2 chain-growth polymerization (201052)

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Chain-growth Polymerization

by Dr. Walaiporn Prissanaroon-Ouajai

Dept. of Industrial Chemistry KMUTNB

411317 Polymer Chemistry (updated 2/2552)411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)

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Chain-growth polymerization Ø formation of polymers via chain reaction

Key factors for chain-growth polymerization

Ø monomersØ initiator (to break π-bond)

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

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Monomers for chain-growth polymerization

Aldehydeor 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 polymerization1. 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 = -

PolymerChain 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 Polymerization1. การเพิ่มของ MW จะเปนไปอยาง

รวดเร็วในชวงแรกของปฏิกิริยา แตเมื่อเวลาผานไป MW จะเปล่ียนแปลงนอย

2. MW α time แต จํานวนโมเลกุล α time

3. ปฏิกิริยาจะเกิดเร็วมาก (10-1-10-6s) และ DP จะเพิ่มขึ้นอยางรวดเร็วในชวงตนปฏิกิริยา (High MW at low conversion)

4. Monomer จะคอยๆ ลดลงอยางชาๆ5. ไมมี by-product

time

MWTime to reach 106 MWPS ~ 7.6s , PMMA ~1.5s, PVC ~0.13s


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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 α time)

High Mn at low conversion(Mn α time but n α 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

Mnor

X n

Chain

Step

Xn Xn

Wt. fr

actio

n

Wt. fr

actio

n

“PER”

Step Chain

4. Rate Growth of chains is usually slow (minutes to days)

Chain growth is usually very rapid (<sec)


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Free Radical PolymerizationMonomer for free radical polymerization

Most alkenes

where R = neutral group or some e-withdrawing character


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Initiator for free radical polymerization

sometimes called "catalysts"

Ø a source of free radicalsØ radicals must be produced at an acceptable rate at convenient

temperaturesØ have the required solubility behaviorØ transfer their activity to monomers efficientlyØ 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)

Ø Low dissociation energy of the O-O bondØ But reagents are unstable.


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3. Redox systems

4. Electromagnetic radiation

Redox initiator = Initiator + Reducing agent

hydrogenperoxide

persulfate

Ø Soluble in water (can also work in organic solvents)

Ø Low dissociation energy then can proceed at relative low Temp → reduce side effect

Ø photochemical initiation involves the direct excitation of the monomer or photolytic fragmentation of initiators

Ø photochemical initiators include a wider variety of compounds411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)

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Free Radical Initiator Efficiency

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

Ø 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.

Ø These species are less reactive than the initiator radicalsØ These species can be recombined with the initiator radicals

Reduce free radical efficiency

Terminate polymerization via chain transfer reaction


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Ø f depends on the conditions of the polymerization, including the solvent.Ø In many experimental situations, f = 0.3-0.8.

Free Radical Initiator Efficiency (f)radicals incorporated into polymerradicals formed by initiatorf =

Note: f should be monitored for each system studied.Evaluation of initiator efficiency1. 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. InitiationMechanism 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 stable411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)

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3. TerminationØ two propagating chains are deactivated, resulting in dead polymer

Two principal modes of termination

Little monomer is left

Low efficiency of active centresin 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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Ø produce one polymer chain with single "head-to-head" linkage

Ø a polymer chain contains two initiator fragments (R) per molecule

Ø higher average MW

Comparison of termination by coupling and disproportionationCoupling Disproportionation

Ø produce two polymer chains Ø one polymer chain contains double bond

and another contains only single bond Ø each polymer chain contains one initiator

fragments (R)Ø lower average MW

Note: - Since the disproportionation requires bond breaking, Etd > Etc- Coupling occurs at lower temperature.

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

H


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Kinetics of Free radical Polymerization1. InitiationStep1: 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 dissociationFrom differential rate law

(for most initiators Ex. Peroxide, azo)

Ri = d[ R. ] = 2 f kd [I] dt

+ 1 d[R.] = - d[I] = kd[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 kd1 = – E* 1 - 1

kd2 RT T1 T2

Arrhenius equation Evaluation of kd at different temperature


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+ 1 d[R.] = - d[I] = kd[I]02 dt dt

Evaluation of kd

d [I] = - kd dt[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 couplingktd = Rate constant for termination by disproportionation

Rp = d[RMn. ] = kp[RMn-1

.][M] = kp[RMn.][M]

dt


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3. Termination

where ktc = Rate constant for termination by couplingktd = Rate constant for termination by disproportionation

where kt = ktc + ktd

From differential rate law

- 1 d[RMn.] = kt[RMn

.]22 dt

Rt = d[RMn. ] = 2 kt[RMn

.]2

dt

Rate of termination (Rt) = Rate of RMn. reduction


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Rp = kp[RMn.][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 constant stationary state

2 f kd[I] = 2 kt[RMn.]2

[RMn.] = [f kd [I] ]

kt

1/2

∆[RMn.] = 0


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Rp = kp[RMn.][M] [RMn

.] = [f kd [I] ]kt

1/2but

therefore Rp = kp f kd [I] [M]kt

1/21/2

Overall Rate of Polymerization (Rpol ) α Rp

Rpol = K [I]0 [M]01/2Initial rate of polymerization

a) Effect of [I] on Rpol ; Rpol α [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 α [M]- if free radicals have low efficiency (f→1) and depend on [M]

Rpol α [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. O’Driscoll

overall values

kp/(kt)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.] = Ri

2 kt

1/2

Ri = Rt = 2 kt[RMn.]2

At stationary state

Rp = kp Ri [M]2kt

1/2

Rp = kp (Ri) [M](2kt)1/2

1/2

Evaluation of Rp when Ri is known

Rpol α (Ri)1/2


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Mean kinetic chain length : ν

ν = number of monomers added into active centers = - d[M]/dt = Rpnumber of active centers - d[I]/dt Ri

number of monomer moleculespolymerized per chain initiated

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

Number-average degree of polymerization (Xn)

1. Coupling

2. Disproportionation

Xn = 2 ν

Xn = ν

Assume f = 1 and no chain transfer reactions


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

• 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/(kt)1/2 (polymerizability) tells the ability of monomer to be polymerizedAt 60oC kp/(kt)1/2 for MMA = 0.678, kp/(kt)1/2 for styrene = 0.0213

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

• Initial Rpol and ν can be evaluated when [I]0 and [M]0 are given


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Average Radical lifetime, τØ Average time of radical exists in polymerizationØ Average time elapsing between formation and termination of active centers

τ = concentration of active center = [RMn.]

rate of loss of active centers Rt

= [RMn.] = 1

2kt [RMn.]2 2kt [RMn

.]

but Rp = kp[RMn. ][M] or [RMn

.] = Rpkp[M]

kp f kd [I] [M]kt

1/21/2

Ri = 2 f kd [I]

Ø Evaluate kp by measuring τ and Rp with known [M]τ = kp[M]2 kt Rp

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

Ø Evaluate kt by measuring τ and Ri


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Example The polymerization of ethylene at 130 °C and 1500 atm was studied using 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 α [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 rate for different [MMA]0 in benzene at 50 oC

“Gel effect”

At low [M]0Effect of [M]0 on conversion → 1st order

(indica

teR po

l)

At high [M]0 (> 40%)Effect of [M]0 on conversion > 1st order

high [M]0 → high initial Rpol→ high viscosity of medium → Difficult to terminate(kt decreases)

Large increase in both Rpol and ν

At low conversion

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


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Problem of auto-acceleration

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

Solving1. stop reaction before gel effect2. reduce medium viscosity by

adding solvent

Large Rpol → large released heatØ Explosion if poor venting systemØ High MWD

Mole fraction of i-mers as a function of Xi for termination by combination for various values of p.

p = %conversion

Xi

Mole

fracti

on


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1. Transfer to monomerDifferent types of CTR (depend on chain transferring agent)

2. Transfer to initiator

3. Transfer to solvent


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Mean kinetic chain length : ν in the presence of CTR

Assume f = 1

Terminations includeCouplingDisproportionationCTR

νtr= Rp = RpΣRt Rt + Rtr, M + Rtr, I + Rtr, S

ktr

tr


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“Mayo-Walling Equation”

1. Coupling + CRT

2. Disproportionation + CRTXn = 2 νtr

Xn = νtr

Evaluation of Xn in the presence of CRT

Reversetr

tr

where (ν)o = ν without chain transferνtr = ν with chain transfer

CM , CI , CS = Rate constants for chain transfer to monomer, initiator,and solvent, respectively

1 = 1 + ∑Ctr x [CT agent]νtr νo [M]


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Evaluation of chain transfer constants

1 = 1 + CS x [S]νtr νo [M]

Assume: CRT to M and I are ignored

1ν

X 105

[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).

1ν0

Slope = Cs


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ØEasy to processØDesirable 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 ν

1 = 1 + CS x [S]νtr νo [M]

1 = 1 + ∑Ctr x [TR agent]νtr νo [M]

Mercaptans (R-SH) have particularly large Ctr for many common monomersand are especially useful for molecular weight regulation. Ex. At 60°C, styrene has Ctr for C4H9-SH = 21 (107 times > Ctr for C6H6 at 60oC)


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Chain transfer to polymer1) Inter-molecular chain transfer

Polymer side chain branching(Graft copolymer)

Monomer

M-M-M-M-M


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graft copolymers have polymeric side chains which differ in the nature of the repeat unit from the backbone.

Graft copolymerization

polybutadiene PS radical

Ex. Butadiene-styrene copolymer (SBS) =High impact PS (HIPS)


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2) Intra-molecular chain transfer

“Back-biting”

Ø 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

0.910–0.940 g/cm3, high degree of chain branching

0.915–0.925 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 polymerizationduring storage by adding either “retarders” or “inhibitors”

depending on degree of protectionblocks polymerization completely until it is removed

slows down polymerization process by competing for radicalsLess protection efficiency

Hydroquinone

Nitrobenzene411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)

2 chain-growth polymerization (201052)

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