Organic Chemistry III

Professor Carl C. Wamser

Chem 336 - Spring 2007

Chapter 29 Notes

Polymers

A. Polymers - macromolecules

Examples of typical polymers:

Special features of polymer chemistry:

1) SIZE

2) POLYDISPERSITY

How big is a macromolecule?

B. Terminology of Polymer Chemistry

polymer - a large molecule consisting of a number of repeating units, with molecular weight typically several thousand or higher

repeat unit - the fundamental recurring unit of a polymer

monomer - the smaller molecule(s) that are used to prepare a polymer (may or may not be equivalent to the repeat unit)

oligomer - a molecule consisting of several repeat units of a monomer, but not large enough to be considered a polymer

Approximate ranges for macromolecules, low polymers, high polymers:

1. Classification of polymers:

copolymer - a polymer prepared from more than one monomer

addition polymer - a polymer that consists of a repeat unit equivalent to its monomer - generally vinyl polymers, e.g., polyethylene, or ring-opening polymers, e.g., poly(ethylene oxide)

condensation polymer - a polymer that differs from its monomer(s) by the elimination of a small molecule during polymerization (Carothers' original definition) e.g., polyamides and polyesters

the definition has been expanded to include any polymer that incorporates new functional group(s) in the chain that were not present in the monomers
(to allow inclusion of polyurethanes)

2. Descriptions of polymer structure:

linear polymer - a polymer consisting of a single continuous chain of repeat units

branched polymer - a polymer that includes side chains of repeat units connecting onto the main chain of repeat units (different from side chains already present in the monomers)

crosslinked polymer - a polymer that includes interconnections between chains, either formed during polymerization (by choice of monomer) or after polymerization (by adding a specific reagent)

network polymer - a crosslinked polymer that includes numerous interconnections between chains such that the entire sample is (or could be) a single molecule

configuration - the three-dimensional structure of a polymer based on orientations that cannot be changed except by breaking of bonds (cis-trans, R&S); this definition is used the same way as in organic chemistry, although there will be further definitions and descriptions of configuration

conformation - the three-dimensional structure of a polymer based on orientations that can be interconverted by simple rotations of bonds; used the same way as in organic chemistry

3. Classification of polymerization reactions:

step-reaction polymerization - a polymerization mechanism in which each reaction between monomers is a discrete and independent step (e.g., formation of polyesters from diols plus diacids)

chain-reaction polymerization - a polymerization mechanism in which monomers add rapidly to a relatively few reactive growth sites (e.g., free radical chain polymerization of ethylene)

ring-opening polymerization - a polymerization mechanism in which a cyclic monomer opens as it is attached in each growth step (e.g., epoxy polymerizations)

4. Descriptions of polymer size:

degree of polymerization (DP) - the number of monomer units incorporated into a polymer chain

monodisperse - describing a polymer sample consisting of molecules ALL of which have the same chain length (an ideal, hypothetical situation, except in natural macromolecules)

polydisperse - describing a polymer consisting of molecules with a variety of chain lengths (and hence molecular weights) - the real life situation for synthetic polymers

number-average molecular weight (Mn) - the average molecular weight of a polydisperse polymer sample, averaged to give equal statistical weight to each molecule; calculated as

Mn = SUM ( Mi Ni ) / SUM ( Ni )

weight-average molecular weight (Mw) - the average molecular weight of a polydisperse polymer sample, averaged to give higher statistical weight to larger molecules; calculated as

Mw = SUM ( Mi^2 Ni ) / SUM ( Mi Ni )

EXAMPLE: consider a sample of 5 molecules of MW= 1, 2, 3, 4, 5

Mn = (1 + 2 + 3 + 4 + 5) / 5 = 3.00

Mw = (1 + 4 + 9 + 16 + 25) / (1 + 2 + 3 + 4 + 5) = 3.67

EXAMPLE: equal moles of polymer of MW 10,000 and MW 100,000

Mn = (0.5 *104) + (0.5 *105) / (1.0) = 55,000

Mw = (0.5 *108) + (0.5 *1010) / (0.5 *104) + (0.5 *105) = 92,000

EXAMPLE: 90% polymer of MW 10,000 and 10% MW 100,000

Mn = (0.9 *104) + (0.1 *105) / (1.0) = 19,000

Mw = (0.9 *108) + (0.1 *1010) / (0.9 *104) + (0.1 *105) = 57,000

5. Descriptions of polymer physical properties:

primary bonds - the covalent bonds that connect the atoms of the main chain

secondary bonds - non-covalent bonds that hold one polymer chain to another, including hydrogen bonding and other dipole-dipole attractions; this term is used similarly in describing protein structure

crystalline polymers - solid polymers with a high degree of structural order and rigidity

amorphous polymers - polymers with a low degree of structural order

semi-crystalline polymers - most polymers actually consist of both crystalline domains and amorphous domains, with properties between that expected for a purely crystalline or purely amorphous polymer

glass - the solid form of an amorphous polymer, characterized by rigidity and brittleness even though there is little order on the molecular level

crystalline melting temperature (Tm) - temperature at which a crystalline polymer converts to a liquid, or crystalline domains of a semi-crystalline polymer melt (increased molecular motion)

glass transition temperature (Tg) - temperature at which an amorphous polymer converts to a liquid, or amorphous domains of a semi-crystalline polymer melt

above Tm - polymer is liquid, although usually very viscous,
with free random motion of polymer chains

at Tm - if polymer can crystallize (orderly structure, slow
cooling), solid crystalline domains appear

at Tg - the remainder of the polymer chains solidify,
without much order, into a glassy state

below Tg - the polymer is a rigid solid, containing some
crystalline and some amorphous domains

polyethylene: Tg = -125°C , Tm = 137°C
(polyethylene is a flexible solid at room temp, consisting of some solid crystalline domains and melted amorphous domains)

polystyrene: Tg = 100°C , Tm = 240°C
(polystyrene is a rigid, glassy solid at room temp)

high-density polyethylene - polyethylene created so as to have a relatively high concentration of crystalline domains; its properties are greater density, rigidity, and strength (e.g., gallon milk cartons)

low-density polyethylene - polyethylene created so as to have mainly amorphous structure, with low density and great flexibility (e.g., squeeze bottles, plastic bags)

thermoplastics (plastics) - polymers that undergo thermally reversible interconversion between the solid state and the liquid state

thermosets - polymers that continue reacting at elevated temperatures, generating increasing numbers of crosslinks; such polymers do not exhibit melting or glass transitions

liquid-crystalline polymers - polymers with a fluid phase that retains some order

elastomers - rubbery, stretchy polymers; the effect is caused by light crosslinking that pulls the chains back to their original state

C. Nomenclature of Polymers

based on monomer source:
the most common method for naming addition polymers
(nearly always unambiguous)
e.g., polyethylene, poly(vinyl chloride) (PVC), poly(methyl methacrylate) (PMMA), etc.

based on polymer structure:
the most common method for condensation polymers, since the polymer typically contains different functional groups than the monomers
e.g., poly(ethylene terephthalate) (Dacron), poly(hexamethylene adipamide) (Nylon 66)

common or trade names:
frequently used for many polymers

IUPAC system:
identify repeat unit, starting with highest priority atoms in chain and putting substituents on lowest number positions

e.g., -(OCHFCH2)- is the preferred repeat unit
(note that there are five other possible choices)
the correct name is poly[oxy(1-fluoroethylene)]

nomenclature is similar to normal IUPAC for organics,
but you need to know names for divalent groups:

-O- oxy

-NH- imino

-CH2- methylene

-CH2CH2- ethylene
(note: polyethylene is polymethylene by IUPAC)

-CH(CH3)- ethylidene

-CH(CH3)CH2- propylene

-CH2CH2CH2- trimethylene

-C(CH3)2- isopropylidene

-C6H4- phenylene (need o,m,p or numbers)

nomenclature examples:

-(OCH2)n- polyformaldehyde
poly(oxymethylene)

-(OOCC6H4COOCH2CH2)n- poly(ethylene terephthalate)
poly(oxycarbonyl-1,4-phenylenecarbonyloxyethylene)

D. Examples of polymer structures

essentially all of the functional groups possible in organic chemistry have been incorporated into polymer structures, leading to a tremendous variety of possible compounds and properties

1. Linear polymers (one main chain)

2. Linear polymers that include rings (cyclolinear polymers)

poly(p-phenylene)
polyaniline
cellulose

3. Branched polymers

generally modified form of the usual linear polymers
(e.g., branched polyethylene with 3° branch points)

multiple functionality (> 3) at branch points lead to star polymers

starches are branched polysaccharides

4. Network Polymers

5. Copolymers

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B
BBBBBBBBBBBBBBBBBBBBBBBBB
(generate reactive site on existing polymer chain, grow new chain)

E. Orientation and configuration in polymer chains

1. Head-to-tail orientation of monomer units predominates

2. Orientation and stereochemistry at double bonds


(isoprene monomer) -----> cis - natural rubber

(Note - natural rubber is head-to-tail: "The Isoprene Rule")

3. Tacticity - configuration at chiral centers

CH2=CHX ---> -CH2-CHX-CH2-CHX-CH2-CHX- ...

isotactic - all chiral centers same

syndiotactic - chiral centers alternate

atactic - chiral centers random

F. Polymerization Mechanisms

chain growth reactions - vinyl polymerization initiated by radicals, cations, anions, or organometallic catalysts

step growth reactions - normal functional group reactions of multifunctional monomers

Chain Growth Polymerization

 Step Growth Polymerization
 The only growth reaction is addition of monomer to a growing chain with a reactive terminus  Reaction can occur independently between any pair of molecular species
 The reaction mixture consists of high
polymer and unreacted monomers, with very few actively growing chains		  The reaction mixture consists of oligomers of many sizes, in a statistically calculable distribution
 Monomer concentration decreases steadily as reaction time increases  Monomers disappear early, in favor of low oligomers
 High polymer appears immediately, average molecular weight doesn't change much as reaction proceeds  Oligomers steadily increase in size, polymer average molecular weight increases as reaction proceeds
 Increased reaction time increases overall product yield, but doesn't affect polymer average molecular weight  Long reaction times are essential to produce polymer with high average molecular weight

Radical polymerization

Ionic Polymerization

Ziegler-Natta Catalysis

G. Brief history of polymer chemistry

1833 - Berzelius - first uses term "polymeric" to describe the relationship of ethylene (called olefiant gas, C2H4) to butene (called oil of wine, C4H8) and higher homologs of empirical formula CH2

1861 - Graham - measured diffusion rates of various substances through parchment, identifying a number of unusually slow materials (both inorganic and organic), which he called "colloids" (Greek kolla - glue)

1921 - Staudinger - demonstrated that many organic "colloids" were actually macromolecules, unlike many inorganic colloids, which could be made to permeate by dilution (deaggregation); prepared many oligomers of formaldehyde diacetate

AcO(CH2O)nAc
n = 1-5 are liquids isolated by distillation
n = 8, 10, 12, 14, 15, 16, 17, 19 are solids isolated by fractional crystallization

he compared mp, densities, and crystal structures of oligomers with the polymer
(in 1937 succeeded in obtaining a single crystal structure of polymer)

Staudinger established the following major principles:
1) atoms in polymers maintain their normal valences;
2) ends of polymers are normal functional groups;
3) end group analysis can lead to estimate of chain length

1930's - Carothers - performed and characterized numerous polymer syntheses by condensation reactions, ultimately leading to the introduction of the Nylon stocking in 1940

1940's - Flory, Mark, others - rapid expansion of polymer science, including detailed studies of polymer structure and properties and applications to a variety of materials of increasing commercial value