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Organic Polymers Synthetic and Natural

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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A polymer is a high molar mass molecular compound made up of many repeating chemical units.

Naturally occurring polymers

• Proteins

• Nucleic acids

• Cellulose

• Rubber

Synthetic polymers

• Nylon

• Dacron

• Lucite Silverstone®: polytetrafluoroethylene

Tyvek

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The simple repeating unit of a polymer is the monomer.

Homopolymer is a polymer made up of only one type of monomer

( CF2 CF2 )n

Teflon

( CH2 CH2 )n

Polyethylene

( CH2 CH )n

Cl

PVC

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Copolymer is a polymer made up of two or more monomers

Styrene-butadiene rubber

( CH CH2 CH2 CH CH CH2 )n

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R groups on same side of chain

Isotactic

R groups alternate from side to side Syndiotactic

R groups disposed at random

Atactic

Stereoisomers of Polymers

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Polymerization: Addition reactions

•  Involve unsaturated compounds containing double or triple bonds •  Particularly C=C and C≡C •  Examples: −  Hydrogenation −  Reactions of hydrogen halides and halogens with alkenes and alkynes −  Polymerization

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Mechanism of addition polymerization

initiator

repeating unit (monomer)

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Vulcanization: Properties of rubber

before vulcantization after vulcantization

relaxation after stretching

stretched

sulfur cross-links

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Polymerization: Condensation reactions

Conduc'ng  Polymers  

Introduction

"   Polymers (or plastics as they are also called) are known to have good insulating properties.

                         

   

 •         Polymers  are  one  of  the  most  used  materials  in  the  modern  

world.    Their  uses  and  applica'on  range  from  containers  to  clothing.    

•         They  are  used  to  coat  metal  wires  to  prevent  electric  shocks.                      

 

What  is  conduc-vity?    

Conduc'vity  can  be  defined  simply  by  Ohms  Law.    V=  IR    

Where  R  is  the  resistance,  I  the  current  and  V  the  voltage  present  in  the  material.  The  conduc'vity  depends  on  the  number  of  charge  carriers  (number  of  electrons)  in  the  material  and  their  mobility.In  a  metal  it  is  assumed  that  all  the  outer  electrons  are  free  to  carry  charge  and  the    impedance  to  flow  of  charge  is  mainly  due  to  the  electrons  "bumping"  in  to  each  other.    

 

Insulators  however  have  'ghtly  bound  electrons  so  that  nearly  no  electron  flow  occurs  so  they  offer  high  resistance  to  charge  flow.    So  for  conductance  free  electrons  are  needed.    

What  makes  the  material  conduc've?  Three  simple  carbon  compounds  are  diamond,  graphite  and  polyacetylene.  

They  may  be  regarded  as  three-­‐  two-­‐  and  one-­‐dimensional  forms  of  carbon  materials  .  

   

Diamond,  which  contains  only  σ  bonds,  is  an  insulator  and  its  high  symmetry  gives  it  isotropic  proper'es.  Graphite  and  acetylene  both  have  mobile  π  electrons  and  are,  when  doped,  highly  anisotropic  metallic  conductors.  

Ethylene C2H4 H-(CH=CH)-H -103,7 °C - 169 °C

Butadiene C4H6 H-(CH=CH)-(CH=CH)-H - 4,4 °C - 108,9 °C

Hexatriene C6H8 H-(CH=CH)-(CH=CH)-(CH=CH)-H 78 °C - 12 °C

Octatetraene C8H10 H-(CH=CH)4-H + 50 °C

...

Polyenes  Polyacetylenes  

1°  J.C.S.  Chem.  Comm.,  1977      received,  16th  May  1977  

2°  P.R.L.,  1977      received,  23  June,  1977  

3°  J.A.C.S.,  1978            received,  September  6,  1977  

   

Yet  Alan  J.  Heeger,  Alan  G.  MacDiarmid  and  Hideki  Shirakawa  have  changed  this  view  with  their  

discovery  that  a  polymer,  polyacetylene,  can  be  made  conduc've  almost  like  a  metal.      

Two  condi'ons  to  become  conduc've:  1-­‐The  first  condi'on  for  this  is  that  the  polymer  consists  of  alterna'ng  single  

and  double  bonds,  called  conjugated  double  bonds.    In  conjuga'on,  the  bonds  between  the  carbon  atoms  are  alternately  single  

and  double.  Every  bond  contains  a  localised  “sigma”  (σ)  bond  which  forms  a  strong  chemical  bond.  In  addi'on,  every  double  bond  also  contains  a  less  strongly  localised  “pi”  (π)  bond  which  is  weaker.  

       

Basis  of  Soliton  Theory  Coulson-­‐Rushbrooke  Theorem  

The  π-­‐molecular   orbitals   of   non-­‐zero   binding   energy   (ε   ≠   0)   appears   in   pair   (conjugate  molecular  orbitals)  with  opposite  energies.  

C.A.  Coulson  and  G.S.  Rushbrooke,    Proc.  Cambridge,  Phil.Soc.  36,  193  (1940)  

Polyacetylenes    with  even  number  of  carbon  atoms  

Polyacetylenes    with  odd  number  of  carbon  atoms  

Mol.  Phys.  5,  15  (1962)  

The  existence  of  misfits  should  by  no  means  be  limited  to  odd  polyenes.  One  can  conceive  of  a  series  of  such  misfits,  at  regular  distances,  in  a  large  even  polyene.    

Such  unpaired  electrons  may  be  detectable  by  electron  spin  resonance.  The  defect  is  able  to  travel  through  the  molecule  since  its  mo?on  depends  only  on  a  small  displacement  of  one  carbon  atom.  This  is  illustrated  in  Figure  3.  (p.19)    

A   strong   concentra'ons   of   spins   –   one   per   5000   atoms   at   room   temperature   has   been  observed  by  ESR  in  extremely  long  even  conjugated  polymers    [M.  Nechstein,  J.  Polymer  Sci.  C1,  1367-­‐1376  (1963)]    

2-­‐The  second  condi'on  is  that  the  plas'c  has  to  be  disturbed  -­‐  either  by  removing  electrons  from  (oxida'on),  or  inser'ng  them  into  

(reduc'on),  the  material.  The  process  is  known  as    Doping.    •  There  are  two  types  of  doping:  

           1-­‐oxida'on  with  halogen  (or  p-­‐doping).    

                                                                                                                                                                                                                                                         

(CHn)  +  3x/2  I                          (CHn)+  +  I3-­‐  

                               

   2-­‐  Reduc'on  with  alkali  metal  (called  n-­‐doping).      

(CHn)  +  xNa                          (CHn)x-­‐  +  xNa+  

PA+      Oxida've  p-­‐doping                    Conduc'on  without  spin  

PA-­‐      Reduc've  n-­‐doping                    Conduc'on  without  spin   !

CH[ ]n +3x2

I2 " CH[ ]n

x+ + x I3#

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CH[ ]n + x Na " CH[ ]n

x# + x Na+

!

" e "# $ $ $

!

+ e "# $ # #

!!!  If  mobile,  conduc'vity  without  spin  

!!!  Red  shiq  upon  doping  

Undoped  PA   Doped  PA  

Doping  process  •  The  halogen  doping  transforms  polyacetylene  to  a  good  conductor.    

Oxida'on  with  iodine  causes  the  electrons  to  be  jerked  out  of  the  polymer,  leaving  "holes"  in  the  form  of  posi've  charges  that  can  move  along  the  chain.  

DOPING - FOR BETTER MOLECULE PERFORMANCE

•  Doped  polyacetylene  is,  e.g.,  comparable  to  good  conductors  such  as  copper  and  silver,  whereas  in  its  original  form  it  is  a  semiconductor.  

Conduc-vity  of  conduc-ve  polymers  compared  to  those  of  other  materials,  from  quartz  (insulator)  to  copper  (conductor).  Polymers  may  also  have  conduc-vi-es  corresponding  to  those  of  semiconductors.  

Factors  that  affect  the  conduc'vity  

1-­‐  Density  of  charge  carriers.  2-­‐  Their  mobility.  3-­‐  The  direc'on.  4-­‐  Presence  of  doping  materials  (addi'ves  that  facilitate  the  polymer  conduc'vity)  

5-­‐  Temperature.      

Applications Conduc'ng  polymers  have  many  uses.    The  most  documented  are  as  

follows:  •  an'-­‐sta'c  substances  for  photographic  film  •  Corrosion  Inhibitors    •  Compact  Capacitors    •  An'  Sta'c  Coa'ng    •  Electromagne'c  shielding  for  computers                    "Smart  Windows"    A  second  genera'on  of  conduc'ng  polymers  have  been  developed  

these  have  industrial  uses  like:  •  Transistors    •  Light  Emitng  Diodes  (LEDs)    •  Lasers  used  in  flat  televisions    •  Solar  cells    •   Displays  in  mobile  telephones  and  mini-­‐format  television  

screens  

Photographic  Film  

smart"  windows  Shield  for  computer  screen  against  electromagne'c  "smart"  windows  radia'on  

Light-­‐emitng  diodes    Solar  cell  

Examples  of  Conduc'ng  Polymers  

Prac-cal  Applica-ons    Conduc-ve  plas-cs  used  in,  or    being  developed  industrially  for:  

 -­‐  an'-­‐sta'c  substances  for  photographic  films    -­‐  shields  for  computer  screens  against  electromagne'c  radia'on    -­‐  "smart"  windows  that  can  exclude  sunlight  

 Semi-­‐conduc-ve  polymers  recently  developed  in:  

 -­‐  light-­‐emitng  diodes  (LEDs)    -­‐  solar  cells      -­‐  displays  in  mobile  telephone  and  mini-­‐format  televisions  screens  

Applica-ons  ⇒  Plas'c  Electronics    Polyaniline      

 conductor    electromagne'c  shielding  of  electronic  circuits    corrosion  inhibitor  

 Poly(ethylendioxythiophene)  –  PEDOT  

 an'sta'c  coa'ng  material  on  photographic  emulsions    hole  injec'ng  electrode  material  in  PLED  

 Poly(phenylene  vinylidene)  

 ac've  layer  in  electroluminescent  displays  (GSM)    Poly(dialkylfluorene)    

 emissive  layer  in  full-­‐colour  video  matrix  displays    Poly(thiophene)    

 field-­‐effect  transistor    Poly(pyrrole)      

 microwave-­‐absorbing  "stealth"  (radar-­‐invisible)      screen  coa'ngs    ac've  thin  layer  of  sensing  devices  

Conclusion    •  For  conductance  free  electrons  are  needed.  •  Conjugated  polymers  are  semiconductor  materials  

while  doped  polymers  are  conductors.  •  The  conduc'vity  of  conduc've  polymers  decreases  

with  falling  temperature  in  contrast  to  the  conduc'vi'es  of  typical  metals,  e.g.  silver,  which  increase  with  falling  temperature.  

•  Today  conduc've  plas'cs  are  being  developed  for  many  uses.  

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•  Termed polypeptides •  Play a key role in nearly all biological processes −  Enzymes, the catalysts of biochemical reactions −  Transport of materials −  Storage of vital substances −  Coordinated motion −  Mechanical support −  Protection against diseases.

Proteins Proteins are polymers of amino acids

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Amino acids are the basic structural units of proteins.

•  Contain at least one amino group (-NH2)

•  And at least one carboxyl group (-COOH)

•  Existing form is pH dependent

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Amino acids are joined in a protein by the formation of a peptide bond

+H3N C C N C C O- + H2O

H

R1

H

R2

O O

H

+H3N C C O- + +H3N C C O-

H

R1

H

R2

O O

Peptide (amide) bond

Dipeptide – contains two amino acid residues

planar

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20 amino acids can form 202 or 400 dipeptides.

Protein with 50 amino acid residues can be arranged in 2050 or 1065 ways.

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Polypeptide chain: repeating amide bonds

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Carbon

Nitrogen

Oxygen

R group

Hydrogen

The structure is held in position by intramolecular hydrogen bonds (………)

Protein Structure: α-helix

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Hydrogen bonds in parallel and antiparallel β-pleated sheets

Protein Structure: β-Pleated Sheets

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primary structure

Protein Structure

secondary structure

tertiary structure

quaternary structure

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Protein Structure

Intermolecular Forces in a Protein Molecule

ionic forces

ionic forces

hydrogen bonds dispersion

forces

dispersion forces

dispersion forces

dipole-dipole forces

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Protein Structure

The structural changes that occur when oxygen binds to the heme group in hemoglobin.

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Denatured proteins: no longer exhibit normal biological activities.

Denaturation can be caused by pH, denaturants (special reagents) or temperature

Can be reversible or irreversible

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Nucleic Acids

Nucleic acids are high molar mass polymers that play an essential role in protein synthesis.

1.  Deoxyribonucleic acid (DNA)

2.  Ribonucleic acid (RNA)

DNA molecule has 2 helical strands. Each strand is made up of nucleotides.

Electronmicrograph of DNA

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The Components of the Nucleic Acids DNA and RNA

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Base-Pair Formation by Adenine and Thymine and by Cytosine and Guanine