Polymers

1
Polymers

• Traditional polymer synthesis
• Initiation, using a catalyst that creates a free
  radical
• Propagation
• Termination

unpaired electron
R ? CC ? R C C ?
R C C ? CC? RC C C C ?
R C C ? ? C CR ? R (C-C)n R
2
Polydispersity

• Traditional synthesis ? large variation in chain
  length

Average chain length
molecular weight
number average
of polymer chains of Mi total number of chains
weight average
weight of polymer chains of Mi total weight of
all chains
weight fraction
by number

• Degree of polymerization
• Average of mer units/chain

mer molecular weight
by weight
3
New modes of synthesis

• Living polymerization
• Initiation occurs instantaneously
• Chemically eliminate possibility of random
  termination
• Polymer chains grow until monomer is consumed
• Each grows for a fixed (identical) period

4
Polymers

• Homopolymer
• Only one type of mer
• Copolymer
• Two or more types of mers
• Block copolymer
• Long regions of each type of mer
• Bifunctional mer
• Can make two bonds, e.g. ethylene ? linear
  polymer
• Trifunctional mer
• Can make three bonds ? branched polymer

5
Polymers
H
H
C C

• Linear
• Branched
• Cross-linked

H
H
C
C
C
C
C
C
C
C
C
6
Polymers
H
H
C C
H in
H out
H
H
C
C
C
C
109.5
C
C
C
C
C
R
Placement of side groups is fixed once polymer is
formed Example side group styrene
R
7
Isotactic
Atactic
8

• Thermal Properties
• Thermoplastics
• Melt (on heating) and resolidify (on cooling)
• Linear polymers
• Thermosets
• Soften, decompose irreversibly on heating
• Crosslinked
• Crystallinity
• Linear more crystalline than branched or
  crosslinked
• Crystalline has higher density than amorphous

9
Defects

• Metals
• Vacancies, interstitials, impurities
• Semiconductors
• Ionic compounds / Ceramics
• Charge balance considerations
• Thermodynamics gt concentration
• G H - TS