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Ch 5: Copolymerization TEXTBOOK NOTES
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Key Concepts:
Terms in this set (26)
the various forms of copolymers
statistical (random)
alternating
block
graft
Copolymerization is useful because it allows
various monomers to be combined such that it provides materials with useful and sometimes unique properties
chain growth polymerization can be used to make statistical copolymers
including monomers A and B in the reaction vessel
active site is a radical, ionic species , or coordination complex
chain growth: the distribution of a species in the chain depends upon
the rate at which one monomer adds to an active site relative to the other
usually results in statistical distribution of units in a chain
-described in terms of rate constants (their ratios)
chain growth polymerization can be used to make block copolymers
adding monomers sequentially, making sure monomer is used up in each stage before adding next batch
Using ONLY a LIVING polymerization technique (anionic)
chain growth polymerization can be used to make graft copolymers
first copolymerize a unit with functional group X into a chain
graft onto these sites a different macromer with a distinct but complementary functional group at each of its ends
ex: thermoplastic elastomers
step copolymerization of different types of units in the same vessel
usually leads to truly random copolymers with the same composition as the feed (due to external reactions)
may not hold true when chain growth is a rapid, irreversible, step-growth reaction (kinetics is the deciding factor as is with chain growth)
knowledge of kinetics allows
the prediction of the instantaneous composition of the resulting copolymer
The copolymer equation is used for 2 monomers because
while it is applicable to each type of site, there may be no way to distinguish separate chains produced from one site from those produced at another
free radical copolymerization
involves same steps as ordinary free radical polymerization:
-initation
-propagation
-termination
propagation step gives copolymerization its special character
polymerization of binary copolymer: 4 possible propagation rxns
when active site (radical) is type 1:
-add to another unit of type 1
~M1
+ M1 -(k11)-> ~M1
-add to another unit of type 2
~M1
+ M2 -(k12)-> ~M2
when active site (radical) is type 2:
-add to another unit of type 1
~M2
+ M1 -(k21)-> ~M1
-add to another unit of type 2
~M2
+ M2 -(k22)-> ~M2
fundamental assumption of the terminal model
the rate of addition of monomers depends only upon the nature of the radical species at the end of the growing chain (M1 or M2)
Limiting condition: k11>>k12 and k22>>k21
type 1 monomer active sites always prefer to add another 1
type 2 monomer active sites always prefer to add another 2
result: BLOCk copolymers or HOMOpolymers
Limiting condition: k12>>k11 and k21>>k22
large tendency for the copolymers to be ALTERNATING
Limiting condition: k12=k11 and k21=k22
copolymers will be truly RANDOM
steady state assumption and copolymerization
the radical species ~M1
and ~M2
are for,ed and removed at equal rates
generate of new species type 1 = rate of removal
k12[M1
][M2] = k21[M2
][M1]
instantaneous copolymer equation (mole ratios)
Tells us the ratio of monomers in the polmer at some instant relative to the ratio of concentration of monomers in the reaction mass
y=d[M1]/d[M2]=(1+r1x)/(1+(r2/x))
where:
r1=k11/k12
r2=k22/k21
x=[M1]/[M2]
F1 and f1
F1 is the mole fraction of M1 being incorporated into copolymer at som instant of time
F1=d[M1]/(d[M1]+d[M2])
f1 is the mole fraction of monomer left in the reaction mass at that same instant
f1=[M1]/([M1]+[M2])
compairing F1 and f1
F1 = (r1(f1)^2 + f1f2)/(r1(f1)^2 + 2f1f2 + r2(f2)^2)
Where
F2=1-F1
f2=1-f1
why r1 and r2 are important
they are a measure of the relative preference of a radical species for the monomers
they represent the only two independent rate variables that we need to know
experimental methods allow for measurements of r1 and r2
composition drift
after polymerization has proceeded for awhile the relative proportions of unreacted monomer may be very different from at the start of the polymerization
copolymer composition usually varies through polymerization and differs from monomer feed concentration
[M1]/[M2] cannot be treated as constant in the instantaneous copolymerization ratio
Reactivity ratios special case 1: r1=r2=0
type 1 never wants to add to itself (k11=0)
type 2 never wants to add to itself (k22=0)
They can add to each other
result: perfectly ALTERNATING copolymer
continues until all monomers are used up or until one of the monomers is used up
Reactivity ratios special case 2: r1=r2=infinity
if r1=infinity, k12 must equal zero
if r2=infinity, k21 must equal zero
monomer 1 will only add M1 radicals
monomer 2 will only add to M2 radicals
thus, two HOMOPOLYMERS are produced
Reactivity ratios special case 3: r1=r2=1
monomers 1 and 2 add with equal facility to either type of radical active site
resulting copolymer has truly random distribution of monomers
copolymer composition is exactly the same as that of the initial monomer concentration and stays so throughout the course of the polymerization
extremely rare, only case in which there is NO composition drift
Reactivity ratios special case 4: r1r2=1
ideal copolymerization
-distribution of monomers in the chain at any point in the polymerization truly random
copolymer composition not usually the same as the composition of the monomers in the reaction mass
each radical displays the same preference for one of the monomers over the other
composition drift occurs; sequence distribution in the polymer is random
k11/k12 = k21/k22
plotting F1 against f1
r1=r2=1 gives straight diagonal line over plot
curve above diagonal is always richer in units of type 1 than the monomer mixture from which it was polymerized (r1>1 and r2<1)
r1 and r2 are both <1: curve crossing the diagonal can be obtained
-point where curve intersects the diagonal is the azeotrope (no composition drift)
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