Chem 336 - Spring 2000
Organic Chemistry III
Dr. Carl C. Wamser

Chapter 23 - Polymers

A. Polymers - macromolecules

• polymers are abundant and important, both naturally and synthetically
• estimate up to 50% of practicing chemists work on polymers
• polymer chemistry encompasses all other areas of chemistry:
  organic, inorganic, physical, biological

Examples of typical polymers:

• inorganic: natural
  sand
  clays
• inorganic: synthetic
  fiberglass
  ceramics
• organic: natural (biopolymers)
  polysaccharides (wood, starch)
  polypeptides (proteins)
  polyisoprenes (natural rubber)
  nucleic acids (RNA, DNA)
• organic: synthetic
  polyolefins
  polyesters, polyamides

Special features of polymer chemistry:

1) SIZE

• molecular weights typically 10,000 - 1,000,000
• nonvolatile (tend to decompose at high temp.)
• chains become entangled easily (lots of conformations)

2) POLYDISPERSITY

• synthetic polymers always involve a distribution of structures & molecular weights
• technically a mixture of molecules, although the properties are virtually identical for all species
• must deal with average molecular weights

How big is a macromolecule?

• generally, a single molecule (held together by covalent bonds) with at least several hundred atoms (molecular weight of several thousand)
• special cases: aggregates of smaller molecules that behave as a unit often have properties similar to macromolecules - e.g. colloids, micelles, vesicles, liposomes

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

• a molecular interpretation of polymer melting/freezing behavior:

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)

• all C-C bonds in the main chain:
  saturated main chain (polyethylene, polypropylene)
  unsaturation in the main chain (natural rubber, polyacetylene)
  polar side groups (PVC, PMMA, Teflon)
  
• with O in the main chain:
  polyformaldehyde (acetal)
  epoxy polymers (PEO, PPO)
  polyesters
  polycarbonates
  
• with N in the main chain:
  polyamines
  polyamides (polypeptides)
  polyurethanes
  
• with other heteroatoms in the main chain:
  polysulfones
  polysiloxanes (silicones)
  polyphosphazenes

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

• loose networks - few crosslinks, insoluble but swellable in solvents
  (rubbers, soft contact lenses, polyacrylamide gels)
  
• dense networks (resins) - many crosslinks, rigid, insoluble
  (epoxies, alkyd resins)

5. Copolymers

• homopolymer from a single monomer
• copolymer from two (or more) monomers
• terpolymer from three monomers
• multicomponent polymer from more than two monomers
  
• chain copolymerization (different monomers incorporated during the growth of the polymer chain) can lead to:
  • alternating copolymer - ABABABABABABABABABABABA
  • block copolymer - AAAAAAAABBBBBBBBBBBBAAA
  • random copolymer - BAABBAABABBABABBBAABBAB
  • graft copolymer - branches of a different polymer are attached to a polymer chain

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
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

• polymer structure has substituents on alternate carbons because each addition is preferred in the same direction
  (revised Markovnikov Rule - form the more stable intermediate)
• a few sites of head-to-head will be present, either due to occasional kinetic competition with the normal head-to-tail addition (leads to one isolated reversed monomer)
  or due to the termination step of recombination (leads to one point where the order reverses, where the chains had been growing in opposite directions)

2. Orientation and stereochemistry at double bonds

• conjugated dienes generally polymerize with 1,4 addition
  (via allylic radical intermediate)

(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- ...

• Note that achiral monomers can generate new chiral centers in the polymer.
• However, chiral centers in most vinyl polymers are asymmetric only by virtue of the differing chain lengths and end groups in either direction.
  - usually not designated as (R) or (S), only relative stereochemistry

isotactic - all chiral centers same

syndiotactic - chiral centers alternate

atactic - chiral centers random

• different polymerization methods lead to different types and amounts of tacticity - requires that the previous position on a growing chain influences the stereochemistry of the next addition
• physical properties of the polymer depend on tacticity because it influences the ease of crystallization

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

• initiation by radical sources ( peroxides, azo compounds)
• termination by coupling or disproportionation
• chain transfer leads to branching (e.g., H-abstraction in a growing polyethylene chain)

Ionic Polymerization

• electron-rich alkenes can be polymerized by acids or Lewis acids via cation intermediates
  • termination by addition of nucleophile or loss of H+
• electron-deficient alkenes can be polymerized by bases or nucleophiles via anion intermediates
  • termination by addition of proton source (H2O)

Ziegler-Natta Catalysis

• heterogeneous (solid-phase) metal catalyst
• leads to highly linear polymers with minimal branching
  • e.g., high-density polyethylene
• "living" polymerization - reactive chain terminus remains active even after all monomer is used up (e.g., Ziegler-Natta or anion intermediates in absence of terminating agents - will continue to grow if "fed" more monomer)

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