Download CHAPTER 2 – POLYMERS

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CHAPTER 2 – POLYMERS
SYNTHETIC ADDITION POLYMERS
 polymers are made up by connecting many small subunits called
monomers to form polymers (usually >10)
 the chemical process to achieve this is called polymerization
 the properties of polymers are varied and are determined by the
properties of the monomers involved, and of their functional
groups
 monomers can be linked together by C-C, C-O, or C-N bonds
 addition polymers are a result of addition reactions from adding
monomers containing unsaturated C-C bonds
Polyethylene (common name)
 made up of repeating units of ethene (polyethene)
 the double bonds in the alkene are changed into single bonds
freeing up a pair of electrons that can form a single C-C bond with
another monomer
 used for insulating wires and making plastic containers
Other addition polymers:
Polypropylene
 monomers of propene undergo addition reactions to form
polypropene (commonly called polypropylene)
 used in making rope and carpet
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Polyvinyl chloride
 commonly known as PVC
 formed by addition reaction of monomers of chloroethene
 used for insulation of electric wire, coating on raincoat fabric and
upholstery materials
Polystyrene
 formed when a benzene ring is attached to ethene (vinyl benzene)
 addition polymer of styrene is polystyrene
(see sample problem on page 102)
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Addition Polymerization Process
 3 stages in addition polymerization process;
1. initiation-an initiation molecule (e.g. a peroxide) with an
unpaired electron forms a bond to one of the C atoms in
the double bond-this causes the electrons to shift which
now has an unpaired electron at the other end of what
was the double bond-this unpaired electron is available to
form a covalent bond with another atom or group
2. propagation-chain grows as the electron shifts are
continuing on
3. termination-polymerization stops when 2 unpaired
electron ends combine forming a covalent that links 2
chains together
Properties of Plastics
 polymers of substituted ethene (or vinyl monomers) are generally
categorized as plastics
 plastics are used widely as containers for chemicals, solvents and
foods because they are chemically unreactive
 remember we are going from C=C to C-C
 plastics are generally flexible and mouldable solids or viscous
liquids
 forces of attraction are mostly van der Waals attractions with
some electrostatic attractions due to substituted groups
 van der Waals forces are numerous but individually weak so that
they can slide along each other making them flexible and
stretchable-can be softened & moulded by heating
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Effect of Substituted Groups on Polymer Properties
 e.g. Teflon is made up of tetrafluoroethene, F2C=CF2 , ethene
with all H’s substituted by F’s
 lack of C-H bonds and presence of C-F bonds make Teflon very
unreactive and therefore no “sticking” of foods will occur at high
temperatures
 Plexiglas is a replacement for glass
 produced by addition polymerization of an alkene monomer
containing a carboxymethyl group (-COOCH3)
 (-COOCH3) responsible for optical properties including
transparency
 Plexiglas can be damaged by organic solvents like acetone
because the carbonyl group in the monomer makes the polymer
soluble in other carbonyl groups (see Fig. 5)
Strengthening Polymers with Crosslinking
 monomers with >1 double bond have different possibilities and
properties
 1.3-butadiene is a monomer in several common polymers
 neoprene (wet suits) is an addition polymer of 2-methylbutadiene
(see Fig. 6)
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 the monomer used in neoprene is 2-chloro-1,3-butadiene
 the highly electronegative Cl atom makes neoprene very polar and
therefore less miscible with hydrocarbons
 gives the advantage of being more resistant to substances like oils
and gasoline than natural rubber
 having more than 1 double bond gives “dienes” the ability to have
other molecules in 2 locations rather than 1
 dienes can be incorporated into 2 separate polymer chains at one
time by joining them together by forming bridges between the 2
chains
 these bridges or “crosslinks” can be formed at different points in
the chains
 the more “crosslinks” the stronger the attraction between the 2
chains
 these links are formed by covalent bonding and are much stronger
than van der Waals that may hold the chains together
 crosslinked polymers form strong materials
 degree of crosslinking depends on how many of the monomers
are dienes, although they do not have to be the main ingredient of
the polymer
 selected diene monomers can be added as is necessary to produce
the desired properties in a polymer
 e.g. of a diene used in crosslinking is p-divinylbenzene (1,4diethenylbenzene)
 in this example each of the 2 vinyl groups have a double bond and
each double bond can be incorporated in a separate polymer chain
 here the main monomer is styrene and in the polystyrene polymer
shown below you can see the crosslinking of the p-divinylbenzene
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 the crosslinking monomer holds the chains together with covalent
bonds
 the relative concentrations of the 2 monomers determine the
degree of crosslinking and therefore the degree of rigidity in the
polymer
inorganic crosslinking agents can be used also
sulphur can be used to harden latex rubber from the rubber tree
Charles Goodyear accidentally discovered this
natural rubber is made up of the monomer 2-methyl-1,3butadiene (soft & chemically reactive)
 Goodyear found that when S is added with heat the rubber
becomes harder
 process called vulcanization
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 in plastic manufacturing the heat resistance determines how they
can be produced and used
 thermoplastics are plastics that can be heated and moulded taking
on a new shape
 these plastics are not crosslinked and are held together by weak
van der Waals forces
 thermoset polymers are those that are highly crosslinked and
therefore will not get softer with heat as the chains are held
together with strong covalent bonds
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Synthetic Condensation Polymers
 monomers that contain C=C form polymers through addition
reactions
 molecules with 1 functional group can react with one other
molecule to form a dimer
 to form a chain, molecules must be able to link at each end of the
molecule e.g. carboxylic acids react with alcohols to form esters
and with amines to form amides in condensation reactions
 when join end to end in ester or amide linkages, polyesters and
polyamides form
 these are what we call condensation polymers
Polyesters from Carboxylic Acids & Alcohols
 when a carboxylic acid reacts with an alcohol, a water molecule is
eliminated and an ester is produced (esterification)
 when this reaction repeats itself over and over we get the
formation of a polyester
 note how both molecules have reactive functional groups on the
ends
 the functional groups don’t have to be identical either
 Dacron (a clothing fabric) is made with the monomers p-phthalic
acid (1,4-benzene dicarboxylic acid) and the diol ethylene glycol
(1,2-ethanediol)
 resulting polymer is long and contains polar carbonyl groups at
regular intervals which helps hold the chains together making
them very strong
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Polyamides from Carboxylic Acids & Amines
 polyamides are polymers consisting of many amides
 amides are formed from the condensation reaction between a
carboxylic acid and an amine with the elimination of a water
molecule
 therefore the monomers must contain 2 functional groups: a
carboxyl groups, amine groups, or one of each
 an example is the production of nylon 6,6 (the 6,6 refers to the
number of C atoms in each of the monomers)
 nylon was made was used as a substitute for silk which has a
similar structure
 nylon was produced even more during wartime as it was used to
make parachutes, ropes, cords for airplane tires, etc.
 the amide groups are responsible for the strength of nylon
 when spun the polymer chains line up parallel to each other and
the amide groups make hydrogen bonds with the carbonyl groups
on nearby chains
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 another example of crosslinking in polyamides is found in Kevlar
 lightweight, stronger than steel and heat resistant Kevlar is used
in sports equipment, protective clothing for firefighters, bullet
proof vests for police officers, etc.
 Kevlar has a strong network of H atoms holding adjacent chains
together in a sheet-like structure
 “sheets” of Kevlar are stacked together and can be woven
together to become highly resistant to damage and can even “stop
bullets”
(see sample problem on page 110)
Read “Keeping Baby Dry with Polymers”
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Proteins – Natural Polyamides
 all living things are composed of large molecules which have high
molecular mass, are composed of polymers of somewhat similar
subunits
 4 groups of macromolecules are proteins, carbohydrates, nucleic
acids and fats & lipids
Amino Acids
 20 natural amino acids each with different R groups
 composed of an amine group and a carboxylic acid attached to a
central C atom
 simplest is glycine which has an H atom as its R group
 amino acids form the basis of proteins as each protein is a
polymer composed of many amino acids in sequence that is
specific to that protein
 only 20 amino acids make up all known proteins so there is a
limitless array of possible combinations
Chiral molecules
 molecules containing a C atom that has 4 different attached
groups can exist as 2 different isomers (mirror images)
 all amino acids (except glycine) can exist as either L and D
configurations
 natural amino acids appear in only 1 configuration designated as L
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Polypeptides from Amino Acids
 polypeptides are formed when 2 amino acids link together with
the amine group of one reacts with the acid group of the other
eliminating a water molecule
 this bond is called a peptide bond and these molecules are called
polypeptides
 proteins differ from polypeptides in that proteins can consist of
several polypeptide chains while proteins are long and flexible
which can form bonds to themselves or to other protein molecules
 using glycine and alanine as examples:
(see page 119 for a more detailed look at the 20 basic amino acids)
Read the section on page 120 on Artificial Sweeteners**
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Protein Structure
Primary Structure of Proteins
 proteins are polypeptides…long strands of amino acids arranged in
very specific order
 this is what’s called a primary structure of protein
 this order can be changed through a change or mutation in the
DNA which can cause the protein to be useless for the task it was
intended for
Secondary Structure of Proteins
 can be in an alpha-helix structure i.e. seen in alpha-keratin in hair
& nails, collagen in tendons and cartilage
 or the secondary structure can be in a pleated sheet (or betapleated sheet) i.e. seen in silk, protein in spider webs, used in
products like parachutes
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Tertiary Structure of Proteins
 tertiary structures of protein are very specific and related to the
role of the organism i.e. “antifreeze protein” in flounder off coast
of Newfoundland
Quaternary Structure of Proteins
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form when several protein subunits join together
found in regulatory proteins like insulin, etc.
best known is haemoglobin which has 4 protein subunits
interactions between subunits permit responses to changes in
concentration of the substance being regulated i.e. oxygen in
haemoglobin
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Denaturation of Protein
 when the bonds in secondary & tertiary structures are broken and
the protein loses its 3-D structure, process is called denaturation
 cooking, change of pH, organic solvents are some of the causes of
denaturation
 cooking can disrupt van der Waals forces and H bonds (cooking of
fish)
 changing pH affects electrostatic forces and disrupts H bonding
(curdling of milk in orange juice or vinegar
 even mild denaturation of a protein is accompanied by severs loss
of function
Starch and Cellulose – Polymers of Sugars
 natural polymers belonging to a group of compounds called
carbohydrates
 sugars (monosaccharides or disaccharides), as well as, starches
and cellulose (polysaccharides composed of sugar monomers)
 carbohydrates are a major source of energy-from plants mostly
i.e. cane sugar, beets, starch from potatoes, etc.
 differences are in molecular size and shape, sugars being smallest
and starches and cellulose being polymers of glucose (simplest
sugar)
Monosaccharide Sugars
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all carbohydrates have the empirical formula C6(H2O)6
name is derived from “hydrated carbon”
can undergo complete combustion to produce CO2 and H2O
usually chains of C’s with functional groups attached to the C’s
i.e. 1st C may have a C=O forming an aldehyde or on the 2nd C
forming a ketone, etc. other C`s may hold OH groups
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 glucose is monomer used to form larger carbohydrates
 due to its aldehyde group is part of sugars called aldoses
 fructose has a ketone group and is part of group of sugars called
ketose (found in fruits-sometimes called fruit sugars)
 both glucose & fructose are single sugar units and are called
monosaccharides
 OH groups and C=O on same molecule can provide for formation
of ring structures
 the C=O group will react with the OH group to form an oxygen
link which results in a 5 or 6 C ring
 a straight chain sugar is flexible but once a ring structure is
formed there is no longer any free rotation of any attached
group or atoms
 the ring is not flat but is said to have a “chair” conformation
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 functional groups can be “above” or “below” the ring and the
orientation of these groups determines the orientation of further
bonding
Disaccharide Sugars
 glucose monomer + fructose monomer = a disaccharide called
sucrose ( sugar we use for cereal or coffee)
 lactose is also a disaccharide (glucose + galactose)
 when we “eat” a disaccharide, enzymes break down the dimer into
its constituent monomers by breaking the bonds holding them
together
i.e. lactase breaks down lactose and people who don’t have this
enzyme are “lactose intolerant”
 sugars are soluble in water (think pop) and have a high melting
point due to abundance of OH groups which means H bonding
with other sugar molecules and with water molecules (see Table 1
on page 126)
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Starch for Energy; Cellulose for Support
 starches are polymers of glucose either branched or un-branched
and are also polysaccharides
 glycogen in animals is also starch-like
 humans can’t break down cellulose which is a straight chain rigid
polysaccharide with a different kind of glucose to glucose bond
 provides support and structure for plants
 difference lies in the orientation of functional groups that are
attached to C atoms
 in starch & glycogen, glucose monomers are added at angles that
lead to a coiled structure, maintained by H bonds between OH
groups on the same chain
 in cellulose, glucose monomers are added to produce linear chains
that align side by side, which helps bonding between chains
 these bonds between chains provide a rigid structure of layered
sheets of cellulose, which also make them insoluble in water.
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Nucleic Acids
 includes DNA, deoxyribonucleic acid
 its function is to put amino acids in the correct order before
peptide bonds form between each pair
 DNA must code for 20 amino acids and so uses a system where 4
different nucleotides (monomers of the large DNA polymer) are
used
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 RNA (ribonucleic acid) is another type of nucleic acid, similar to
DNA and serves an important role in protein synthesis
 mRNA carry genetic code from DNA and tRNA bring amino acids to
the site where the amino acids are aligned in proper sequence
 3 parts of the nucleotide are; a phosphate group, a 5-carbon
sugar called ribose and a nitrogenous base which contains an
amino group
 the nucleotides differ only in the composition of the bases:
adenine, thymine, guanine & cytosine
 in Fig. 2(b), the sugar is called deoxyribose because it is missing
an “O”, due to an OH group being replaced by an H atom
 condensation reactions combine these nucleotide monomers to
form polynucleotides called nucleic acids
 the condensation reaction occurs between the –OH of the
phosphate group of one monomer and the ribose of another
monomer
 the overall shape is staggered giving it a helical shape
 DNA is also known to be a double helix
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 The 2 chains are held together by pairs of amine bases one from
each DNA chain which H bond
 cell replication is also a result of this paired arrangement where
the chain is “unzipped” and a reverse copy of DNA is made
 i.e. if DNA has the nucleotides AGCT the reverse copy would be
TCGA
Denaturation of DNA
 heat and pH change can denature DNA as they can proteins
 H bonds can cause the 2 strands in DNA to unwind and separate
but if the DNA is cooled this can be reversed
 mutations in DNA can occur spontaneously or can be accelerated
by chemical agents
 even a minor error in the DNA sequence can lead to an incorrect
protein being made or no protein being made at all which can lead
to a major malfunction in the organism
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Fats and Oils
 fats – solid at room temperature
 oils – liquid at room temperature
 fats & oils are triglycerides: formed by an esterification reaction
between glycerol (an alcohol) (Fig. 2) and long-chained carboxylic
acids called fatty acids (Table 1)
 belong to class of compounds called lipids
 the fatty acids can be identical (simple triglyceride) or nonidentical (mixed triglyceride)
 most fats & oils are mixed triglycerides
Fatty Acids
 can be 4-36 carbons long
 generally unbranched, can be saturated or unsaturated and have a
carboxylic acid group at one end
 very similar to hydrocarbons and burn as efficiently
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 when fatty acids “burn” they release great amounts of energy
i.e. equal to burning of fossil fuels and greater than an equal
mass of carbohydrates
 lipids which can metabolize into fatty acids are an efficient form of
energy storage
Triglycerides
 simplest example is palmitin, which has 3 identical fatty acids,
found in most fats & oils like butter, lard, olive oil, etc.
 formed by combining palmitic acid and glycerol
 another example of a simple triglyceride is stearin which has
stearic acid (CH3(CH2)16COOH) as its fatty acid
 just as esters can be broken down (hydrolyzed) into their alcohols
and acids, so can triglycerides
 when triglycerides are hydrolyzed with NaOH, we get glycerol and
the Na salt of the fatty acids
 these salts are what we call soap and this process is called
saponification
 soaps are effective because they have a polar “head” and a nonpolar “tail”
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Structure and Properties of Fats and Oils
 insoluble in water, due to the nonpolar nature of the triglyceride
molecule i.e. all the C=O and OH groups are bound in the ester
linkages while the fatty acids contain long nonpolar hydrocarbon
chains
 these chains also determine the physical state of the fat or oil and
the shape determines how closely the molecules can be packed
together which affects their melting points
 when saturated hydrocarbons are present, the chain can rotate
around the C-C bonds, chain is more flexible, allows chains to pack
together more tightly, increasing van der Waals forces therefore
increasing the thermal energy needed to separate the saturated
fatty acids (leaving these triglycerides solid at 25°C
 unsaturated hydrocarbons can’t rotate around the double bond
and this restriction causes a “bend” in the molecule in the –cis
configuration, so that these molecules can’t pack together tightly,
results in weaker van der Waals forces and these unsaturated
fatty acids are generally oils
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 vegetable oils contain more unsaturated fatty acids than animal
fats and are called polyunsaturated compounds
 when oils (like canola or corn) are used to make margarine, the
oils are hydrogenated (H2 added across the double bonds to make
single bonds)
 increased saturation of the hydrocarbon chains leads to a
decrease in bending of the chain which results in closer packing
changing the liquid oil into a solid
 the high caloric value of fats & oils make them a good energy
source but their poor solubility in water makes them less easily
converted to energy than carbohydrates
 this is why bears can hibernate in the winter, with CO2 and water
being produced by the “burning” of fat
 also why camels can cross the desert using energy and water
released from the stored fat in their humps