controlled radical polymerization

8/20/2019 Controlled Radical Polymerization

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Controlled Radical

Polymerization

ATRP and RAFT Processes

Controlled Radical Polymerization

Controlled Radical Polymerization


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Nitroxide (SFR) Controlled Radical Polymerizations

Scope and Limitations

Controllable Monomers

Styrene, Butadiene, Methyl Methacrylate, Styrene Sulfonate(Aq)

Problem Monomers

Alkyl Acrylates, Vinyl Acetate, Vinyl Chloride

Initiator Structure

Initiator formed in situ using commercial initiators

SFR adduct can be introduced to preformed structures including

dendrimers, functionalized polymers and telechelics

Useful Additives

Camphorsulfonic acid - deactivates styrene dimer

Chain Transfer Chemistry

• 1. Thiols:

•

•

•

•To produce monofunctional polymers, use functionalized thiols:

P .

H S(CH2)3CH3+ P H + S(CH2)3CH3

.

CH2=CH

XCH3(CH2)3-S CH2CH

X

.

CH2=CH

XCH3(CH2)3-S P

.

HO-CH2CH2-SH HO-C-CH2-SH

O

2. Disulfides

+ S SX XP

.

P S

X+ S

.

SSX X

Can lead to difunctional telechelics

Dithiauram disulfide extremely efficient

NCH3

CH3C S

S

S C

S

N CH3CH3


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P .

NCH3

CH3C S

S

S C

S

N CH3CH3

+N

CH3

CH3C S

S C

S

N CH3CH3

S

P

.

.NCH3

CH3C S

S C

S

N CH3CH3

S

P

+ CS

S

NCH3

CH3

.

N

H3C

H3CC

S

S

P

NH3C

H3CC

S

S S C

S

NCH3

CH3

Dithiauram Chain Transfer Process

Resultant polymer is difunctional iniferter , Useful in

controlled free radical polymerization

Iniferter Polymerization Technique

Initiator Transfer Terminator = Iniferter

NCH3

CH3C S

S

S C

S

N CH3CH3

uv lightN

CH3

CH3C S

S. S C

S

N CH3CH3

.

S C

S

N CH3CH3

.+

Primary radical termination

NCH3

CH3C S

S. N

CH3

CH3C S

S

CH2 CH.

Styrene

NCH3

CH3C S

S

CH2 CH S C

S

NCH3

CH3

Otsu, et. al. Makromol. Chem., Rapid Commun., 1982. 3: 127-132; 133-

140.

Formation of Carbon Centered Radicals

NCH3

CH3C S

S

CH2

CH S C

S

NCH3

CH3 UVN

CH3

CH3 C S

S

CH2 CH

.

S CS

N

CH3

CH3

.

+

Alternate Initiators

NCH3

CH3

C S

S

CH2 CH S C

S

NCH3

CH3

CH2 CH3

CH3N

S

CS

BDC NCH3

CH3C

S

S

CH2

CH2CH2

CH2S

S

CCH3

CH3N S C

S

N CH3CH3

S C

S

N CH3CH3

Durene tetrakis (N,N-diethyldithiocarbamate

(DDC)


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Generic RAFT Agent

X

Z

X R

Reactive

double bond

Z modifies addition and

fragmentation rates

R is free radical

leaving group (R.)

It must be able to

reinitiate

polymerization

Monomer inserted into

weak single bond

X = S

Chain transfer agent that acts by fragmentation

Reversible Addition-Fragmentation Chain

Transfer Polymerization (RAFT)

Controlled with designed chain transfer agents (RAFT agents)

Raft agent

Chain transfer

Reinitiation

CZ

S S R P +

. k add

k -add

.P SCZ

S R

M

+k

P SCZ

S R .

MR M

..R

CZ

S S PnPm +. k add

k -add

.Pm S CZ

S Pn

M

+k

Pm S CZ

S Pn.

Chain Equilibration

Apparent Chain Transfer Constants

k tr = k add

k

k -add + k

Ctr = k tr/k p where

Useful Raft agents have Ctr > 2

26

10

2.3

0.72

0.01

Z = Ph

Z = CH3

Z = OC6F6

Z = OPh

Z = NEt2

Ctr at 80 CRaft agent

Styrene monomer

with

Impact of Z substituents on reactivity

S

Z

S CH2


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Apparent Chain Transfer Constants

• Impact of R substituents on reactivity

13

10

2

0.4

0.16

0.03

0.03

R = C(CH3

)2

CN*

R = C(CH3)2Ph*

R = C(CH3)2CO2Et

R=C(CH3)2CH2C(CH3)3

R = CH(CH3)Ph

R = C(CH3)3

R = CH2Ph

Ctr at 80 CRaft agent

Styrene monomer

with

S

Ph

S R

* Dependent upon initial concentration of RAFT agent

Concentration effect of RAFT agent

S

Ph

S C

CH3

CH3

Thermal initiated polymerization of

styrene at 110C for 16 hr

Mn, PDI, conv: control, 324K, 1.74, 72%;

0.0001RA, 189K, 1.59, 59%; 0.0004RA, 107K, 1.21, 60%

0.0010 RA, 48K, 1.07, 55%; 0.0029RA, 14.4K, 1.04, 55%

Factors responsible for retardation

• Slow fragmentation of initiator adduct

Slow fragmentation of polymer adducts during propagation

Slow reinitiation by expelled radicals

Selectivity for the expelled radical (R .) to add to the RAFT

agent rather than to monomer

Selectivity for the propagating radical (Pn.) to add to the RAFT

agent rather than to monomer, (i.e. transfer constant to high)


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C O S S Ph

SO

4

styrene, 110 C

C O S S Ph

SO

4

Ph

n

Star Polystyrene Synthesis

Addition/fragmentation

S SPh

S

+

Ph

Ph

n

Ph

Ph

Ph

n

Ph

2

Ph

Ph

n

Ph

2

+ Disproportionation

Byproduct formation during star synthesis

Using UV detector, no shoulder detected

RI and LS detectors

General Mechanism for Atom Transfer

Radical Polymerization (ATRP)

R X + Mtn

YLigand

k act

k deact

R

k pMonomer

Mtn+1

YX Ligand+K eq =

k actk deact

k tTermination

Keq determines polymerization rate

Rp = kp[M][P*] =kpKeq[M][I]o x [CuI/[X-CuII]


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Mechanism of ATRP Process

Initiation via Redox activation of alkyl halide

R X R X

Y

Mtn

Mtn+1

X

R R

Y

k i

Y


R X

Y

Mtn

Mtn+1

X

R

R

Y

k p

Y

z

R

Y

Yz

k p

R

Y

X

Yz

Propagation via reversible activation steps


R

Y

Yz

Mtn+1

X2

R

Y

Yz

H

+ R

Y

Yz

+ 2 Mtn+1

X

T

e

Termination reactions lead to enhanced concentration of

persistent radicals--PRE

Disproportionation

and coupling

Persistent radicals shift equilibrium and

lower concentration of propagating radicals


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Polymerization of Methyl Acrylate (MA)

Evolution of Mn and MWD, Mw/Mn with monomer

conversion for bulk polymerization of MA at 130

C[1-(PE)Cl]= [CuCl] = 0.038; [bpy] = 0.11

Polymerization of Methyl Acrylate (MA)

Mn and MWD, Mw/Mn on monomer conversion for

bulk polymerization of MA at 130 C

[1-(PE)Cl]= [CuCl] = 0.038; [bpy] = 0.11

Optimized MA Polymerization

Evolution of molecular weight and polydispersity in the ATRP of MA,

T = 90 C,[MA] = 11.2 M; [MA]/[MBP]= 1513;

[MBP/[CuBr]/[dtbpy] = 1/1/2

MBP = methyl 2-bromoporpionate, dTbpy = 4,4’-di-tert-butyl-

2,2’-bypyidine


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Components of ATRP

• 1. Monomers—Stabilizing groups (phenyl or carbonyl)

adjacent to carbon radicals

2. Initiators-- Alkyl halides or psuedohalides

Structure should be similar to propagating halide

3. Catalysts-- Transition metal with two readily available

oxidation states

Ligands control solubility and dynamics of

ATRP equilibrium

4. Solvents—inert to chain transfer, non-poison to

catalysts but solvate catalyst initiator complex

5. Temperature—high temperature better but side

reactions and catalyst stability limit choice

ATRP Monomers

• Styrenes

Methacrylates

ATRP Monomers

• Acrylates


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ATRP Monomers

• Acrylonitrile—need solvent like ethylene carbonate

(Meth)acrylamides—tend to deactivate catalystsAcrylamide not successful

Methacrylic acids --poison catalysts

Can polymerize protected monomers

Initiators

• Halogenated Alkanes and Benzylic Halides

α-Haloesters, a-Haloketones, Alkyl and Aryl Sulfonyl Chlorides

Catalysts

• Transition metal complexes of:• Rhenium,

• Ruthenium and Iron

• Rhodium,

• Nickel and Palladium

• Copper


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Ligands for Copper Complexes

Bipyridines < < Me5DETA < Me6TREN

Reactivity

Block Copolymers

Best results with monomers in same class (i.e. acrylates)

Block Copolymers

• Sequence of addition critical for monomersfrom different classes

Methyl acrylate then methyl methacrylate

poor transfer, broad PDI

Methyl methacrylate then methyl acrylate

good transfer, narrow PDI

Crosspropagation activity AN > MMA > Sty ~~MA


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Functionalized Polymers via ATRP

Functionalized Polymers via ATRP

Nucleophilic modification of active halide end group

Molecular Architecture via ATRP


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controlled radical polymerization

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