chapter 4 radical polymerization
TRANSCRIPT
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Radical Polymerization
Chapter 4
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Ch 4 Sl 2Chain polym’n step vs chain polym’n step ~ growth by reaction of functional groups
chain ~ growth by addition of monomer to active center
I R* RM* RMM* --- RMn* RMn+1* I ~ initiator [開始劑]
* ~ active center [活性㸃] * = • (free-radical) ~ radical polym’n
* = + (cation) ~ cationic polym’n Chapter 5
* = − (anion) ~ anionic polym’n Chapter 5
* = coordination site ~ coordination polym’n Chapter 6
M ~ monomer [單量體, 單位體]
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Ch 4 Sl 3Monomers for chain polym’n monomer ~ unsaturated compound with either
double bond (C=C, C=O)
ring ~ ring-opening polym’n Chapter 7
polymerizability of monomer Table 5.1 p124
C=O by ionic only
CH(X)=CH(Y) ~ hardly polymerize steric hindrance
CH2=C(X)(Y) ~ can polymerize
vinyl monomers CH2=CH(X) with e− donating X ~ cationic or radical
with e− withdrawing X ~ anionic or radical
Most C=C monomers can be radical-polymerized.
CH2
CH
C
N
CH2
C
N
CH
CH2
CH
C
N
CH2
C
N
CH
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Ch 4 Sl 4Radical polym’n two types of radical polym’n (conventional) free-radical polym’n
(newer) living radical polym’n Section 4.5
mechanism of free-radical polym’n initiation I R• RM•
propagation RM• RMM• --- RMn•
termination RMn• + RMm• RMn+mR or RMn + RMm
chain transfer RMn• + TA RMnT + A•
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Ch 4 Sl 5Initiation 2 steps I R• RM•
1st step ~ formation of radical homolysis of initiator thermal initiators ~ containing peroxide or azo linkage thermolysis at (usually) 50 – 100 ºC Table 4.2 p86
BPO
AIBN
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Ch 4 Sl 6
photochemical initiators ~ UV sensitive photolysis ~ time- and space-controllable
redox initiation redox = reduction + oxidation oxidant + reductant
fast, at variety of Temp, in aqueous medium useful for low Temp polym’n and/or emulsion polym’n Section 4.4.4
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Ch 4 Sl 7
2nd step ~ addition of one monomer much faster than 1st step
Not all R• initiate, since R• reacts with other R• within the cage ~ ‘cage effect’
other R• or polymer radical termination
solvent, polymer, initiator chain transfer
monomer initiation
initiator efficiency, f ~ 0.3 < f < 0.8 < 1
modepredominant steric + mesomeric
mesomeric = resonance
CH2
CH
F
CH2
F
CH
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Ch 4 Sl 8
addition of M to growing •
mode
head-to-tail configuration favored by > 98% proved by analysis
may be < 98% for very small X with little mesomeric effect like F PVF ~ 90%; PVDF ~ 95% PVA > 98%
Propagation
predominant
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Ch 4 Sl 9Termination disappearance of radical polym’n stops
two modes combination [coupling] one dead polymer
(x+y)-mer
disproportionation two dead polymers
x-mer + y-mer
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Ch 4 Sl 10
Combination requires low Ea. usually favored, esp at low Temp
Mode highly dep on type of monomer. vinyl monomers favor combination eg PS
α-methylvinyl monomers favor disproportionation eg PMMA
additional C-H’s
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Ch 4 Sl 11Chain transfer transfer of kinetic chain to other molecule transfer of active center from active chain
abstraction of T (typically H) from other molecule
A•, if reactive, may initiate a new kinetic chain.
Other molecule can be initiator, monomer, solvent, chain transfer agent, or polymer.
CT lowers MM. # of chains per # of initiations
except for CT to polymer
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Ch 4 Sl 12
CT to initiator
CT to solvent
CT to chain transfer agent CTA ~ add to control MM
What if using CTA as solvent? telomerization [popcorn polym’n]
telomer - telogen
Fig 4.1 p67
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Ch 4 Sl 13
CT to monomer
not popular vinylic H not reactive
autoinhibition [degradative CT to monomer]
leading to long-chain branch
H2C CH
CH2Y
+ H2C CH
CH2Y
H2C CH2
CH2Y
+ H2C CH
CHY
H2C CH
CHY
H2C CH
CH2Y noinitiation
CH2CH
X
+ H2C CH
X
CH2CH2
X
+ H2C C
X
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Ch 4 Sl 14
CT to polymer branching intramolecular CT and intermolecular CT
MM not change (both), PDI increases (intermol)
Radical polym’n of ethylene gives LDPE. Fig 4.2 p69
intramol CT [back-biting] short-chain branching butyl, ethyl, ethylhexyl branches
intermol CT long-chain branching
lower crystallinity [low density], lower mp, and lower shear-rate sensitivity than HDPE
SCB LCB
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Ch 4 Sl 15
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Ch 4 Sl 16
CT to polymer (cont’d) PMA vs PVAc Read p68 and see Fig 4.3 & 4.4 p70
polymer from 1,2-disubs vinyl monomers [CH2=CXY] no 3° backbone H negligible CT to polymer
polymer from 1,3-diene Section 4.6.2.2
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Ch 4 Sl 17Kinetics: rate of free-radical polym’n initiation
propagation
termination
Rt =
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Ch 4 Sl 18
steady-state condition [approximation] Ri = Rt
true in early stage, except for very early stage
rate of polym’n = rate of propagation
thermolysis of initiator
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Ch 4 Sl 19
photoinitiation
redox initiation
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Ch 4 Sl 20
assuming no chain transfer, (xn)0
thermolysis of initiator
photoinitiation
redox initiation
Kinetics: molar mass
and (s-s approx)
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Ch 4 Sl 21
kinetic chain length, ν ν = # repeat units/radical = Rp/Rt
Rp and MM
[I] Rp and MM ~ trade-off in Rp and MM
instantaneous equations ~ change with conversion
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Ch 4 Sl 22Diffusion-controlled kinetics
s-s approx valid only at conversion < 15-20% at higher conversion, [I] and [M] → Rp
actually, Rp
become diffusion-controlled
Fig 4.5 p77conv vs time at various [M]0
[MMA]0 in benzene
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Ch 4 Sl 23
IA ~ steady-state
IB ~ [M] → Rp ~ may or may not occur
II ~ autoacceleration viscosity → diffusion of polymer → kt
kp and kd not changed ( small monomer)
Rp, xn → viscosity → heat diffusion →Temp → Rp
AA, gel effect, Trommsdorff-Norrish effect
IIIA ~ at higher conv (>50%), kp also affected
Rp levels off
IIIB ~ glass effect Tg of mixture [M-plasticized polymer]
becomes higher than polym’n Temp
Rp decreases to stop
Rp vs convOdian pp282-287
%polymer
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Ch 4 Sl 24MM with chain transfer
When kr < ki Rp and xn
When kr ≈ ki only xn
C ~ chain transfer constant
kr
<cf> autoinhibition
Mayo(-Walling) Eqn
Mayo(-Lewis) Eqn ~ copolym’n
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Ch 4 Sl 25
CM ~ small, little effect on xn
CI ~ larger than CM, but lower effect on xn
CS or CCTA ~ dep on bond strength <cf> telomerization
Table 4.1 p79
C’s for radical polym’n of ST
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Ch 4 Sl 26Catalytic chain transfer special type of chain transfer to monomer Co(II) abstracts H• from growing radical
Co(III) gives H• to monomer
very effective [high C] for monomer with α-methyl
resulting C=C end group
macro(mono)mer
with controlled MM
p80
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Ch 4 Sl 27
substance reacts withradical, and does not reinitiate ~ inhibitor
slowly reinitiate ~ retarder
O2
harmful in polym’n awa in use
Inhibition and retardation differs only in degree. eg, quinone ~ inhibitor for ST, retarder for MMA
inhibition
retardation
normal
inductionperiod Fig 4.7 p81
Inhibition and retardation
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Ch 4 Sl 28
may be added deliberately for transportation, storage
remove before or feed more initiator in polym’n
quinone, hindered phenol, radical scavenger
<cf> autoinhibition
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Ch 4 Sl 29Molar mass distribution MMD and its prediction much more complex than step polym’n modes of polymer formation; parameters change with conv
At low conversion, ([M], [I], k’s constant) [M] and [I] decrease a little, actually. Problem 4.1
β ~ probability of growth of a radical β = Rp / (Rp+Rt+RCT)
same to p in step polym’n
termination by disproportionation probability of i-mer,
xn = 1/(1 - β)
xw = (1 + β)/(1 - β)
PDI or D,
PDI 2 when β 1 [Rp >> Rt, RCT]
for polymer fraction only, not for whole mixture!!
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Ch 4 Sl 30
low conv (cont’d) termination by combination (j-mer) + (i – j-mer) (i-mer)
2 propagating chains terminated by 1 combination
Mi = i M0
(i – 1) ways
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Ch 4 Sl 31
weight fraction, wi
wi = i Ni / N0 = i N P(i) / N0
PDI,
PDI 1.5 when β 1 [Rp >> Rt, RCT]
smaller PDI than disproportionation
eqn 3.11 p35
Mi = i M0
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Ch 4 Sl 32
At higher conversion, MMD gets much broader. [I] decrease faster than [M] does. larger molecule at later stage
PDI ~ 3 - 5
may be controlled by multiple charge of I
autoacceleration PDI ~ 5 - 10
CT to polymer PDI ~ 20 - 50
Hard to control AA and CT to polymer avoid bulk polym’n or stop at low conversion
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Ch 4 Sl 33Determination of kinetic parameters Rp
separation of reaction product step polym’n ~ distillation of small molecules
chain polym’n ~ isolation of polymer (ppt in nonsolvent)
chemical or spectroscopic analysis step ~ disappearance of functional group
chain ~ disappearance of double bond
dilatometry monitoring volume change
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Ch 4 Sl 34
kd
[I] by spectroscopy
Table 4.2 p86
f f kd
using radical scavenger color change by UV/vis
end-group analysis low conc’n in polymer
0.3 < f < 0.8 = Ri
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Ch 4 Sl 35
kp and kt
from kp/kt½ and kp/kt
kp/kt½
using [M]0 and [I]0 knowing Rp and f kd
at s-s (low conv)
kp/kt
τ, avg life-time of radical
τ measured using‘rotating sector method’ ~ photopolym’n with on/off
0.1 < τ < 10 sec Odian pp263-267
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Ch 4 Sl 36
kp and kt separately by using pulsed-laser polym’n p87-88 initiation/termination by short pulse (10 ns) laser
MM of the product measured
chain transfer constants
one by one. pp78-79 CI (AIBN) ≈ 0 plot 1/xn vs [I]0
½/[M] CM
with other I CI
with S CS
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Ch 4 Sl 37
Odian p269
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Ch 4 Sl 38
As T up, [M] up, and [M]2 up more rapidly. (p89) ?
Effect of Rxn Temp ~ Activation energy
> 0 ~ As T up, Rp up.
< 0 ~ As T up, xn down.
Table 4.4 p89
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Ch 4 Sl 39
photopolym’n Ed = 0
Actually, may not. Photoinitiators are thermally decomposed.
chain transfer
> 0 ~ As T up, C up, and xn down.
> 0, but small ~ lower effect
> 0 ~ As T up, xn up! (p89)
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Ch 4 Sl 40Equilibrium thermodynamics Polym’n – depolym’n is an equili rxn
∆G = ∆H – T∆S ∆Hp < 0 (–50 ~ –100 kJ/mol)
H(formation of σ bond) > H(breaking π bond)
∆Sp < 0 (–100 ~ –120 J/mol K)
loss in DOF by decreased number of molecules
disfavors polym’n
|∆Hp| must be > |T∆Sp| for polymerization. T must be lower than Tc.
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Ch 4 Sl 41
ceiling Temp Tc
Tc largely depends on ∆H ∆S similar ∆H depends on structure Table 4.5 p90
low Tc for 1,1-disubstituted monomers small ∆H due to steric
critical in carbonyl polym’n (ionic polym’n)
∆G = ∆G0 + RT ln K = 0
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Ch 4 Sl 42
Floor Temp ∆Sp > 0 and ∆Hp > 0 (both very small)
depolym’n vs thermal stability vs heat resistance Tg vs HDT (or VSP)
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Ch 4 Sl 43Radical polym’n processes 4 types of process bulk, solution, suspension polym’n ~ same kinetics
emulsion polym’n ~ somewhat different
bulk polym’n simple and pure [M] fixed at max
[I] and Temp controlled to compromise Rp and MM
problems at high conversion heat, shrinkage, and autoacceleration
stop at low conversion recovery of monomer needed
polymer may not be soluble in monomer ~ ppt polym’n eg, PVC; vs suspension
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Ch 4 Sl 44
bulk polym’n (cont’d) 2-stage polym’n for PMMA (large) sheet 1st to a low conversion viscous solution
2nd in a sheet mold taking care of heat and shrinkage problems taking care (actually taking advantage) of autoacceleration
Alternatively, polymer + monomer in the cast [mold]
‘cast polym’n’
Plexiglas® (cell casting)
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Ch 4 Sl 45
solution polym’n use of solvent that dissolve M, I, and polymer no heat and autoacceleration problem
low rate of polym’n low [M]
CT to solvent
isolation of polymer needed evaporation of solvent or precipitation in non-solvent
useful only for polymer used as solution (paint, adhesive)
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Ch 4 Sl 46
suspension polym’n M+I suspended in (heated) water to form droplets with vigorous stirring determine size (.05 – 2 mm)
with dispersion stabilizer (water-soluble polymer like PVA)and surfactant
bulk polym’n in droplet problems of bulk polym’n resolved
droplet bead ~ ‘bead polym’n’
commonly used industrially for many polymers esp for PVC
not for low-Tg polymers
What if monomer and/or polymer water soluble? inverse suspension polym’n, eg acrylamide [PAM]
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Ch 4 Sl 47Emulsion polymerization emulsion = colloidal dispersion
particle size < few µm (suspension < few mm)
monomer micelles in water
polym’n to form latex (= polymer particles in water)
recipe water-insoluble M
water-soluble I ~ usually redox (like persulfate)
surfactant [emulsifier] (like SLS)
water
3 phases
monomer-swollenmicelle (10 nm)(when conc’n > CMC)
monomer droplet(10 µm)
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Ch 4 Sl 48
mechanism Interval I: particle nucleation I R oligomer radical [O]
micellar nucleation (z-mer) O into micelle and polym’n M from droplet thru aq phase surfactant from inactive micelle
homogeneous nucleation (j-mer) less water-insoluble M to form primary particle
Fig 4.9 p95
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Ch 4 Sl 49
mechanism (cont’d) Interval II: particle growth polym’n in the particles
M from droplet through aq phase
radical from aq phase: repeat initiation/termination
Interval III: completion no more monomer droplet
polym’n of residual M
autoacceleration may occur
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Ch 4 Sl 50Emulsion Kinetics ~ Smith-Ewart theory rate of emulsion polym’n
Np ~ # of particles [active micelles at the end of interval I] n ~ avg # of radicals/particle 0.5 [M]p ~ monomer conc’n in a particle
at Interval I, Np Rp
at Interval II, Rp constant [M] constant
at Interval III, [M]p Rp
Fig 4.10 p96
Rp
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Ch 4 Sl 51
MM
ρ = molar rate of radical formation from initiator
possible to enhance Rp and MM at the same time. by increasing Np increasing conc’n of surfactant
reason ~ compartmentalization
applications of emulsion polym’n water-borne paints, adhesives, coatings
core-shell particles
water soluble monomers? inverse emulsion polym’n
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Ch 4 Sl 52Mini-emulsion polym’n start with small (~100 nm)
monomer droplet polymer part’l
compared to macroemulsion; high shear
costabilizer ~ highly water-insoluble like C16H34 or oligomer
osmotic P against Ostwald ripening
applications water-insoluble additives
in-situ polym’n in droplet eg, epoxy latex
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Ch 4 Sl 53Microgel micron-size crosslinked polymer
temperature-sensitive microgel water-insoluble at polym’n Temp > LCST ~ deswell
water-soluble at lower Temp < LCST ~ swell
eg, pNIPAAm ~ useful in drug delivery
Fig 14.2 p321
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Ch 4 Sl 54LIVING RADICAL POLYMERIZATIONS living polym’n polym’n with no termination originally in anionic polym’n for limited monomers
gives low PDI enables (block) copolymer structure control
living radical polym’n became available very recently not truly living, though termination reduced, not absent quasi- or pseudo-living
living radical polym’n = controlled radical polym’n= reversible-deactivation radical polym’n (IUPAC)
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Ch 4 Sl 55Two types of strategies
Fig 4.11 p100
S1. reversible end-capping
very low [M]
very low Rt (∝ [M]2)
low Rp also
S2. reversible CT [M] similar to
conventional
Rp also similar
Rt very low
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Ch 4 Sl 56
quasi-living radicals very effective and fast activation-deactivation
very short (transient) life-time trad ~ .01 - .1 ms conventional τ ~ .1 - 10 s
gives short chain/period x = kp [M] trad ~ .02 - 35 repeat unit/period p100
termination suppressed MM increase linearly with conversion
narrow MMD
block copolymer can be prepared
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Ch 4 Sl 57
nitroxide-mediated radical polym’n
living-like polym’n possible due to stable nitroxide ~ no self rxn ~ ‘persistent radical’
weak C-O bond
small K most chains dormant very low [M] negligible termination
equal chance to growth, no AA low PDI
NMP
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Ch 4 Sl 58
developments ST polym’n with TEMPO at 125 ºC with conventional initiator
‘stable-free-radical(-mediated) polym’n (SFRP)’
ST, acrylates, dienes at lower Temp with TIPNO, SGI
use of monomer-alkoxyamine as initiator better control of [M]
current standard
Fig 4.13 p102
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Ch 4 Sl 59
kinetics of NMP
plot linear only in early stage; positive curvature later ‘persistent radical effect (PRE)’ termination [M] , (relative) [NO] termination
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Ch 4 Sl 60
modified kinetics
AA possible, but not critical (kt-1/3)
MM in NMP
c ~ (fractional monomer) conversion
PDI? should be ≈1; actually 1.1 - 1.3
~ linear
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Ch 4 Sl 61
side reactions in NMP
serious in MMA ~ NMP not for MMA yet
purely thermal (self) initiation of ST (at 80 ºC)
Practically, compensate ‘persistent radical effect’
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Ch 4 Sl 62ATRP atom-transfer radical polymerization by Krzysztof Matyjaszewski
CuBr(L) CuBr2(L)
CuBr(L)
CuBr2(L)
initiator
‘activator’
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Ch 4 Sl 63
initiators
resembles monomer structure ~ slightly larger activity
activators
Cu dominant
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Ch 4 Sl 64
features similar as NMP chain growth through
repeated ‘activation-propagation-deactivation’
fast activation-deactivation reactions
small K, most chains dormant, low [M]
low Rt, low Rp
low PDI, radicals live for 2nd monomer
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Ch 4 Sl 65
Kinetics of ATRP
Like NMP, 1st order also be modified by ‘persistent radical effect’
Unlike NMP, [activator]/[deactivator] controls kinetics.
PDI? should be ≈1; actually 1.1 - 1.2
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Ch 4 Sl 66
Compared to NMP, ATRP is for more monomers styrenics, (meth)acrylates, --- even E, VAc ~ still developing
Acidic H (like in AA) should be ionized or protected.
has more choice of initiators, activators, and deactivators
is better controllable reverse ATRP, SR&NI, ICAR, AGET, ARGET
to control K, Rp, MM, MMD
use metal ~ should be removed
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Ch 4 Sl 67RAFT reversible addition-fragmentation chain transfer
radical polym’n in (otherwise normal) radical polym’n
use of very effective CT agent [RAFT agent]
CCTA ~ as high as 1000
A and R of similar reactivity
fragmentation addition
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Ch 4 Sl 68
RAFT mechanism
use [RAFT]0 >> [I]0 (like 10-fold)
all chains with RAFT end-group no termination
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Ch 4 Sl 69
mechanism (cont’d)
fast and effective A-F living-like
MM
all chains with RAFT end-group and living-like
narrow MMD by that all chains grows equal rate
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Ch 4 Sl 70
kinetics Rp should be the same to conventional radical polym’n.
Actually, slower. Probably due to radical formation by thermal initiator only
[RAFT]0 >> [I]0
stable adduct radical slow fragmentation [M]
termination by rxn of adduct radical and other radical [M]
Compared to NMP and ATRP, RAFT is much faster
is for much more versatile monomers
color
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Ch 4 Sl 71
RAFT agents Fig 4.18 p112
adduct stability (Z)
stabilizing radical ~ reactive
thiobenzoate > thiocarbonate > thiocarbamate > xanthate
leaving radical stability (A) stable radical ~ better leaving ~ reactive
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Ch 4 Sl 72
RAFT agents (cont’d)
more reactive RAFT agent for more reactive monomer monomer reactivity ↔ its radical stability important in copolymerization Chapt 9
reactive monomer ~ stable radical ~ need reactive RAFT agent
for PS
p212
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Ch 4 Sl 73Non-linear radical polym’n with crosslinking monomers Degree of Xlinking depends on
content of Z. monomers Fig 4.20
(i) DVB for PS gel for GPC ion-exchange resin -SO3
(-)
bead for protein synthesis
(ii) (EG)DMA for acrylates ‘hydrogels’ hydrophilic network soft lenses
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Ch 4 Sl 74
(iv) graft-linking monomer with different reactivity
to graft copolymer
for core-shell particles
(v) bis-GMA for dental material
(vi,vii) for higher Xlinking density
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Ch 4 Sl 75
with unsaturated polymers unsaturated polyester diol + diacid + unsat’d diacid unsat’d prepolymer
prepolymer + styrene + initiator crosslinking
useful for composite
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Ch 4 Sl 76
in (1,3-)diene polym’n Chapt 6
copolymerization of double bonds along the chain ~ from 1,4-polym’n
at side group ~ from 1,2- (or 3,4-)polym’n
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Ch 4 Sl 77
in diene polym’n (cont’d) CT to polymer abstraction of allylic H (4 H’s/ru)
uncontrollable? by conversion or CT agent