View
2
Download
0
Category
Preview:
Citation preview
Position Deputy Dean (Research and Innovation)
Field of Study Basic Chemistry, Organic Chemistry
Research Focus Organic Synthesis, Polymer Chemistry
Room No. Deputy Dean Office (Research and Innovation)
(PM51, FSSA)
Tel. No. 5878, 5785
Email add. msani@ums.edu.my
Ts. ChM. Dr. Mohd. Sani SarjadiPhD (Sheffield, UK)
Radical Chain Polymerization: bead
“Molecule ‘Empire Building’ by ‘Radical’ Groups”
Chain-Growth Polymerization
(Addition) Processes
Chapter 3 - Part 1
1. Free radical Initiation Processes
2. Cationically Initiated Processes
3. Anionically Initiated Processes
4. Coordination Polymerization
Characteristics of Chain-Growth Polymerization
1. Only growth reaction adds repeating units one at a
time to the chain
2. Monomer concentration decreases steadily throughout
the reaction
Characteristics of Chain-Growth Polymerization
3. High Molecular weight polymer is formed at once;
polymer molecular weight changes little throughout
the reaction.
4. Long reaction times give high yields but affect
molecular weight little.
5. Reaction mixture contains only monomer, high
polymer, and about 10-8 part of growing chains.
The Chemistry of Free Radical Polymerization
Radical Generation:
Initiator RadicalsR R 2 R
Initiation:
Monomers
R + C C R C C
Propagation: R C C + C C C C CR
Termination: R C C + CCC R
R C C C C C R
Polymer
Free Radical Polymerization Mechanisms
1. Overview – Free radical polymerization processes
involve at least three mechanistic steps.
A. Initiation
1. Radical Formation (Generation)
DIn In
hv , etc.In + In
2. Initiation
In MIn + M
C. Termination
1) Radical Coupling (Combination)
In + In In In
2) Disproportionation (-hydrogen transfer)
In MxC
H
C
H
H H
+ InMyC
HC
H
HH
H3C CH2 My InCH2CHIn Mx +
In-MX. + .MY-In In-MX-MY-In
D. Chain Transfer (sometimes) – An atom is transferred
to the growing chain, terminating the chain growth
and starting a new chain.
Px R Px + RH+
Chain Transfer to Chain Transfer Agent:
Chain Transfer to Monomer:
Px. + H2C=CH-(C=O)OR
Chain Transfer Agent
Monomer
H2C=CH-(C=O)OR
P + x
Linear Polymer
Linear Polymer
D. Chain Transfer (sometimes) – An atom is transferred
to the growing chain, terminating the chain growth
and starting a new chain.
Px+ Py
H
Px Py+
Chain Transfer to Polymer: Causes Branching
Polymer
E. Inhibition and Retardation – a retarder is a substance
that can react with a radical to form products incapable
of reacting with monomer. An inhibitor is a retarder
which completely stops or “inhibits” polymerization.
2. Monomers that are susceptible to free radical addition
A. Vinyl Monomers
H2C CHX H2C CH Cl
Vinyl chloride
H
H
Y
X
F
FH
H
Vinylidene fluoride
B. Allyl Monomers
C. Ester Monomers
OH
O
OR
O
Acrylic Acid Acrylate Esters
X Cl
Allyl Chloride
1) Acrylates
2) Methacrylates
OH
O
OR
O
Methacrylate Esters
3) Vinyl Esters
O
O
Vinyl Acetate
D. Amide Monomers
NH2
O
NH2
O
Acrylamide Methacrylamide
Methacrylic Acid
3. Monomers that are not susceptible to Free Radical Addition
A. 1,2-a-olefins (Polymerize to oils only)
B. Vinyl ethers
OR
Omethyl vinyl ether
x
C. 1,2-disubstituted Ethylenes
H
Cl
H
Cl
1,2-dichloroethylene
1. Benzoyl Peroxide
C
O
O O C
O80-90
0 C
C
O
O 2 + 2 CO2
(continued)
4. Initiation - “Getting the thing started!”
A. Radical Generators (Initiators)
2) Di tert-Butyl Peroxide
H3C C
CH3
CH3
O O C
CH3
CH3
CH3
1200-140
0C
H3C C
CH3
CH3
2
H3C C
CH 3
CH 3
+
O
O
O
O
New Active Site
Initiator End-Group
Initiator
End-GroupVinyl Acetate
3) Azobisisobutyronitrile (AIBN)
CH3 CH3
H3C – C – N=N – C – CH3
CN CN
~60oC
or hn
H3C C
CH3
CN
+ N2 H3C C
CH3
CN
CH2
CHPh
eliminating a molecule of nitrogen, form two 2-cyanoprop-2-yl radicals:
Hydroperoxides can generate radicals by “induced
decomposition” from growing polymer chains:
P + H O O R
PH + O O R R OO2
R-OO-OO-R 2 RO + O2
What effect does this have on the polymerization process?
Acting as a chain-transfer agent, it reduces the degree
of polymerization and molecular mass!
Acrylic Acid
5. Propagation - “Keeping the thing going!”
A. The addition of monomer to an active center
(free radical) to generate a new active center.
R CH2
CH
X
X R CH2
HC
X
CH2
CH
X
X X
etc. etc.R C
H2
HC
X
CH2
CH
Xn
(continued)
Examples:
R CH2
CH
Ph
Ph R CH2
HC
Ph
CH2
CH
Phn
R CH2
CH2
CH
C
O
CH 3
O
OCH 3
O
R CH2
CH2
HC
C
O
CH 3
O
CH2
CH
C
O
CH 3
O
Polystyrene
Polymethyl
Acrylate
B. Configuration in Chain-Growth Polymerization
1) Configuration Possibilities
-
attack
-
attack
P
sterically and electronically unfavored
favored
H2C CH
X
HC CH2
P CH2
C HX
PHC CH2
X
X
X
.
2) Radical Stability Considerations
Which possible new active center will have the greatest stability?
P CH2
CH2
P CH2
CH
P CH2
CH
-attack produces resonance
stabilized free radical
.
PHC CH2 X No resonance stabilization
P
______________________________________________
HC C
O
O CH3
CH2
H2C CH
C
O
O CH3
X
P CH
CH
C O CH3O
PH
CH2
CH
C O
O CH3
PH
CH2
CH
C O
O CH3
Secondary radical
is resonance stabilized
(more examples)
Cl
Cl
H
H
H
H
Cl
Cl
P
X P C
Cl
Cl
CH2
P CH2
C
Cl
Cl
P CH2
C
Cl
Cl
P CH2
C
Cl
ClTertiary radical is resonance stabilized
3) Steric Hinderance Considerations
P
HC CH2
X
H2C CH
X
X
For large X, -substitution
is sterically favored
4) Radical Stability
3o > 2o > 1o
5 ) “Bottom Line”
Resonance and steric hinderance considerations lead to the
conclusion that -substitution (head-to-tail) is strongly
preferred in chain-growth polymerization.
CH2
HC C
H2
HC C
H2
HC C
H2
HC
X X X X
Alternating configuration
6. Termination - “Stopping the thing!”
A. Coupling (most common)
Px CH2
C
H
X
+ PyCH2
C
X
H
PyCH2
C
X
H
Px CH2
C
H
X- occurs head-to-head
- produces two initiator fragments (end-groups)
per chain.
B. Disproportionation
In MxC
H
C
H
H H
+ InMyC
HC
H
HH
H3C CH2 My InCH2CHIn Mx +
- Produce one initiator fragment (end-group) per chain
- Production of saturated chain and 1 unsaturated chain
per termination
C. Factors affecting the type of termination that will take place
1) Steric factors - large, bulky groups attached directly
to the active center will hinder coupling
2) Availability of labile -hydrogens
3) Examples – PS and PMMA
+Px CH2
C
H
C CH2
Py
H
Combination (coupling)Polystyrene
(continued)
P yP x CH 2
HC
HC C
H 2Ph Ph Ph =
CH3 H3C
~~~PX – CH2-C. + . C-CH2- PY~~~
C=O O=C
O O
CH3 CH3
PMMA
1. Sterically
hindered
2. 5 -Hydrogens
3. Disproportion-
ation dominates
(continued)
CH3 H3C
~~~PX – CH2=C + HC-CH2- PY~~~
C=O O=C
O O
CH3 CH3
4) Electrostatic Repulsion Between Polar Groups –
Esters, Amides, etc.
~~~PX – CH2-CH. + . HC-CH2- PY~~~
d+ CN d- d- NC d+
Polyacrylonitrile (PAN)
One might assume electrostatic repulsion in this case.
BUT, how about electrostatic attraction from the
nitrogen to the carbon? Also, steric hindrance is
limited.
At 60oC, this terminates almost exclusively by
coupling!
D. Primary Radical Termination
~~~PX – CH2-CH. + . In
X
~~~PX – CH2-CH-In
XMore Likely at
High [In.]
So molecular mass can be controlled using chain-transfer
agents, hydroperoxide initiators, OR higher levels of
initiator!
7. Chain-Transfer - “Rerouting the thing!”
A. Definition – The transfer of reactivity from the
growing polymer chain to another species. An
atom is transferred to the growing chain,
terminating the chain and starting a new one.
~~~PX – CH2-CH. + X-R → ~~~PX – CH2-CHX + R.
Y Y
~~~PX – CH2-CH. + CCl4 → ~~~PX – CH2-CHCl + Cl3C.
Y Y
B. Chain-transfer to solvent:
Propylene – Why won’t it polymerize with Free Radicals?
~~~PX – CH2-CH. + HCH=CH
CH3 CH3
~~~PX – CH2-CH2-CH3 + CH2=CH-CH2.
H2C-CH-CH2
Chain-transfer occurs so readily that propylene won’t polymerize
with free radicals.
D. Chain-transfer to polymer:
~~~PX – CH2-CH2-CH2. + ~~~CH2-CH2-CH2~~~
~~~PX – CH2-CH2-CH3 + ~~~CH2-CH-CH2~~~
Increases branching and broadens MWD!
E. Chain-transfer to Initiator (Primary Radical
Termination):
~~~PX – CH2. + R-O-O-R → ~~~PX – CH2-OR + . OR
Definition – The transfer of reactivity from the
growing polymer chain to another species. An
atom is transferred to the growing chain,
terminating the chain and starting a new one.
F. Chain-transfer to Chain-transfer Agent:
Examples: R-OH; R-SH; R-Cl; R-Br
~~~PX – CH2-CH2. + HS-(CH2)7CH3
~~~PX – CH2-CH3 + . S-(CH2)7CH3
H2C CHX . CXH-CH2- S-(CH2)7CH3
etc., etc., etc.H2C CHX
8. Inhibition and Retardation - “Preventing the thing
or slowing it down!”
Definition – Compounds that slow down or stop poly-
merization by forming radicals that are either too
stable or too sterically hindered to initiate poly-
merization OR they prefer coupling (termination)
reactions to initiation reactions.
~~~PX – CH2-CH. + O= =O
para-Benzoquinone
~~~PX – CH2-CH2-O- -O.Will Not
Propagate
~~~PX – CH2-CH. + O=O ~~~PX – CH2-CH-O-O .
Kinetics of Free Radical Polymerization
1. Initiation
I 2 R. Radical Generationkd
R. + M M1. Initiation
ki
Assuming that ki >>kd and accounting for the fact that two
Radicals are formed during every initiator decomposition,
The rate of initiation, Ri, is given by:
Ri = d[Mi] = 2fkd[I]
dt
f = efficiency of the initiator and is usually 0.3< f >0.8
(RDS)
2. Propagation
M1. + M M2
.
M2. + M M3
.
M3. + M M4
.
.
.
.
Mx. + M Mx+1
.
Rp = - d[M] = kp[M .][M]
dt
kp
kp
kp
kp
We assume that the
reactivity of the growing
chain is independent of the
length of the chain.
3. Termination
Mx. + . My Mx-My (Combination)
Mx. + . My Mx + My (Disproportionation)
ktc
ktd
Since two radicals are consumed in every termination,
then:Rt = 2kt [M .]2
4. Steady State Assumption
Very early in the polymerization, the concentration of radicals becomes constant because Ri = Rt
2fkd[I] = 2kt [M .]2
2fkd [I] = 2kt [M .]2
Solve this equation for [M .]:
[M .] = (fkd [I]/kt)1/2
Substituting this into the propagation expression:
Rp = kp[M .][M] = kp [M](fkd [I]/kt)1/2
Since the rate of propagation, Rp, is essentially the
rate of polymerization, the rate of polymerization is
proportional to [I]1/2 and [M].
7. Qualitative Effects – a Summary
Factor Rate of Rxn MW
[M] Increases Increases
[I] Increases Decreases
kp Increases Increases
kd Increases Decreases
kt Decreases Decreases
CT agent No Effect Decreases
Inhibitor Decreases (stops!) Decreases
CT to Poly No Effect Increases
Temperature Increases Decreases
Thermodynamics of Free Radical Polymerization
DGp = DHp - TDSp
DHp is favorable for all polymerizations and DSp
is not! However, at normal temperatures, DHp
more than compensates for the negative DSp term.
The Ceiling Temperature, Tc, is the temperature above
which the polymer “depolymerizes”.
At Tc , DGp= 0. DHp - Tc DSp = 0
DHp = Tc DSp Tc = DHp/ DSp
Cationic Chain-Growth Polymerization
Polymerization of isobutylene (2-methylpropene)
by traces of strong acids is an example of cationic
polymerization.
This process is similar to radical polymerization,
Chain growth ceases when the terminal
carbocation combines with a nucleophile or loses
a proton, giving a terminal alkene (slide No. 62).
Monomers bearing cation stabilizing groups, such
as alkyl, phenyl or vinyl can be polymerized by
cationic processes.
These are normally initiated at low temperature
in methylene chloride solution.
Strong acids, such as HClO4 , or Lewis acids
containing traces of water (as shown before) serve
as initiating reagents.
At low temperatures, chain transfer reactions are
rare in such polymerizations, so the resulting
polymers are cleanly linear (un-branched).
Anionic Chain-Growth Polymerization
Treatment of a cold THF solution of styrene with
0.001 equivalents of n-butyllithium causes an
immediate polymerization.
This is an example of anionic polymerization, the
course of which is described by the following
example.
Chain growth may be terminated by water or
carbon dioxide, and chain transfer seldom occurs.
Only monomers having anion stabilizing
substituents, such as phenyl, cyano or carbonyl
are good substrates for this polymerization
technique.
Many of the resulting polymers are largely
isotactic in configuration, and have high degrees
of crystallinity.
Species that have been used to initiate anionic
polymerization include alkali metals, alkali amides,
alkyl lithium and various electron sources.
+ C4H9-Li+
A practical application of anionic polymerization
occurs in the use of superglue.
This material is methyl 2-cyanoacrylate,
CH2=C(CN)CO2CH3.
When exposed to water, amines or other
nucleophiles, a rapid polymerization of this
monomer takes place.
Coordination Polymerization:
Ziegler-Natta Catalytic Polymerization
An efficient and stereospecific catalytic
polymerization procedure was developed by Karl
Ziegler (Germany) and Giulio Natta (Italy) in the
1950's.
Their findings permitted, for the first time, the
alkenes synthesis of un-branched.
High molecular weight polyethylene (HDPE),
laboratory synthesis of natural rubber from
isoprene, and configurational control of polymers
from terminal like propene (e.g. pure isotactic
and syndiotactic polymers).
In the case of ethylene, rapid polymerization
occurred at atmospheric pressure and moderate
to low temperature, giving a stronger (more
crystalline) product (HDPE) than that from
radical polymerization (LDPE).
For this important discovery these chemists
received the 1963 Nobel Prize in chemistry
Ziegler-Natta catalysts are prepared by reacting
certain transition metal halides with organometallic
reagents such as alkyl aluminum, lithium and zinc
reagents.
The catalyst formed by reaction of triethylaluminum
with titanium tetrachloride has been widely studied,
but other metals (e.g. V & Zr) have also proven
effective.
The following diagram presents one mechanismfor this useful reaction.
Others have been suggested, with changes toaccommodate the heterogeneity or homogeneityof the catalyst.
Polymerization of propylene through action of thetitanium catalyst gives an isotactic product;whereas, a vanadium based catalyst gives asyndiotactic product.
Copolymers
The synthesis of macromolecules composed of
more than one monomeric repeating unit has
been explored as a means of controlling the
properties of the resulting material.
In this respect, it is useful to distinguish several
ways in which different monomeric units might
be incorporated in a polymeric molecule.
The following examples refer to a two component
system, in which one monomer is designated A
and the other B.
Statistical Copolymers:
Also called random copolymers. Here themonomeric units are distributed randomly, andsometimes unevenly, in the polymer chain:
~ABBAAABAABBBABAABA~.
Alternating Copolymers:
Here the monomeric units are distributed in aregular alternating fashion, with nearly equimolaramounts of each in the chain:
~ABABABABABABABAB~.
Block Copolymers:
Instead of a mixed distribution of monomeric units,
a long sequence or block of one monomer is joined
to a block of the second monomer:
~AAAAA-BBBBBBB~AAAAAAA~BBB~
Graft Copolymers:
As the name suggests, side chains of a given
monomer are attached to the main chain of the
second monomer:
~AAAAAAA(BBBBBBB~)AAAAAAA(BBBB~)AAA~
Recommended