Chemistry Reference
In-Depth Information
species in an equilibrium. The result of the equilibrium is that all polymer
chains grow at a similar rate resulting in a relatively narrow molecular weight
distribution. The molecular weight is governed by the amount of monomer
and not by chain termination reactions. This polymerization method is espe-
cially valuable for preparing block copolymers.
Pol
Pol
Pol
S
S
S
+
C
12
H
25
C
12
H
25
C
12
H
25
S
S
S
S
S
S
CN
CN
CN
Dormant species
Radical that can
reinitiate
polymerization with
available monomer
Raft agent
Pol
Pol
S
S
S
Pol
2
C
12
H
25
S
S
C
12
H
25
C
12
H
25
+
Pol
S
S
Pol
2
S
S
Pol
2
Radical that can
reinitiate
polymerization with
available monomer
Dormant species
Dormant species
The molecular weight of a polymer has a pronounced effect on many
important physical properties. This can be rationalized by thinking about a
plate of spaghetti. The strands are entangled among each other and it can be
difficult (and messy) to pull them apart. If the spaghetti represents a high
molecular weight polymer, macaroni might represent a low molecular weight
polymer. A macaroni noodle is readily removed from the other noodles and
is not entangled. In this simplistic analogy, the longer the strand, the greater
is the entanglement. Polymers behave similarly and longer strands (higher
molecular weight) are more entangled. The greater the entanglement of
a polymer, the better is the toughness. Polymer properties such as tensile
strength (strength required to pull apart a polymer of a defined cross-sectional
area), impact resistance (strength required to fracture a polymer or cause
it to yield), and tear strength are all improved (stronger) with increasing
molecular weight. However, this is a trade-off with processability. For most
plastic fabrication flow in the molten state is necessary and often high flow
(low viscosity) is desirable. As a general trend, the greater the polymer
molecular weight, the higher the viscosity (and the poorer the flow). Flow
is more complicated than this and people have spent their careers studying
polymer flow, also known as the rheology of polymers. Polymer molecular
weight is a critical parameter for modifying flow.
Because of the pronounced effect of molecular weight on properties, it
is well studied. The molecular weight of a monomer is easy to calculate. If
we consider ethylene (C
2
H
4
) we can easily calculate the molecular weight
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