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Mechanisms of polyemerization
(Note: Polyethylene falls under the category of chain-growth polymerization)
POLYMERIZATION REACTIONS
Types of Polymerization
Polymerization can proceed according to two different mechanisms, referred
to as chain-growth and step-growth polymerization. In chain-growth
polymerization (also called addition polymerization) reaction occurs by
successive addition of monomer molecules to the reactive end (e.g. a radical
end) of a growing polymer chain.
The most important group of chain-growth polymerizations is
polymerization of vinyl monomers such as ethene, propene, styrene, and vinyl
chloride:
nCH2= CHX → - (-CH2-CHX)n –
in which X = H, CH3, C5H6, or Cl, respectively.
An initiator or a catalyst is usually required to start the chain-growth reaction.
Step-growth
polymerization
involves
the
reaction
between
the
functional groups (HO-, HOOC-, etc.) of any two molecules (either monomers
or polymers). By repeated reaction long chains are gradually produced. Most
commonly, the reactions are condensation reactions resulting in the
elimination of a small molecule like water or methanol. Examples are the
production of poly (ethene terephthalate) by reaction of ethene glycol with
either terephthalic cid or dimethyl terephthalate (see Scheme 1).
Scheme 2 shows the principle reactions taking place when two
different functional groups are present on the same molecule (indicated as
AB. e.g. in the production of caprolactam), and when two different molecules
each contain two identical functional groups (indicated as AA and BB, e.g. in
production of PET).
O
||
n HO-C--
O
||
___ C-OH + n HO-CH2-CH2-OH →
terephthalic acid
O
||
-O-CH2-CH2-O-C-
ethene glycol
O
||
-C-
+ 2n-1 H2O
poly-ethene terephthalate
O
||
n H3C-O-C -
O
||
___ C-O-CH3 + n HO-CH2-CH2-OH →
dimethyl terephthalic
O
||
-O-CH2-CH2-O-C-
ethene glycol
O
||
-C-
+ 2n-1 CH3OH
poly-ethene terephthalate
Scheme 1
Reactions for the production of polyethene terephthalate (PET)
In chain-growth polymerization, a high molecular weight product is
produced right from the start, while the monomer quantity deceases slowly
with time. In contrast, in step-growth polymerization there is a slow increase in
average molecular weight of the product. The molecular weight is usually not
as high as in chain-growth polymerization, and relatively small amounts of
unreacted monomer are present after the start of the reaction.
Chain-growth polymerization generally is fast, irreversible, and
moderately
to
highly
exothermic.
On
the
other
hand,
step-growth
polymerization is usually slow, equilibrium-limited, and isothermal to slightly
exothermic.
Polymerization of bifunctional monomer (A,B: two different functional groups):
AB + AB
→
ABAB (or AB)2 )
AB + (AB)2
→
(AB)3
AB + (AB)3 →
(AB)4
(AB)2 + AB)2 →
(AB)4
…
(AB)r + (AB)s →
(AB)r+s
Polymerization of two monomers with different functional groups:
AA
+
BB
→
AABB
AABB
+
AA
→
AABBAA
AABBAA
+
BB
→
AABBAABB
AABB
+
AABB →
AABBAABB
Scheme 2
Reactions in step-broth polymerization.
Mechanisms of Chain-Growth Polymerization
Chain-growth polymerization can be classified (in order of commercial
importance) as radical, coordination, anionic, or cationic polymerization,
depending on the type of initiation. The next two sections will briefly discuss
radical and coordination polymerization respectively.
Radical polymerization
Scheme 3 outlines a typical reaction scheme for radical polymerization. Most
radical polymerizations need an initiator to produce the first radical and thus
start the chain of addition reactions. The most common initiation reaction is
the thermal decomposition of molecules containing weak bonds, e.g.
peroxides (-O-O-) or azo compounds (-N=N-). The formed radicals then react
with the monomers. Once initiated, a chain will grow by repeated additions of
monomer molecules with simultaneous creation of a new radical site. This
propagation is very fast, so very long polymer chains will form already in the
earliest stage of the reaction.
Termination can occur by disproportionation or recombination.
Initiation:
R-R
→ R•
R• + M
→RM1•
Propagation: RM1• + M → RM2•
ki ≈ 10-4 – 10-6 s-1 (300-350 K)
kp ≈ 102 – 104 m3 kmol-1 s-1(300 350 K)
RM2• + M → RM3•
Etc.
RMn-1• + M → RMn•
Termination
by disproportionation:
RMn• + RMn• → RMm= + RMn
Termination
by recombination:
RMm• + RMn• → RMm-MnR
Transfer to solvent:
RMn• + S → RMn + S•
Transfer to monomer:
RMn• + M → RMn + RM•
Scheme 3
Steps in radical polymerization; M1, Mn, number of monomers in
chain.
H
H
|
|
C–CH2–CH2–CH2–CH2- C •
|
H
CH2 – CH2
H
|
C
H
H
C
|
H
H
|
CH2
CH2
H
|
CH = CH
2
2
C –CH2–CH2–CH2–CH2-CH3 ⎯⎯⎯
⎯⎯
→
•
C-CH2-CH2-CH2-CH2-CH3
|
CH2
|
H–C–H
C
•
Figure 1 Backbiting in the synthesis of LDPE by radical polymerization.
In the first case, the final polymers on an average have the same length as
the growing chains. Termination occurs by transfer of a hydrogen atom from
one of the radicals to the other, leading to unsaturation at one chain end.
Recombination results in polymers with on average double the length of the
growing chains.
Chain transfer is common in many radical polymerization processes. It
involves the transfer of the radical end of a growing polymer chain to another
species, for instance a monomer or a solvent. Chan transfer reduces the
average polymer size and molecular weight. It is possible to add a special
agent, a modifier, in order to control the average degree of polymerization.
In the production of low-density polyethene (LDPE), side chains are
generated by internal chain transfer, in which the end of the chain abstracts a
hydrogen atom from an internal –CH2- group, a process termed backbiting.
Figure 1 illustrates this process. A branch starts to grow from the internal
carbon radical. In this branch, backbiting is also possible resulting in widely
branched chains. Backbiting has a significant influence on the structure, and
hence, the properties of the final polymer.
Coordination polymerization
In coordination polymerization, usually transition-metal catalysts are involved.
Figure 2 indicates the main features of chain propagation in the coordination
polymerization of ethene. A growing polymer chain is coordinatively bound to
a metal atom that has another coordinative vacancy (partially empty dorbitals). A new ethene molecule is inserted by the creation of bonds between
one of its carbon atoms and the metal and between the other carbon atom
and the innermost carbon atom of the existing chain. Branching will not occur
through this mechanism since no radicals are involved; the active site of the
growing chain is the carbon atom directly bonded to the metal. High density
polyethene (HDPE) is produced by this type of polymerization.
When higher 1-alkenes are added, the resultant polymer chain will
have a few short branches. These are all of the same length since they are
simply the rest of the 1-alkene molecule. Figure 3 shows the incorporation of
1-butene in a growing polyethene chain The ethene copolymer known as
linear-low-density
polyethene
(LLDPE)
is
produced
polymerization of ethene and 1-butene or 1-hexane.
by
coordination
The most important catalysts for coordination polymerization are socalled Ziegler-Natta or Ziegler catalysts, and ‘Phillips’ catalysts, both
discovered to be effective for alkene polymerization in the 1950s. Ziegler
catalysts combine transition-metal compounds such as titanium and vanadium
with organometallic compounds. An important property of these catalysts is
that they yield stereoregular polymers when higher alkenes are polymerized,
e.g. polymerization of propene produces isotactic polypropene with high
selectivity. On Ziegler catalysts, the polymer chains grow to a very long
length. Therefore, a chain-transfer agent is added to the system. Most
commonly H2 is used, which donates a hydrogen atom to terminate and
detach the chain from the metal atom.
In catalyst systems developed at Phillips Petroleum Co., chromium
oxide supported on silica is the most important constituent. Phillips catalysts
are less active than Ziegler catalysts, and therefore are limited to the
production of polyethene. They produce a polymer with a large molecularweight distribution, with chains varying in molecular weight from 103 to 106
g/mol.
It is not surprising that polymerization catalysis draws a lot of attention,
and many new systems have been discovered. Even plastic materials that
only melt at temperatures of over 770 K (!) can be produced.