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Chemistry 1002 Chapter 14Polymers |
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Introduction
Polymers are composed of enormous molecules containing
anywhere from thousands to heptillions (1024)
of atoms per molecule. Molecules which are this large are called
"macromolecules."
Some macromolecules have a variety of different types
of atoms connected together willy-nilly without much apparent
order (coal).
Some macromolecules made of only one or two types of
atoms which are bonded together in simple exact 3-D crystalline
structures (diamond or quartz).
Polymers are macromolecules which have situation in
between simple crystals and haphazard structures.
Although they may contain millions of atoms, these
millions of atoms are typically organized as repeating patterns
of smaller groups of atoms, known as "monomer units,"
which are all covalently bonded together one after another like
railroad cars in a train.
Monomer units themselves typically contain a group
of from two to fifty atoms bonded to one another in well-defined
covalent structures.
If you look at the structural formula for a molecule
and notice same pattern of atoms and bonds repeats itself over
and over again, then you are looking at the structural formula
for a polymer.
Plastic, skin, muscle, rubber, starch, DNA, nylon,
cotton, and dacron are all polymers; some are man-made and some
are "natural."
Organic Chemistry
In order to understand polymers (both natural and man-made)
it is first necessary to understand a little more about organic
chemistry. Most polymers are organic chemicals (especially natural
ones).
Organic chemicals are chemicals with molecules (or
macromolecules) which are made of carbon atoms covalently bonded
to other carbon atoms (catenation) and/or several other kinds
of atoms. The atoms which are found in organic (macro)molecules
live in the upper right portion of the Periodic Table (the "nonmetals"),
plus hydrogen.
Fossil fuels are all organic chemicals.
Almost all organic chemicals made from fossil fuels
(exceptions: small quantities complicated drugs sometimes obtained
from plants or bacteria).
In recent past virtually all manmade organic chemicals
obtained from petroleum ("petrochemicals"). Today,
however, technology exists for making complete set of organic
chemicals from coal (via gateway chemical methanol).
Fossil fuel first broken down with catalysts and heat
("cracked"), then converted to alkenes & alkynes
(petroleum), or aromatics (coal or petroleum) with appropriate
catalysts.
These "feedstocks" then turned into blistering
array of different organic chemicals by techniques known as "organic
synthesis."
In past light alkene & alkyne feedstocks only made
from petroleum. Now make from coal via methanol or methane (coal
gasification).
VALENCES
The valences (number of attached bonds) of each of
the common organic atoms are given in table below.
| Atom | Attached Bonds
|
| |
| B
| 3
|
| Al
| 3
|
| C
| 4
|
| Si
| 4
|
| N
| 3
|
| P
| 3 or 5
|
| O
| 2
|
| S
| 2, 4, or 6
|
| F
| 1
|
| Cl
| 1
|
| Br
| 1
|
| I
| 1
|
FUNCTIONAL GROUPS
Certain particular combinations of atoms are found
so often in organic chemicals that they are given special names.
The structures of these "functional groups" and names
of classes of molecules containing them given in Table 14.3 on
pg. 415.
The beauty of functional groups is that over the centuries
organic chemists have figured out how to use inorganic
chemicals (don't need to obtain from fossil fuels) to interconvert
them into one another.
Double bonds, triple bonds, and aromatic rings (not
shown on table) are functional groups. Once you have created
these from fossil fuels you can go wild and make anything you
want. The science of interconverting functional groups into one
another is known as organic synthesis.
ALPHA CARBON
Any organic molecule is simply a hydrocarbon chain
(linear, branched, or containing rings) with functional groups
attached at various places. The carbon atom of a hydrocarbon
chain to which functional group is attached is called an alpha
carbon. Alpha carbon is not part of functional group
(ie. carboxylic acid or alkene carbons).
Hydrogens attached to an alpha carbon are called alpha
hydrogens.
Functional groups often classified according to how
many hydrocarbon branches are attached to their alpha carbon.
If alpha carbon is attached to one other carbon (not including
functional group carbons), functional group and alpha carbon called
primary (1_). If alpha carbon
has two carbon neighbors term secondary (2_)
used; three neighbors called tertiary (3_).
PROPERTIES & REACTIONS OF SOME FUNCTIONAL GROUPS
Reactions of Hydrocarbons
Addition. Must have multiple bonds. Two molecules
combine to make one molecule. Triple forms double and double
forms single bond.
Substitution. Two molecules react, rearrange fragments,
and form two different molecules. Parts of original molecules
"swap partners."
Combustion. Hydrocarbon molecule reacts completely
with oxygen during combustion (burning) to make several molecules
of carbon dioxide and water.
Properties & Reactions of Alcohols
Alcohols classified as primary, secondary, or tertiary
according to how many carbon neighbors their alpha carbon has.
Methanol (CH3OH) is a 0_
alcohol because its alpha carbon has no carbon neighbors. Ethanol
(CH3CH2OH) is a primary
alcohol.
Idiosyncratic terminology involving alcohols: "proof"
ratings, "denaturing," and commonly-used names of ordinary
every-day alcohols. Other issues: hydrogen bonding (very important)
and organic chemistry of alcohols.
Proof Ratings.
Ethanol and water mixture (booze) will burn if the
amount of ethanol in the mixture is higher than 50%. Back before
turn of century flammability used as "proof" of strength
of booze.
Pure ethanol is what proof?
Since 50% ethanol will burn, it is said to be "100
proof." Therefore the proof rating is simply double the
purity. 100% ethanol is 200 proof.
Denaturing Alcohol.
During prohibition in the US ethanol could not be made
completely illegal because it was useful as a solvent for industrial
and research purposes. In order to discourage people from drinking
lab alcohol back then the government required that all lab alcohol
be contaminated with enough methanol to make people go blind if
they drank it (the origin of the phrase "blind drunk").
Since it is the "nature" of ethanol to make you drunk
without losing your eyesight, the process of contaminating lab
ethanol with methanol was called "denaturing" it. "Denatured"
ethanol can still be found but not very common now.
Common Names of Alcohols.
Origins of common names: Wood alcohol (methanol) originally
distilled from hardwood, ethylene glycol (1,2-ethanediol) alcohol
made from ethylene which tastes sweet ("glyco" usually
refers to sugar, ie. "glycolysis), grain alcohol (ethanol)
originally distilled from fermented grain, rubbing alcohol (2-propanol)
rubbed on body to kill bacteria and ease fever, glycerol (1,2,3-propanetriol)
like (ethylene) "glycol" but needed modified word.
Hydrogen Bonding in Alcohols.
A hydrogen bond is a weak temporary bond formed between
a hydrogen atom attached to nitrogen, oxygen, or fluorine (electronegative
atom which sucks away electron material from hydrogen leaving
it positive charged and hungry for electrons) and a nonbonding
pair of electrons belonging to a different nitrogen or oxygen
atom.
A water molecule can form two hydrogen bonds because
it has two hydrogen atoms capable of participating in this type
of bonding.
Most alcohols can form only one hydrogen bond. Important
exceptions:
When we talk about how many hydrogen bonds a molecule
can form we are talking about how many hydrogens it has capable
of being "donors."
The more hydrogen bonds a molecule can form the more
strongly it sticks to water & other molecules like itself.
Alcohols having less than 4 carbons are all totally
water-soluble because the alcohol molecules stick well to water
molecules.
Alcohols with 4 or more carbons become less and less
water-soluble because their hydrocarbon parts are "oily"
(oil is hydrocarbon and oil and water don't mix).
Balancing act between "hydrophobic" carbon
fragment and "hydrophilic" hydrogen-bonding OH piece
determines water solubility. Soluble as long as there are at
least 1/3 as many OH's as C atoms.
Sucrose (table sugar) is water-soluble even though
it has 12 carbons because it has 8 OH groups capable of forming
hydrogen bonds.
The more hydrogen bonds a molecule can form the stronger
this hydrogen bonding is and the harder it is to separate these
molecules from one another. More heat has to be added to change
substance whose molecules stick together tenaciously from liquid
into gas (high boiling point).
Hydrogen bonding in alcohols makes alcohols good anti-freeze
agents. Liquids contaminated with very soluble impurities are
hard to boil or freeze. The most effective anti-freeze agents
are nonvolatile (ie. salt) and have low molecular weights.
Although table salt is better at keeping water from
freezing or boiling than most alcohols (low molecular weight and
completely nonvolatile), brine solutions are corrosive to metal
surfaces (engines) whereas alcohol solutions are not.
Ethylene glycol (1,2-ethanediol) is the best compromise
for an alcohol antifreeze. It can form two hydrogen bonds which
gives it a high boiling point (nonvolatile) and it has a reasonably
low molecular weight (62 g/mol vs. average molecular weight of
about 29 g/mol for the ions in table salt).
Organic Chemistry of Alcohols.
For this course need to know oxidation of alcohols,
hydration of alkenes to make alcohols, and reaction of alcohols
with carboxylic acids to make esters.
Water (HOH) is byproduct of this substitution reaction.
The majority of substitution reactions you will see in remainder
of this course involve two molecules coming together to form a
third molecule after a molecule of water is removed. Esterification
reaction is merely one example of this.
Aldehydes and Ketones
Aldehydes and ketones not as important as alcohols
in fuel chemistry, making polymers, and biochemistry. Important
thing to understand is difference between them.
Both aldehyde and ketone molecules contain a molecular
fragment called a "carbonyl group" which made from an
oxygen atom connected to a carbon atom with two bonds.
Remember that oxygen likes two bonds and carbon likes
four bonds. Oxygen is satisfied here but carbon needs two more
neighbors.
If both neighbors are carbons, molecule is a "ketone."
If neighbors are C & H or H & H, molecule is an "aldehyde."
Carboxylic Acids
Like aldehydes and ketones carboxylic acid molecules
also have carbonyl groups. If one of carbonyl carbon's two other
neighbors is hydroxide (OH) then molecule called "carboxylic
acid."
Unlike aldehydes and ketones carboxylic acids very
important in polymer chemistry and biochemistry.
Two most important reactions to learn involving carboxylic
acids are conversion of carboxylic acids into esters and
amides.
Reactions which make esters and amides from carboxylic
acids are substitution reactions which involve creation of water
molecules as byproducts.
Substitution reactions used to make polymers called
"condensation" reactions ("condensed" milk
is milk which has had water removed from it).
Esters, Amides, and Anhydrides
These are three more classes of organic compounds which
contain the carbonyl group. They have the atom connection patterns
shown below.
ORGANIC CHEMICALS FROM COAL
Show Fig. 14.3
Polymers
Remember that polymers are made of repeating "monomer
units" (small molecular fragments) linked together one after
another like railroad cars in a train.
CATEGORIZING POLYMERS
there are three different ways of categorizing polymers:
The kind of chemical reaction used to make them, the manufacturer
(man or nature), and the kind of structure the polymer has.
Chemistry.
Polymers can be made by three general types of chemical
reactions.
"Addition" polymers are made by combining
monomer molecules via addition reactions. All of the atoms originally
present in the monomer molecules retained in polymer (2+2=4).
"Condensation" polymers (involves reduction
of total number of atoms) are made by substitution reactions.
When a monomer molecule reacts with a developing polymer
molecule a removable fragment attached to monomer leaves along
with a removable fragment attached to growing polymer. This leaves
both monomer and growing polymer bereft of partners; they attach
to one another.
Condensation polymers give small-molecule byproducts
(two removed functional groups combined) when synthesized (8+12=17
polymer atoms).
"Rearrangment" polymers are made when a monomer
molecule attaches to a growing polymer molecule and then one or
more atoms attached to the polymer decide they are unhappy where
they are and have to jump around to different locations on the
polymer before the situation can stabilize.
PROBLEM 41.
What common to all condensation rxns?
@ A small molecule is eliminated.
Manufacturer
Polymers are called "synthetic" if they are
made by man and "natural" if they are made by nature.
Man often makes identical polymers to nature. In this case the
synthetic polymers are indistinguishable from the natural polymers.
Nature uses the same three basic kinds of chemical rxns outlined
above to make polymers that man does.
Structure
Polymers are categorized according to structure by
two different criteria, molecular shape and arrangement
of monomer units
Molecular Shape.
Polymers whose monomer units are attached to one another
like railroad cars have a shape like very long strands of spaghetti,
or like string. Polymers with this kind of structure called "linear"
polymers.
Another kind of architecture exists for polymers.
Many polymers built like lots of separate strands of string laid
side by side and then connected together at multiple places to
give structures looking like fishing nets. These polymers said
to be "crosslinked." Connections between strands =
"crosslinks."
Arrangement of Monomer Units.
Polymers made of only one kind of monomer unit called
"homopolymers" ("homo" means same).
Polymers made up of two or more different kinds of
monomer units called "copolymers" ("co" means
in collaboration with).
ADDITION POLYMERS
"Addition" polymers made by addition reactions.
Monomers are alkenes (double bond necessary). Total number
atoms in polymer can be calculated by simple addition of
atoms in monomers.
PROBLEM 24.
What structural feature do all addition monomers share?
@ They are all alkenes
Show Table 14.10
In general a monomer must have at least two reactive
sites (places where covalent bonds can form) in order to be able
to become part of a polymer. This because each monomer must attach
to two other monomers (one in front and one behind) to become
part of polymer. Polymers have monomer units arranged like railroad
cars in a train.
One C-C double bond contains two reactive sites (one
at each C atom). This because when second C-C bond in alkene
is broken each carbon can is now free to bond to another alkene
monomer unit (M).
Remember: carbon has 4 "sticks"!
Addition Polymerization Mechanism
First an unstable reactive molecular fragment is created
which has one atom which is devoid of a partner (doesn't have
enough bonds to neighbors to be happy). This fragment either
a "free radical," "cation," or "anion"
based on the charge held by the unhappy atom.
Next the unhappy atom finds the alkene part of an addition
monomer molecule. It breaks the second bond of the C-C double
bond, attaches itself to one of the two carbons, and leaves the
other carbon devoid of a partner (unhappy).
The unhappy carbon atom finds the alkene portion of
a second monomer molecule, breaks the second bond of the C-C double
bond, attaches itself to one of the two alkene carbons, and leaves
the second carbon to fend for itself (unhappy). This process
repeats self indefinitely till polymer created.
Read text page 428.
Recognizing Addition Polymers
If main chain of polymer contains only catenated carbon
atoms polymer was made by addition reaction. If main chain contains
other atoms (O, N, Si, P) then polymer made by substitution reactions
and is condensation polymer.
To figure out what monomer an addition polymer was
made from first draw a box around any two carbon atoms in the
main chain and include any functional groups attached to either
of these carbons. Next erase everything outside of the box you
drew. Finally make a double bond between the two main-chain carbons.
Remove the prefix "poly" from the polymer name to make
monomer name.
PROBLEM 34.
Determine monomer Orlon made from.
@ "Orlon" is a trade name for polyacrylonitrile.
CROSSLINKING
Polymers that have many crosslinks between linear chains
result in a structure resembling either a fishing net, or, more
often, a 3-D crosslinked "honeycomb" structure which
more closely resembles a block of Swiss cheese.
Process of crosslinking polymers started with "vulcanizing"
of rubber discovered accidentally by Goodyear in 1839 (p. 434
of text).
In order for a polymer to form crosslinks at least
one of the monomers used to make it must have more than two reactive
sites. Two reactive sites per monomer are needed like couplers
on railroad cars to produce an linear polymer with no crosslinks.
Extra reactive sites from different linear chains can be connected
(forming crosslinks) bringing many different linear chains together
in crosslinked structure.
RUBBER
Rubber is an addition polymer made from a monomer called
"isoprene."
Notice that isoprene has two double bonds and it only
needs one to form a linear polymer. When this monomer is polymerized
the carbons on the left-hand side and the right-hand side become
the reactive sites, both double bonds are broken, and a new double
bond forms between the two carbons in the center. The net result
is that one double bond is used in making linear polymer, and
one double bond remains, but has moved to center of molecule.
Show rubber from isoprene.
Read text p. 434 - 436 about rubber.
Nature makes two different addition polymers out of
isoprene which have a subtle structure difference but different
material properties.
Nature doesn't crosslink linear isoprene polymers via
extra alkene carbons after doing polymerization. Therefore the
natural linear isoprene polymers still have extra C-C double bonds
(at every fourth position in polymer chain).
If these double bonds have polymer chain running through
them in a cis fashion then the polyisoprene is called "natural
rubber." If the extra double bonds have polymer chain running
through them in trans fashion then the polyisoprene called
"gutta-percha."
Since 1839 (Charles Goodyear) natural rubber (cis)
has had some of extra alkene carbons crosslinked together with
sulfur by melting rubber and sulfur together to make rubber for
car tires, etc. Crosslinked gutta-percha used for golf ball covers
& electrical insulation.
PROBLEM 27.
Sketch & describe properties of cis & trans
polyisoprene. 
PROBLEM 28.
Describe the role of sulfur in vulcanization.
@ Forms crosslinks.
PROBLEM 29.
Effect of vulcanization on physical properties of rubber?
@ Makes rubber high melting, stretchable, and tough
rather than gooey.
PROBLEM 30.
Describe nature of polymer which is extensively crosslinked
(small rings like diamond structure).
@ Hard and brittle.
PROBLEM 35.
Which monomers can undergo addition polymerization
and why?
@ a. Styrene. Yes. Has alkene group.
b. Propene. Yes. Also is an alkene.
c. Ethane. No. Not an alkene.
THERMOSETTING POLYMERS
Because crosslinking a polymer holds its macromolecules
together better, these polymers are tougher, stiffer, more resilient,
more heat resistant, harder to tear, higher melting, and longer
lasting than ordinary polymers.
If a polymer is an oily liquid before crosslinking,
it will probably become a rubbery stretchable solid afterwards.
If a polymer is a low-melting solid plastic before crosslinking,
it will probably become a rock-hard, brittle ceramic-like material
after crosslinking.
Some polymers become crosslinked while the monomer
molecules are reacting with one another but most are formed in
two steps.
First step in forming two-step crosslinked polymers
involves making linear polymer with excess unreacted reactive
sites (removable fragments in condensation monomers or double
bonds in addition monomers).
Linear polymer formed in first step is solid; too stiff
to enable excess reactive sites to be able to find each other
& make crosslinks till polymer is melted.
Second step: When a solid condensation polymer with
excess reactive sites is heated, it melts, enabling the excess
reactive sites to move around and find each other, forming crosslinks.
Once the crosslinks form the polymer solidifies and will not
melt again even when heated.
This crosslinking process is called "thermosetting"
(thermo = heat, setting = stiffening). Ordinary linear polymers
with no excess reactive sites cannot be thermoset. When they
are heated they just melt and never stiffen up.
For this reason ordinary polymers are called "thermoplastic"
rather than thermosetting ("plastic" means soft or workable;
thermoplastics become soft or melt when heated).
Addition polymers normally cannot be crosslinked without
the help of added vulcanizing agents (sulfur in rubber) which
form the crosslinks.
Alkene carbons not reactive enough to form bonds to
one another just by melting the polymer (this is why an initiator
needed to start addition polymerization rxns).
Special crosslinking agents (sulfur in rubber) need
to be added to the melted addition polymer to connect together
the excess reactive sites (alkene carbons) and form crosslinks.
The more extensively crosslinked a polymer is (small
ring sizes, tight weave or mesh) the more brittle a polymer becomes.
Remember diamond is made of interlocking 6-membered
rings, whereas in rubber the ring sizes can be as large as several
hundred atoms, depending on how thoroughly vulcanized the rubber
is.
CONDENSATION POLYMERS
Formed by removal of fragments of functional groups
from two or more different kinds of monomers, which allows monomers
to bond to one another and generates small-molecule byproducts
composed of fragments removed from monomer molecules.
If they can crosslink normally they are thermosetting;
they generally solidify before they can crosslink completely.
Polyesters
Usually made from diacids and dialcohols condensed
together. Major uses: clothing and plastic bottles.
Polyamides
Also called Nylons. Made from diamines and diacids.
Extremely strong because hydrogen bonding creates pseudocrosslinks.
Most commonly known Nylon is Nylon 66 made from adipic acid
and hexanediamine. Called Nylon 66 because the diacid (adipic
acid) and the diamine each have 6 carbons.
PROBLEM 42
Starting materials for Nylon 66?
@ adipic acid and hexanediamine
Show non-ball-and-stick version of Fig. 14.12 with
H bonds acting like pseudocrosslinks in Nylon 6.
Alkyds
Like polyesters except that glycerol (a triol) used
rather than a diol. Since the alcohol monomer has more than two
(3) reactive sites (alcohol functional groups) a crosslinked polymer
is formed. Used for tough factory paint jobs for cars (Fact-O-Bake).
Final step requires baking because the polymer needs to be melted
to allow it to make all of its crosslinks (thermosetting).
Silicones
Formed by reacting chlorosilanes (have chlorine atoms
bonded to silicon atoms) with water. Initially OH group from
water replaces all chlorines attached to silicon. SiOH groups
not stable- SiOSiOSiOSiOSiOSiOSiOSi polymers
form by loss of waters. Each OH attached initially
to a silicon is a reactive site.
If monomer starts with only two OH groups attached
to silicon a linear polymer produced which is an oil. If monomer
starts with three OH groups bonded to silicon crosslinks can form,
so resulting polymer is a thermosetting tough rubbery solid called
silicone rubber. If monomer starts with four OH groups attached
to silicon sand is produced. Generally this is only done to produce
a superfine sand called silica gel.
Silly Putty is produced from a mixture of diclorosilanes
(produces two OH groups attached to each Si) and trichlorosilanes
(three OH groups per Si). The resulting polymer has properties
midway between oil (linear polymer) and silicone rubber (crosslinked
tough rubbery polymer). Not a liquid oil but you can pull it
apart; mushy, yet you can bounce it.
Silicone rubber much more heat-stable than isoprene
rubber because Si-O bonds are extremely strong.
PROBLEM 32.
Show how polymers prepared from monomers given.
@:
a. CH3CH=CHCH3 addition
with self makes poly-2-butene
b. HOOCCH2CH2COOH condensation
with dialcohol makes polyester.
b. HOOCCH2CH2COOH condensation
with diamine makes polyamide (or nylon).
c. H2NCH2CH2CH2CH2NH2
condensation with diacid makes polyamide
(nylon).
d. (CH3)2Si(OH)2
condensation with self makes silicone oil
(remember the rubber needs 3 OH groups attached to Si so crosslinking
can happen).
Comments?
Last Revised : Sunday, October 5, 1997
Copyright © 1997
Louisiana State University, Department of Chemistry.
All rights reserved.
http://www.chem.lsu.edu/lucid/courseinfo/chem1002/ch14.html
