פילמור רדיקלי

א"תשע/אדר ב/ז"ט

1

Free Radical Polymerization

Addition Polymerization

• Free radical polymerization is a chain reaction

process.

• Polymerization is occur at the reactive end of

the growing chain

– High Mw is obtained

• Free radical is a specific initiator that change

polymer structure and therefore will be treated

separately.


2

Vinyl Polymers

• Polymer that derived from monomer

containing a vinyl group C C

H H

HX


3

Free Radical Initiators

• Certain monomers (styrene, methyl

methactylate) undergo polymerization upon

heating in the absence of radical.

– However most polymerization processes requires

free radical

• 4 major types of initiators:

– Peroxides (ROOR) & Hydroperoxides (ROOH)

– Azo compounds

– Redox initiators

– Photo initiators

Peroxides & Hydroperoxides

• ROOR and ROOH decomposed at elevated

temperature. The rate of decomposition is

depends on the structure

– i.e. benzoyl peroxide

(BPO)

• The peroxide not necessarily reacts with

monomer but it may also undergo other

“waste” reactions


4

Choosing Peroxides

• The extent of which side reaction occur

depends on the structure of peroxide, radical

stability and reactivity of the monomer.

BPO: t½ = 30 min at T = 100 °C ; tend to react better with monomers. Few side reactions.

Diacetyl peroxide

Radicals not too stable.

Tend to be wasted.

Very stable! React with monomers.

Almost no side reactions. (but, long t½ and high Temp)

Di-t-butyl peroxide (half-life:10hours at 120℃)

Low temperature Peroxide

• Initiation can occur at low temperature by

adding promoters.

– For example the use of N,N-dimethylaniline

Ph C

O

O O C

O

Ph + Ph N(CH3)2

Ph N O C

CH3

CH3 O

Ph + Ph CO2

Ph N

CH3

O

CH3

C

O

Ph Ph N(CH3)2 + Ph C

O

O


5

Azo Compounds

• Azo compound having cyano groups on the

azo linkage will decomposed at low

temperature to give nitrogen and radical

– α,α'-Azobis(isobutyronitrile) (AIBN). Stable by resonance.

– Can also go through side reaction (waste)

Half-life of : 1.3 hours at 80℃.

Redox Initiators

• Low temperature initiation can be introduce by

electron transfer to form free radical.

– Useful for emulsion polymerization as with cumyl

hydroperoxide

• Rates can be easily control by changing

component concentration


6

Photo Initiators

• Reaction is independent of temperature.

• Easy to control rate (wavelength) and easy to

stop.

Initiators Summary

PhotoRedoxAzoPeroxidesInitiator Type

Independent

on

temperature

LowModerate to

low

VersatileWorking

Temperature

VersatileVersatileModerateVersatileReactivity

Low

temperature

application.

Very convinent

Low

temperature

for emulsion

polymerization

VersatileAdvantage


7

Thermal Polymerization

• Some monomers (styrene) can polymerize in

the absence of initiators.

– Generation of free radical moieties.

CH

CH

Polymerization Techniques

• Bulk polymerization

– Only monomer and monomer soluble initiator

• No contaminations

– The viscosity is increase as reaction proceed

• Problem with exothermic reaction due to heat transfer

• Difficult to remove non reacted monomer

• Polymer may precipitate and viscosity does not changed

much

– Use in the polymerization of PMMA, PS and low

Mw vinyl polymer


8


• Solution Polymerization

– Solution of monomer, initiator and solvent

• May be use directly for solvent casting or spraying

– Maintain low viscosity

• Easy to control heat transfer

– Difficult to remove solvent


• Suspension Polymerization

– Monomer (+initiator) is break into droplet in a

non soluble liquid (water) by continuous mixing

• Form Granular beads

• Efficient heat transfer

• May cause agglomeration (problem with tacky polymers)

• Contamination


9


• Emulsion Polymerization

– Monomer is dispersed by an emulsifier (soap)

• Good heat transfer

• Used directly as latex

• Contamination

– Only one initiator diffused into

micelle

• High Mw

• Maybe too high…



10

Free Radical Mechanism

• Initiation

– Involve two steps:

• Formation by decomposition (rate controlling step)

• Addition of radical to monomer

Initiator 2R.


• Propagation

– Addition of monomer radical to monomer

molecule follow by addition of macro radical to

monomer.

– Head-to-tail addition due to steric and electronic

effects is predominant.

• But not occur exclusive

(13-17% of head to head) (5-6%) (19%)


11


• Termination

– Two principles routes of termination

• Coupling (also known as combination)

• Disproportion – Transfer of atom (usually hydrogen)

from one chain to another

– Both reaction are diffusion control


• The mechanism in which termination occur

depends on the structure of the monomer.

– Coupling of stericaly hindered is less favorable

– Electrostatic repulsion of polar group may raise activation

energy for coupling

– Availability of α-hydrogen for hydrogen transfer

2 CH2C

CH3

CO 2CH3

CH 2CH

CH 3

CO 2CH 3

+ CH=C

CH 3

CO 2CH 3

or CH 2 C

CH 2

CO 2CH 3

.


12


• Another termination mechanism is primary

radical termination

– Significant at

• High initiator concentration

• High viscosity limit diffusion of high Mw moieties

• The termination mechanism will effect the end

group nature of polymer chains

– Double bond (disproportion)

– Head-to-head (coupling)

Chain Transfer Reaction (CTR)

• Transfer of reactivity from growing polymer

chain to another species.

– One chain is terminated but at the same time

generate new radical

• The results is low Mw products and broad distribution

• Typical for all free radical polymerization

• Chain end radical abstract hydrogen from a

chain leading to a reactive site for branching


13


• The hydrogen abstracting may also occur in

the intramolecular level, causes ‘backbiting’

– Leads to small chain branching in polyethylene


• CTR can occur with monomer or initiator

• PS CTR with solvent (carbon tetrachloride)


14



• CTR in monomer contain allylic hydrogen (PP)

is very important, since the formation of a

resonance stable allylic radical is favorable

– Difficult to achieve high molecular weight PP with

free radical technique

– Not a problem in PMMA since addition is more

favorable


15


• CTR is used to prevent polymerization (for

example in styrene monomers)

– Inhibitors, like alkylated phenols, transfer

hydrogen to form a new radical that undergo

coupling reaction.

– Before polymerization the inhibitors need to be

distilled or consume by adding excess of initiator

• The time till reaction start called induction period

R

R

R

OH

Free Radical Kinetics

• Assumptions:

– Rates of the different stages are different.

– Each propagation stage is head-to-tail and occur at

the same rate (true when chain is longer then n=4)

– Rate of termination is independent of chain length

– Rate of decomposition is very slow compare to

addition

• Initiation:Initiator

KdR.

R.Ki

+ M M1.


16


• Taking into account that two radical are

formed:[ ]

[ ]Ifkdt

MdR di 2==

•

f : Initiator efficiency.(fraction of initiator that initiate polymerization)

f usually in the range of 0.3-0.8

kd : Decomposition rate constant.

[I] : molar concentration of initiator.

[M ·] : molar concentration of radical.

f = radicals formed from initiator

radicals that initiate a polymer chain


• Termination rateRt= dt

-d[M·]= 2kt[M·]

2

kt = ktc+ ktd

Rp = dt

-d[M]= kp[M][M·]

Rp = dt

-d[M]= kp[M]

t

d

k

Ifk ][[M·]=t

d

k

Ifk ][

Ri=Rt

][2][2 ⋅= MkIfk td2

Propagation rate (Rp) = Polymerization rate

Early on in the process, the rates of initiation and termination become equal.(from empirical observations)

We reach a Steady state concentration of free radicals.(The steady state assumption)


17


• Polymerization rate is therefore the

propagation rate:

Rp is proportional to the square root of [I].

Rp is proportional to [M].

Rp = dt

-d[M]= kp[M]

t

d

k

Ifk ][


• Average kinetic chain length (ν) is used to

described the average number of monomer

units polymerized per chain initiated = rate of

polymerization over rate of initiation

ν = Ri

Rp=

Rt

Rp

ν =2kt[M·]

2

kp[M][M·]=

2kt[M·]

kp[M]=

kp[M]

][2 Ikfk dt

Steady state: Ri = Rt

initiation of rate

tionpolymeriza of rateν =


18


• So, in the absence of side reaction:

• But we already seen that CTR change the

chain growth kinetics

Disproportionation :

Combination :

DP = ν

DP = 2ν


• In case CTR occur we need to combined rates

of termination and CTR

trt

ptr

RR

Rv

++++==== ]][[ TMkR trtr ⋅⋅⋅⋅====

Transfer agent

eventually, plugging (CT = chain transfer constant) we get:p

trT

K

KC ====

][

][11

M

TC

vv

T

tr

Σ++++====

As [T] increase or the rate of transfer increase the Mw is decrease


19


• There are cases where no termination or CTR

occur. This known as living (free radical)

polymers

– Adding more monomer, after monomer is fully

consumed, will increase Mw

– Adding different monomer will create block

copolymer

• Example for this kind of reaction is the atom

transfer radical polymerization (ATRP)


• In the case of living (free radical) polymers

the degree of polymerization is simply:

– Since all chain are initiated at the same time we get

a low polydispersity (as low as 1.05)

0

0

][

][

I

MDP ====


20


• The kinetic of emulsion polymerization is

different then bulk or solution polymerization

• Increasing rate of polymerization increases

Mw

– Due to the fact that, not like in bulk, the initiation

is not competing with termination, but rather with

diffusion rate of terminator radicals.

– Polymerization depends on rate and number of

particles (N)


• Emulsion kinetics


21


• Auto acceleration

– Also known as “gel effect”

• The increase viscosity in bulk polymerization

(or precipitating in solution) make it hard to

macro radicals to act as terminators

– Therefore polymerization rate is increase and high

Mw obtained

– High heat is generated

– May cause even cross linking

Free Radical Kinetics - Summary

Average chain lengthRate of PolymerizationMechanism

Simple

polymerization. No

CTR.

CTR polymerization

Living free radical

polymerization

(ATRP)

Emulsion

polymerization

Rp = dt

-d[M]= kp[M]

t

d

k

Ifk ][ ν =kp[M]

][2 Ikfk dt

DP = ν DP = 2ν

For disproportion For combination

][

][11

M

TC

vv

T

tr

Σ++++====

0

0

][

][

I

MDP ====


22

Stereochemistry

• Free radical mechanism lack significant steric

and electro repulsive forces to generate a

regular stereochemistry.

• Most free radical polymers are atactic

• There are some special cases of syndiotactic

structure

• This is based on balance between propogation

energy and free rotation energy

• Regularity decreases as temperature

increases

Stereochemistry of Dienes

• Cyclopolymerization

• 1,3-butadiene goes 2 kinds of additions:

– 1,2

– 1,4

CH2 CH CH CH2RCH2 CH CH CH2

R .

CH2CH

CH

CH2

CH2

C C

CH2

H H

CH2

C C

H

H CH2

1,4-Addition cis 1,4-Addition trans


23

Stereochemistry of Dienes

Date post:	26-Nov-2014
Category:	Documents
Upload:	itamar-politzer
View:	141 times
Download:	1 times

פילמור רדיקלי

Documents