2 Chain-Growth Polymerization

8/13/2019 2 Chain-Growth Polymerization

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Chain-growth Polymerization

by Dr . Wal a i por n Pr i s s anar oon- Oua j a i

Dept . o f I n dus t r i a l Chemi s t r y KMUTNB

411317 Polymer Chemistry (updated 2/2552)

411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)


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Chain-growth polymerization

formation of polymers via chain reaction

Key factors for chain-growth polymerization

monomers

initiator (to break -bond)



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Monomers for chain-growth polymerization

Aldehyde

or ketone

Alkene

(olefins & vinyl monomers)

except

Acetylene

H2C C C CH2

H XH2C

C C CH2

H H

H2C C C CH2

H Cl

HC CH

Ring-opening polymerization

Diene



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Mechanisms of chain-growth polymerization

1. Initiation

2. Propagation

3. Termination (Dependent on type of active center)

Propagating chain(polymer chain with active center)

Addition polymerization

Active center = +

Active species (initiator fragment with active center, can be +, - or radical)

Active center = radical

Active center = -

Polymer

Chain transferring agent Dead chain(polymer chain without active center)

Active center



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Summary Mechanisms of polymerization for polyethylene

R R

Degree of polymerization (DP, Xn)

= number of monomer unit

in a polymer chain



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Characteristics of Chain-growth Polymerization

1. !"#$%&'()*MW +,#-./0-)12!*

"34#"536/723*8"()*-9%%"%1!

8:2#';&)#3&1/8-


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Some comparisons between Step-growth and Chain-growth polymerizations

Step-growth Chain-growth

6. Mn

of polymer High Mn

at high conversion

(Mn time)

High Mn

at low conversion

(Mn time but n time)

2. [M] with time Immediately disappeared

1. Monomer type Contain at least 2 functionalities Contain unsaturated bond

Gradually decrease

3. Reactivity Reactivity of functional end group

is independent on size of polymer

Reactivity of active centre decreases

With longer polymer chain

5. Mixture composition

during reaction

Dimer, oligomer, polymer

and trace monomer (< 1%)

Monomer and polymer with high Mn

time

MnorX

n

Chain

Step

Xn

Xn

Wt.fraction

Wt.fraction

PER

Step Chain

4. Rate Growth of chains is usually slow

(minutes to days)

Chain growth is usually very rapid

(


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Initiator for free radical polymerization

sometimes called "catalysts"

a source of free radicals

radicals must be produced at an acceptable rate at convenient

temperatures

have the required solubility behavior

transfer their activity to monomers efficiently

be amenable to analysis, preparation, purification

Requirements for an initiator



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1. Organic peroxides or hydroperoxides

Cumyl hydroperoxide

2. Azo compounds

Examples of free radical initiation reactions

Benzoyl peroxide (BPO)

2,2'-Azobisisobutyronitrile (AIBN)

Low dissociation energy of the O-O bond

But reagents are unstable.



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3. Redox systems

4. Electromagnetic radiation

Redox initiator = Initiator + Reducing agent

hydrogenperoxide

persulfate

Soluble in water (can also work in organic

solvents)

Low dissociation energy then can proceed at

relative low Temp reduce side effect

photochemical initiation involves the direct excitation of the monomer or photolytic

fragmentation of initiators

photochemical initiators include a wider variety of compounds



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Free Radical Initiator Efficiency

reactive species can undergo as alternatives to adding to monomers tocommence the formation of polymer

two radicals are trapped together in a solvent (cage) resulting in

direct recombination

2-cyanopropyl radicals from AIBN acetoxy radicals from acetyl peroxide

benzoyloxy radicals from BPO

Solvent Cage

Reduce free radical efficiency

(the efficiency with which these radicals initiate polymerization)



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Reactions between the initiator radical and the solvent

For example, carbon tetrachloride is quite reactive towards radicals because

of the resonance stabilization of the solvent radical produced.

These species are less reactive than the initiator radicals

These species can be recombined with the initiator radicals

Reduce free radical efficiency

Terminate polymerization via chain transfer reaction



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f depends on the conditions of the polymerization, including the solvent.

In many experimental situations, f = 0.3-0.8.

Free Radical Initiator Efficiency (f)

radicals incorporated into polymer

radicals formed by initiator

f =

Note: f should be monitored for each system studied.

Evaluation of initiator efficiency

1. Direct method - End-group analysis

Limitation: difficult in addition polymers (very higher MW than condensation polymers)

R Rn

2. Indirect method

Reaction with scavengers

diphenylpicrylhydrazyl radicals411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)


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1. Initiation

Mechanism of Free radical Polymerization

Step1: Dissociation of initiator

Step2: Reaction of radical

with 1st monomer

In case of asymmetry monomer Ex.

There are 2 possible ways for the reaction of radical to 1st monomer

Part I is higher possibility (low Ea

)

and radical can resonance with X group



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2. Propagation

Again! for

there are 2 possible ways for the reaction of propagating chain to next monomer

head-to-tail configuration, H-T

head-to-head configuration, H-H

headtail

Part I (H-T) is higher possibility and more stable



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3. Termination

two propagating chains are deactivated, resulting in dead polymer

Two principal modes of termination

Little monomer is left

Low efficiency of active centres

in long propagating chains

Reasons for termination

1) Combination or Coupling (connect two active centers)

2) Disproportionation (transfer an atom (normally H) from one propagating chain to another)



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produce one polymer chain with

single "head-to-head" linkage

a polymer chain contains two

initiator fragments (R) per molecule

higheraverage MW

Comparison of termination by coupling and disproportionation

Coupling Disproportionation

produce two polymer chains

one polymer chain contains double bond

and another contains only single bond

each polymer chain contains one initiator

fragments (R)

loweraverage MW

Note: - Since the disproportionation requires bond breaking, Etd

> Etc

- Coupling occurs at lower temperature.

Examples:At 60 GC polyacrylonitrile 100% couplingpoly(vinyl acetate) 100% disproportionationPS and PMMA both processes

H



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Kinetics of Free radical Polymerization1. Initiation

Step1: Dissociation of initiator

Step2: Reaction of radical

with 1st monomer

where kd

= Rate constant for dissociation of initiator

where ka

= Rate constant for formation of active center

If f= free-radical efficiency

Rate determining step

1/2 Rate of radical formation = Rate of initiator dissociation

From differential rate law

(for most initiators Ex. Peroxide, azo)

Ri

= d[ R.] = 2 f k

d[I]

dt

+ 1 d[R.] = - d[I] = k

d[I]

2 dt dt

Rate of initiation (Ri) = Rate of radical formation



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kd

and activation energies (Ed) for some initiator decomposition reactions.

Data from J. C. Masson

Effect of temperature on Ri

k = Ae (-E*/RT)

ln k = ln A(E*/RT)ln k

d1= E* 1 - 1

kd2

RT T1

T2

Arrhenius equationEvaluation of k

dat different temperature



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+ 1 d[R.] = - d[I] = k

d[I]

0

2 dt dt

Evaluation of kd

d [I] = - kddt

[I]0

d [I] = - kd

dt

[I]0

t=0

t=t

t=0

t=t

ln [I] = - kd

t[I]

0

where [I] = concentration of initiator at t = t[I]

0= concentration of initiator at t = 0

time

ln [I]

[I]0

Slope = -kd

Assume f = 1



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2. Propagation

where kp

= Rate constant for propagation

Assumption: kp

is a constant independent of the size of the growing chain

(same kp

for every propagation steps)

3. Termination

where ktc

= Rate constant for termination by coupling

ktd

= Rate constant for termination by disproportionation

Rp

= d[RMn

.] = k

p[RM

n-1

.][M] = k

p[RM

n

.][M]

dt



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3. Termination

where ktc

= Rate constant for termination by coupling

ktd

= Rate constant for termination by disproportionation

where kt= k

tc+ k

td

From differential rate law

- 1 d[RMn

.] = k

t[RM

n

.]2

2 dt

Rt

= d[RMn. ] = 2 k

t[RM

n.]2

dt

Rate of termination (Rt) = Rate of RMn

.

reduction



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Rp

= kp[RM

n

.][M]0

In propagation step

Stationary state radical concentration [RMn

.]

At the beginning of polymerization Ri

>> Rt

After a period of time

Ri

= Rt

Lots of RMn

.are formed in Initiation step whereas

lots of RMn

.are disappeared in termination step

Total radical concentration [RMn

.] becomes constantstationary state

2 f kd[I] = 2 k

t[RM

n

.]2

[RMn

.] = [f k

d[I]]

kt

1/2

[RMn

.] = 0



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Rp

= kp[RM

n

.][M] [RMn.] = [f k

d[I] ]

kt

1/2but

therefore Rp

= kp

f kd

[I] [M]

kt

1/21/2

Overall Rate of Polymerization (Rpol

) Rp

Rpol

= K [I]0

[M]0

1/2Initial rate of polymerization

a) Effect of [I] on Rpol

; Rpol

[I]1/2

b) Effect of [M] on Rpol

;

- if free radicals have very high efficiency (f!1) and do not depend on [M]

Rpol [M]

- if free radicals have low efficiency (f!1) and depend on [M]

Rpol [M]3/2



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Rpol

= K [I] [M]1/2

log Rpol

= log K + 1/2 log [I] + log[M]

(b) [AIBN] in MMA (l)

[BPO] in styrene (n)

[BPO] in MMA (p )at constant [M]

Log-log plots of Rp

versus concentration which confirm the kinetic order.

(a) [MMA] varied at constant [I]

a) Data from T. Sugimura and Y. Minoura, J. Polym. Sci.A-1, 2735 (1966)

b) Data from P. J. Flory, Principles of Polymer Chemistry, copyright 1953 by Cornell University,

Slope = 1

Slope = 1/2

a) log Rpol = (log K+1/2 log [I]) + log[M]

b) log Rpol

= (log K+log[M]) + 1/2 log [I])



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Rate constants at 60 C and activation energies for some propagation and

termination reactions

Data from R. Korus and K. F. ODriscoll

overall values

kp/(k

t)1/2 = polymerizability (or ability of monomer to be polymerized)

Rp

= kp

f kd

[I] [M]

kt

1/21/2



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Rp

= kp

f kd

[I] [M]

kt

1/21/2

Rp

= -d[M] = kp

f kd

[I] [M]

dt kt

1/2 1/2

d[M] = - kp

f kd

[I] dt

[M] kt

1/2 1/2

d[M] = - kp

f kd

[I] dt

[M] kt

1/2 1/2

t=0

t=t

t=0

t=t

ln [M] = - kp

f kd

[I]0

t

[M]0 kt

1/2 1/2

where [M]0

and [I]0

= [M] and [I] at t = 0

Evaluation of [M] at any time

[RMn

.] = R

i

2 kt

1/2

Ri

= Rt

= 2 kt[RM

n

.]2

At stationary state

Rp = kp Ri [M]2k

t

1/2

Rp

= kp

(Ri) [M]

(2kt)1/2

1/2

Evaluation of Rp

when Riis known

Rpol

(Ri)1/2



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Mean kinetic chain length :

= number of monomers added into active centers = - d[M]/dt = Rp

number of active centers - d[I]/dt Ri

number of monomer moleculespolymerized per chain initiated

At stationary-state condition, Ri = Rt = Rp =Rt

Number-average degree of polymerization (Xn)

1. Coupling

2. Disproportionation

Xn

= 2

Xn

=

Assume f = 1 and no chain transfer reactions



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Summary: Effect of [M], [I] and their natures on Rpol

and

In the same system of M and I

- high Rpol

and high MW polymer result from high [M]

- high Rpol

and low MW polymer result from high [I]

kp/(k

t)1/2 (polymerizability) tells the ability of monomer to be polymerized

At 60oC kp/(k

t)1/2 for MMA = 0.678, k

p/(k

t)1/2 for styrene = 0.0213

of PMMA > of PS (32 times) when same I (same kd), [I] and [M] are used

Initial Rpol

and can be evaluated when [I]0

and [M]0

are given



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Average Radical lifetime,

Average time of radical exists in polymerization

Average time elapsing between formation and termination of active centers = concentration of active center = [RM

n

.]

rate of loss of active centers Rt

= [RMn

.] = 1

2kt[RM

n

.]2 2k

t[RM

n

.]

but Rp

= kp[RM

n

.][M] or [RM

n

.] = R

p

kp[M]

kp

f kd

[I] [M]

kt

1/21/2

Ri

= 2 f kd

[I]

Evaluate kp

by measuring and Rp

with known [M] = k

p[M]

2 ktR

p

depends on nature of I (Kd) and [I], not [M]

Evaluate ktby measuring and R

i



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Example The polymerization of ethylene at 130 GC and 1500 atm was studiedusing different concentrations of the initiator, 1-t-butylazo-1-

phenoxycyclohexane. The rate of initiation was measured directly and radical

lifetime were determined using the rotating sector method. The following results

were obtained, Evaluate kt.

(data from T. Takahashi and P. Ehrlich, Polym. Prepr., Am.

Chem. Soc. Polym. Chem. Div. 22, 203 (1981)).



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Trommsdorff effect

Auto-acceleration Rpol [M]

0

Rpol

= kp

f kd

[I] [M]

kt

1/21/2

Reprinted from G. V. Schulz and G. Harborth, Makromol. Chem. 1, 106 (1948).

Acceleration of the polymerization ratefor different [MMA]

0in benzene at 50 oC

Gel effect

At low [M]0Effect of [M]

0on conversion 1st order

(indicateR

pol)

At high [M]0(> 40%)Effect of [M]

0on conversion > 1st order

high [M]0 high initial R

pol high viscosity of medium Difficult to terminate

(kt decreases)

Large increase in

both Rpol

and

At low conversion

Note: [M] = 100% Bulk polymerization(no solvent)



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Problem of auto-acceleration

generally,H =10- 30kcal/molMost of free radical polymerizations are exothermic reaction

Solving1. stop reaction before gel effect

2. reduce medium viscosity by

adding solvent

Large Rpol large released heat

Explosion if poor venting system

High MWD

Mole fraction of i-mers as a function of Xifor

termination by combination for various values of p.

p = %conversion

Xi

Molefraction



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1. Transfer to monomer

Different types of CTR (depend on chain transferring agent)

2. Transfer to initiator

3. Transfer to solvent



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Mean kinetic chain length : in the presence of CTR

Assume f = 1

Terminations include

Coupling

Disproportionation

CTR

tr= Rp = RpR

tR

t+ R

tr, M+ R

tr, I+ R

tr, S

ktr

tr



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Evaluation of chain transfer constants

1 = 1 + CS

x [S]

tr

o[M]

Assume: CRT to M and I are ignored

1

X 10

5

[S]

[M]

Effect of CTR to solvent for PS at 100 oC.

Data from R. A. Gregg and F. R. Mayo, Discuss. Faraday Soc., 2, 328 (1947).

1

0

Slope = Cs



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Easy to process

Desirable for particular applications such as lubricants or plasticizers

Controlling Xn

of polymer by CTR

CTR reduces MW

solvent or CT agent is chosen

and its concentration selected to

produce the desired value of

1 = 1 + CS

x [S]

tr

o[M]

1 = 1 + Ctr

x [TR agent]

tr

o[M]

Mercaptans (R-SH) have particularly large Ctr

for many common monomers

and are especially useful for molecular weight regulation.

Ex.At 60GC, styrene has Ctr for C4H9-SH = 21 (107 times > Ctr for C6H6 at 60oC)



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Chain transfer to polymer

1) Inter-molecular chain transfer

Polymer side chain branching(Graft copolymer)

Monomer

M-M

-M-M

-M



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graft copolymershave polymeric side chains which differ in the nature ofthe repeat unit from the backbone.

Graft copolymerization

polybutadiene PS radical

Ex. Butadiene-styrenecopolymer (SBS) =

High impact PS (HIPS)



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2) Intra-molecular chain transfer

Back-biting

Long chain branching can occur at high pressure to produce LDPE.

Short chain branching(normally ethyl or butyl group)

0.941 g/cm3, low degree of branching

0.9100.940 g/cm3, high degree of chain branching

0.9150.925 g/cm3, significant numbers of short branches

(higher tensile strength and higher impact than LDPE).

Common types of PE



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Suppressing polymerization

1. Inhibition:

2. Retardation:

Commercial monomers are required to prevent their premature polymerization

during storage by adding either retarders orinhibitorsdepending on degree of protection

blocks polymerization completely until it is removed

slows down polymerization process by competing for radicals

Less protection efficiency

Hydroquinone

Nitrobenzene

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2 Chain-Growth Polymerization