Transcript Chp17 - University of Nebraska Omaha

Polymers
Basic Definitions
• Polymer: Any long-chain molecule synthesized
by linking together single parts called monomers.
• Monomer: The simplest nonredundant unit from
which a polymer is synthesized.
• Elasticity: The property of a material to return to
its original shape when stretched.
• Plasiticity: The property of a material to stay
deformed with stretched.
• Plastic: A material that exhibits plasticity or a
colloquial term for a polymer.
Synthetic Polymers
Synthetic polymers (that is, not biopolymers like
proteins and DNA) can be divided into two main
categories:
A. Addition polymers, a.k.a. chain-growth polymers:
• Vinyl monomers bond together by splitting or moving pi
bond.
• Polymerization proceeds through a reactive intermediate
as a free radical, carbocation or carbanion.
• Examples are poly(vinyl chloride), polyethylene,
polystyrene.
B. Condensation polymers, a.k.a. step-growth
polymers
• Functional groups on the ends of monomers react to form
new bonds and thus create a polymer.
• Common by-products of condensation polymerization are
H2O or HCl.
• Reactions are often nucleophilic acyl substitution
reactions between monomers that are carboxylic acid
derivatives.
• Examples include polyester, Nylon, polycarbonate.
Drawing Polymer Structures
• Show the structure by placing parentheses
around the repeat unit.
• n = average degree of polymerization
• Note that the chain doesn’t have any extra carbons, just
extra covalent bonds (the black bonds above).
Nomenclature
• To name a polymer, add the prefix poly to the
name of the monomer from which the polymer is
derived.
• If the name of the monomer is one word, no
parentheses are necessary.
• For more complex monomers or where the name of
the monomer is two words, enclose the name of the
monomer in parentheses, as for example poly(vinyl
chloride).
Morphology
• The architecture of polymers is quite varied.
Some have linear and branched chains. Others
have comb, ladder, and star structures.
• Most commercial polymers are linear or lightly
branched chains.
• Some polymers such as rubber and Bakelite are
cross-linked which gives polymers added
strength.
• Polymers in the solid state tend to have both
ordered crystalline domains and disordered
amorphous domains.
• Some polymers that cooled or precipitated tend
to partially crystallize.
• Polymers with complicated and irregular shapes,
which prevent efficient packing into ordered
structures, are usually in the amorphous state.
http://www2.dupont.com/Plastics/en_US/Products/
Zytel_HTN/Zytel_HTN_whitepaper_R8.html
• Polymers with crystalline regions have either:
1. Regular, compact structures so that a chain can fold
back upon itself and be stuck together with
dispersion forces.
2. Polar functional groups so that hydrogen bonding
can bind regions in the same polymer chain or
among different chains.
• As the degree of crystallinity increases, the
polymer becomes more opaque due to
scattering of light by the crystalline domains.
•
Contrary to popular perception, transparent
materials are amorphous, not crystalline.
• Melt transition temperature, Tm: The
temperature at which crystalline regions melt
• as the degree of crystallinity increases, Tm increases
Thermal Properties of Polymers
• Thermoplastic: A polymer that can be melted
and molded into a shape that is retained when it
is cooled.
• Thermoset: A polymer that can be molded when
first prepared, but once it is cooled, hardens
irreversibly and cannot be remelted.
• Highly amorphous polymers, on heating, are
transformed from a hard glass to a soft, flexible,
rubbery state.
• Glass transition temperature, Tg: The
temperature at which a polymer undergoes a
transition from a hard glass to a rubbery solid
Step-Growth Polymers
• Step-growth polymerization: a polymerization in
which chain growth occurs in a stepwise manner
when a functional group from one monomer
reacts with a functional group of another
monomer.
• Monomers are usually difunctional.
• Chain propagation occurs in both directions.
• We’ll examine five types of step-growth polymers.
•
•
•
•
•
Polyamides
Polyesters
Polycarbonates
Polyurethanes
Epoxy resins
Polyamides
• While there are many synthetic routes to make
polyamides (nylons), the reaction of a dicarboxylic
acid with a diamine is common.
• Hexanedioic acid reacts with 1,6-hexanediamine to make Nylon 6,6.
• The raw material base for the production of
Nylon 66 is benzene, which is derived from
cracking and reforming of petroleum.
• Catalytic reduction of benzene followed by air
oxidation gives a mixture of cyclohexanol and
cyclohexanone.
• Oxidation of the mixture by nitric acid gives
adipic acid.
• Adipic acid is in turn used for the synthesis of
1,6-hexanediamine.
• Nylon 6,6 is used in electrical insulation, carpet
fibres, clothing, airbags, zip ties, ropes, conveyor
belts, hoses, among other things.
Nylon 6
• Nylon 6 is synthesized from a six-carbon
monomer in a novel process called ring-opening
polymerization.
• Nylon 6 is fabricated into fibers, brush bristles,
high-impact moldings, and tire cords.
Kevlar
• Kevlar is an aromatic polyamide (an aramid).
• Cables of Kevlar are as strong as cables of steel, but
only about 20% the weight.
• Kevlar fabric is used for bulletproof vests, jackets, and
raincoats, snowboards, drumheads, et. al.
• Much of the strength of Kevlar originates from
the hydrogen bonding that occurs between
chains as well as the phenylene groups that
make the individual chains more rigid.
Polyesters
• Many polyesters are synthesized from a
dicarboxylic acid and a diol.
• Poly(ethylene terephthalate), abbreviated PET
or PETE, is fabricated into Dacron fibers, Mylar
films, and plastic beverage containers. The key
step in formation of this polymer is
transesterification.
• Ethylene glycol is synthesized from ethylene.
• Terephthalic acid is synthesized from pxylene, which is obtained from petroleum
refining.
H2CrO4
or
• Poly(ethylene terephthalate) (PET) can be made
with % crystalline domains ranging from 0% to 55%.
n
• Completely amorphous PET is formed by
cooling the melt quickly.
• PET with a low degree of crystallinity is used for
plastic beverage bottles.
• By prolonging cooling time, more molecular
diffusion occurs and crystalline domains form as
the chains become more ordered.
• PET with a high degree of crystallinity can be drawn
into textile fibers and tire cords.
Polycarbonates
• Polycarbonates, the most familiar of which is
Lexan, are important engineering plastics.
• Lexan is made from the disodium salt of bisphenol A
(BPA) and phosgene. Phosgene is a diacid chloride.
Bisphenol A
• Lexan is a tough transparent polymer with high
impact and tensile strengths and retains its
shape over a wide temperature range.
• It is used in sporting equipment, such as bicycle,
football, and snowmobile helmets as well as hockey
and baseball catcher’s masks.
• It is also used in the manufacture of safety lenses and
unbreakable windows.
Polyurethanes
• A urethane, or carbamate, is an ester of
carbamic acid, H2NCH2COOH.
• They are most commonly prepared by treating an
isocyanate with an alcohol.
• Polyurethanes are mostly synthesized from a
diisocyanate and a diol.
• Polyurethane fibers are fairly soft and elastic and
are used in “stretch” fabrics such as spandex
and Lycra.
Epoxy Resins
• Epoxy resins are prepared by a polymerization
in which one monomer contains at least two
epoxy groups.
• Epoxy resins are produced in forms ranging from low
viscosity liquids to high melting solids.
• The most widely used epoxide monomer is the
diepoxide prepared by treating 1 mol of bisphenol A
with 2 mol of epichlorohydrin.
• Formation of a diepoxide by an SN2 mechanism.
• The diepoxide monomer is then treated with a
diamine that opens the highly strained
epoxide ring by an SN2 mechanism.
Chain-Growth Polymers
• Chain-growth polymerization: A
polymerization in which monomer units are
joined together without loss of atoms.
• From the perspective of the chemical industry, chaingrowth polymerization is the single most important
reaction of alkenes.
• Many commercially available polymers are
produced from vinyl monomers using chain
polymerization.
Free Radical Polymerization
• Free Radical: A molecule or ion containing one
or more unpaired electrons.
• Chemists propose a three-step radical chain
mechanism to account for the polymerization of
alkenes.
(1) chain initiation
(2) chain propagation
(3) chain termination
• Among the initiators used for radical chaingrowth polymerization are organic peroxides,
which decompose as shown on mild heating.
• Fishhook arrow: A curved and barbed (fishhook)
arrow used to show the repositioning of a single
electron.
• Step 1: Chain initiation
• A step in a radical chain reaction characterized by the
formation of radicals from nonradical compounds.
• Step 2: Chain propagation
• Chain propagation: a step in a radical chain reaction
characterized by the reaction of a radical and a
molecule to give a new radical.
• Chain length, n: the number of times the cycle of
chain propagation steps repeats in a chain reaction.
• Chain propagation (cont’d)
• Step 3: Chain termination
• A step in a radical chain mechanism that involves
destruction of radicals.
• One type of chain termination is the combination of
two free radicals.
• Chain termination can occur via disproportionation
where two free radicals meet, but rather than combine,
an electron moves from one chain to another.
• The stability of free radicals is similar to the
stability of carbocations. (methyl < 1< 2< 3)
• Free radical reactions with vinyl groups almost
always give the more stable (the more
substituted) radical.
• Therefore, polymerizations of vinyl monomers
tend to yield polymers with head-to-tail linkages.
• The first commercial polyethylene produced by freeradical polymerization was the soft, tough polymer
known as low-density polyethylene (LDPE).
• LDPE chains are highly branched.
• Because this branching prevents polyethylene chains
from packing efficiently, LDPE is largely amorphous and
transparent.
• Approximately 65% is fabricated into films for consumer
items such as baked goods, vegetables and other
produce, and trash bags.
• High-density polyethylene (HDPE) has fewer
branches and is more crystalline than (LDPE);
therefore, it is a stronger material.
• It’s most common application is to make milk jugs.
Recycling Codes
• Many thermoplastic polymers can be recycled
easily.
• Polymers are commonly sorted according to a
recycling code developed by the Society of the
Plastics Industry in 1988.
• Codes for the sorting of other materials have also
been generated by other industry groups.
• In Omaha, PETE (1), HDPE (2), PVC (3) and
polypropylene (5) are recycled via trash pick-up.
• Includes: water bottles, pop bottles, cooking oil bottles, mouthwash
bottles, shampoo bottles, cleaning product bottles, milk jugs, juice
bottles, margarine tubs, whipped topping tubs, yogurt containers, sour
cream containers, reusable/disposable food storage containers,
clamshell deli containers, DVD cases, microwave meal trays