general polymer synthesis

7/29/2019 General Polymer Synthesis

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General Approaches to Polymer Synthesis

1. Addition Chain Growth

Polymerization of Vinyl Monomers

Ring Opening Polymerization

Heterocylics

Metathesis of Cyclic Olefins

2. Condensation Step GrowthPolymerization of A-B or AA/BB Monomers

3. Modification of Preformed Polymers

Polysaccharides

Peptides and Proteins

Synthetic Precursors

Current Strategies in Polymer Synthesis

Objectives: Precise Macromolecular Design

1 . Control of: Molecular Weight

Molecular Weight Distribution Composition

Sequence of repeat units

Stereochemistry

2. Versatility

Anatomy of Addition Polymerizations

Initiation

Generation of active initiator

Reaction with monomer to form growing chains

Propagation

Chain extension by incremental monomer addition

Termination

Conversion of active growing chains to inert polymer

Chain Transfer

Transfer of active growing site by terminating onechain and reinitiating a new chain.

Polymerizability of Vinyl Monomers

Active Centers must be stable enough to persist

though multiple monomer additions

Typical vinyl monomers

X X X

radical cationic anionic

O ROO

CH3

OEt

O

CN


++++Styrenes

++++1,3-Dienes

+-+-1,2-Dialkylolefins

--+-1,1-Dialkylolefins

+-+/--Propylene

++-+Ethylene

ComplexMetal

AnionicCationicRadicalMonomers


+/-+/-+/-+SubstitutedStyrenes

--+-Vinyl ethers

-+-+Acrylonitriles/ Acrylamides

-+-+Acylates/methacrylates

---+Vinyl esters

+/---+VCl

ComplexMetal

AnionicCationicRadicalMonomers


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Thermodynamics of Polymerization

X X X X

Gp = Hp-TSp

Hp < 0 -bond-bond

Sp < 0 Loss of translational entropy

Polymerization favored below a ceiling temperature, Tc

Tc =

S

Thermodynamics of Polymerization

326 ( 123)

50

12148Isobutylene

318 (45)

61

11035-Methyl

styrene

478(205)

220

10456MMA

600 (327)400

15593Ethylene

Tc, K (C)

Observed

-Sp,

J/K-mole

-Hp,

kJ/mole

Monomer

Free Radical Initiated Polymerization

Classical Free Radical Process

Applied to wide range of monomers

Broad scope of experimental conditions

Molecular weight can be controlled

Mw/Mn > 1.5 2.0

Statistical compositions and sequences

Little stereochemical control

Free Radical Initiated Polymerization

Controlled Free Radical Polymerization

Broad range of monomers available

Accurate control of molecular weight

Mw/Mn 1.05 --Almost monodisperse

Blocks, telechelics, stars

(Controlled molecular architecture)

Statistical Compositions and Sequences

Types of Radical Initiators

Application Temperatures, T1/2

= 10 hr.

150C Hydroperoxides and Alkyl peresters

80C Benzoyl Peroxide, AIBN, Persulfates

25C AIBN + Light, Percarbonates,

Photoinitiators

05C Redox Systems, ROOH + Me++

Thermal Free Radical Initiators

Rate of Decomposition

Temperatures giving half lives of 10 hr

considered optimum use temperatures

Rd = kd [I] where k = A e-Ea/RT and A 10

15 sec-1

To produce 10-7

to 10-6

radicals mole/l.sec,

Ea 30-40 kcal/mole (115-140 kJ/mole)

For 1st

order reactions, half-live, = ln 2/k


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Fate of Initiator Radicals

Radical reactions

Chain initiation, Ri = 2 f kd [I]

Efficiency factor, f = 0.1 - 0.9

Recombination in solvent cageRecombination in mediaReaction with polymer radicals (kt)

Reaction with initiator (MIH)Radical abstraction from polymer chainsReaction with solvent or inhibitor

R +

X

R

Xki

Kinetics of Polymerization

Initiation steps

Rd = kd[I]

Ri = k i [I.] [M]

Where [I.] = 2 kd [I]

Add efficiency factor, Ri = 2 f ki [I] [M}

RO

O

O

R

O

heat

R O

O

2

R-CO2

R +

X

R

Xki

Propagation Steps

Sequential monomer addition

Assume rate of addition is independent of radicalsize, Rp = kp [M

.][M]

R

X

+ M R

X

Mkp1

Rp = p1[M ][M]

Rp = kp2[MM ][M]R

X

M + Mkp2

R

X

M M

R

X

P M + Mkp

R

X

P M

Termination Steps

Termination by coupling,

Rtc = 2 ktc [RPM.]2

Termination by disproportionation

Rtd = 2 ktd [RPM.]2

2R

X

P MR

X

PM

ktc

R

X

P M

R

X

P CH X

H

2ktd

R

X

P CX

H

R

X

P CH

C

X

H H H+

Chain Transfer

Hydrogen transfer to growing polymer chain

Reinitiation of growing chain using transferred

radical

R

X

PCH X

H

+ R SH R

X

PCH X

H

H

+ R S

ktr

R S +

X

R S

Xka

kp

Rate Expressions for Radical Polymerization

Overall growth of polymer Rpoly = R i + Rp Rt

Rpoly ~ R p Rp= kp [M.][M]

Assumptions: Contribution of Ri and Rt negligible for

high degrees of polymerization

Radical concentration based upon Steady State

concentration of radicals, i.e. Ri = Rt

2 ki f [I][M] = 2 (ktc + ktd) [M.]2

[M.] = {(ki f)/ (ktc + ktd)}1/2 [I]1/2 [M]

Assume [M] on initiation is negligible

[M.] = {(ki f)/ (ktc + ktd)}1/2 [I]1/2


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Rate Expressions for Radical Polymerization

Overall rate of polymerization

Rp= kp [M.][M]

[M.] = {(ki f)/ (ktc + ktd)}1/2 [I]1/2

Then Rpoly = kp {(ki f)/ (ktc + ktd)}

1/2[I]1/2[M]

Rate of polymerization is proportional to:

square root of initiator concentration

First order in monomer concentration

Control of Molecular Weight

Impact of initiator concentration

DP = Rp / Rt where is the kinetic chain length

For termination by coupling DP = 2

For termination by disproportionation, DP =

=

Rp

Ri

Rp

Rt=DP =

DP =kp [M

.] [M]

kt [M.]2

1

DP=

kp [M.] [M]

kt [M.]2 =

kd(f kd)1/2 [I]1/2

kp [M]

Control by Chain Transfer

Add chain transfer processes to termination processes

Assume chain transfer to monomer and initiator are small

Where Ctr is the chain transfer constant

1

DP=

kp [M.] [M]

kt [M.]2 + ktr[SH][M

.] ktm[M][M.]+ kti[I][M

.]+

1

DP=

kp [M.] [M]

kt [M.]2 + ktr[SH][M

.]=

kt[M.]

kp[M]+

ktr[SH]

kp[M]

1

DP=

1

DPo+

[SH]

[M]Ctr

Calculation of Ctr

1/DP

[SH]/[M]

1/DPo

Ctr

Types of Vinyl Polymerization

Water in oil latex

formed

Inversion promotes

dissolution in water

Inverse Emulsion

Removal of additives

Coagulation needed

Latex stability

High RpolyLow Temperatures

High Mol. Wt.

High surface area latex

Emulsion

Removal of additivesLow viscosity

Direct bead formation

Suspension

(Pearl)

Lower mol. Wt.

Low RpolySolvent Recovery

Good mixing

Ready for application

Solution

Heat buildup

Gel effectBranched or crosslinked product

Simple equipment

Rapid reactionPure polymer isolated

Bulk (Neat)

DisadvantagesAdvantagesMethod

general polymer synthesis

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