safety helmets, tennis rackets, trampolines and hiking and camping gear. (Incidentally it was invented by a
woman, Stephanie Kwolek, who wanted to go medical school but did not have the funds to do so.)
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A single Kevlar polymer chain could have anywhere from five to a million segments bonded together. Each
Kevlar segment or monomer is a chemical unit that contains 14 carbon atoms, 2 nitrogen atoms, 2 oxygen
atoms and 10 hydrogen atoms. The monomer is
H
O
H N
N C
H
O
C OH
poly(p-phenyleneterephtalamide) Text pg. 110
A group of polymer chains can be organized in a fiber like a plate of spaghetti, or a box of spaghetti. You
can put the polymer chains together randomly in a pile or you can orient them neatly side by side in a row.
It turns out that the orientation of the polymer chains is very important to certain properties of polymers such
as flexibility, rigidity, and strength. Which orientation would you guess would be stronger when made into a
fiber? answer, oriented like the uncooked spaghetti. The individual polymer strands of Kevlar are held
together by hydrogen bonds that form between the polar amide groups on adjacent chains. Also, the aromatic
components of Kevlar polymers have a radial (spoke-like) orientation, which gives a high degree of
symmetry and regularity to the internal structure of the fibers. This crystalline-like regularity is the largest
contributing factor in the strength of Kevlar.
Kevlar is very strong even when compared to other synthetic polymers.
Polymer
Kevlar
Rayon
Nylon 66
breaking strength in g/denier
20-30
1-2
3-10
Denier is a measure of linear density of yarn, the weight in g of 9000 metres of yarn eg: 80 Denier means
that the weight of 9000 metres of the yarn is 80 g.
Proteins: Natural Polyamides.
Proteins make up about half the dry mass of our bodies. Muscles, skin, cartilage, tendons, nails and hair are
all made of protein molecules. And these proteins are made from about 20 different amino acids.
Amino acids are compounds in which an amino group and a carboxyl group are attached to the same carbon
atom.
H NH2
C
O
R
C
HO
Digress to explore chiral molecules.
Use model kits with C attached to four distinct ligands.
All the amino acids except glycine can exist in two different configurations.
Why can’t glycine be optically active?
H
O
H2N C C
H
H
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A peptide bond is the bond formed when the amino group of one amino acid reacts with the acid group of
the next. (We saw this earlier as an amide) This is a condensation reaction which also produces water. If
the starting materials are a just two amino acids the product is a dipeptide. If the product is a polymer made
of many amino acids joined with peptide bonds, it is a polypeptide or protein. It is the structure of the
protein that dictates what it will do biologically.
Primary structure- the sequence of the monomers
Secondary Structure-the three-dimensional organization of segments of a polymer chain (coils/helices or
pleated sheets)
Alpha-helix-a right-handed spiraling structure held by intramolecular hydrogen bonding between groups
along a polymer chain.
Pleated-sheet conformation a folded sheet like structure held by intramolecular or intermolecular hydrogen
bonding between polymer chains.
3 structure-helical section and pleated sheets within the same molecule attract each other, twisting the
ribbon into a specific shape. see pg.122 of Nelson.
Denaturing of Proteins. Any reaction that changes the structure of the protein will affect its ability to do its
work. Changing pH can break the hydrogen bonding, e.g. hair perms or curdling of milk with OJ.
Starch and cellulose, polymers of sugar
All carbohydrates have hydrogen and oxygen in the ratio of 2:1, the same as for water; hence the name of
carbohydrate. All sugars contain a carbonyl group as a ketyl or aldehyde group, and two or more OH groups.
Glucose and Fructose are both 6 carbon sugars; glucose is an aloes and fructose is a ketoses. These sugars
usually form ring structures. The cyclic forms are hemiacetals formed by an intramolecular reaction of the
OH group at C-5 with the aldehyde group.
As an aside: In an attempt to find a more natural sugar substitute, the chlorinated sugar that is now named
Sucralose was studied. It was actually synthesized in the hopes that it would be a good insecticide!! see
overhead comparing the structure of sugar and Sucralose.
H H H OH
O
HO C C C C C C
H OH OH H OH
C
C
C
C O
C
H
C O
C
C
C
C O H
C
C O
H
When two cyclic sugars join, and release also a water molecule, a disaccharide is formed. e.g. glucose +
fructose = sucrose.
Sugars have high melting point and high solubility in water. Both properties are due to the hydrogen
bonding.
Glucose can form a polymer as the result of hydrolysis occurring on two places on each sugar. This polymer
is starch. Cellulose is also a polymer of glucose. Both starch and cellulose can be considered to be
condensation polymers of glucose. But starch and cellulose have such different properties. How can we make
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two very different polymers out of the same monomer? Depending on how the two molecules are oriented,
this is called an alpha-1,4 or a beta-1,4 bond. Starch is made up of alpha-1,4 linked glucose units; cellulose
of beta-1,4 linked glucose units. And that's basically the only difference between them.
DNA: Deoxyribonucleic acid. The basic structure is phosphate-ribose (a sugar)- nitrogenous base. The
base could be adenine, thymine, guanine or cytosine. The linking and structure within the DNA double helix
depends hugely upon hydrogen bonding.
Structure of Proteins.
The primary structure is a result of the sequence of amino acids in the polypeptide chain.
The secondary structure is the alpha helix and results from hydrogen bonds between the amine group and
the carbonyl of an amino acid further along the same chain. (The R groups of the amino acids protrude
outward from the coil.
Tertiary structure may be in the form of pleated sheet sections within the molecule or helical sections.
These sections attract each other, within the molecule, folding a long twisted ribbon into a specific shape.
Shape is critical to enzymes, haemoglobin and hormones.
Quaternary Structure is the result of two or more protein subunits joined by van der Waals forces and
hydrogen bonding between protein subunits. e.g. haemoglobin has four subunits held together in a roughly
tetrahedral arrangement.
Fats and Oils.
Chemically, fats and oils are triglycerides, esters formed between the alcohol glycerol (1.2.3-propanetriol)
and long chained carboxylic acids, called fatty acids. If the hydrocarbon chain in the fatty acid is saturated
with hydrogen, there is freer rotation of the carbon-carbon bonds, and the chains pack tighter, giving
maximum van der Walls interaction (recall that forces decrease with the inverse of the square of the
distance). Thus saturated fats are more apt to be solids that unsaturated ones.
Camels use the fat in the hump to act as a source of energy, and also as a source of water! They use the
combustion product!
What is the fuss about trans fats?
What are trans fats?
H
R COOH
R
C C
R
R COOH
C
H
trans fat
C
H
H
cis fat
The double bond cannot rotate. The trans fats will be more in a straight chain, rather than the more kinked
shape the cis bond will impose. Thus trans fats can pack more tightly, be more dense and thus clog arteries
when plaque is formed. It is believed that biologically, trans fats are not good for you.
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