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Radical Chain Polymerization: “Molecule ‘Empire Building’ by ‘Radical’ Groups”. Chain-Growth Polymerization (Addition) Processes. 1. Free radical Initiation Processes. 2. Cationically Initiated Processes. 3. Anionically Initiated Processes. 4. Group Transfer Polymerization.

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Radical Chain Polymerization: “Molecule ‘Empire Building’ by ‘Radical’ Groups”

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Radical Chain Polymerization:

“Molecule ‘Empire Building’ by ‘Radical’ Groups”

Chain-Growth Polymerization (Addition) Processes

  • 1. Free radical Initiation Processes

  • 2. Cationically Initiated Processes

  • 3. Anionically Initiated Processes

  • 4. Group Transfer Polymerization

  • 5. Coordination Polymerization


Characteristics of Chain-Growth Polymerization

1. Only growth reaction adds repeating units one at a

time to the chain

2. Monomer concentration decreases steadily throughout

the reaction

3. High Molecular weight polymer is formed at once;

polymer molecular weight changes little throughout

the reaction.

4. Long reaction times give high yields but affect

molecular weight little.

5. Reaction mixture contains only monomer, high

polymer, and about 10-8 part of growing chains.


The Chemistry of Free Radical Polymerization

Radical Generation

R

R

2 R

-

Initiator Radicals

Initiation

R

C

C

R

C

C

+

Monomers

Propagation

R

C

C

C

C

R

C

C

C

+

Termination

C

C

C

R

R

C

C

+

R

C

C

C

C

C

R

Polymer


Free Radical Polymerization Mechanisms

1. Overview –Free radical polymerization processes

involve at least three mechanistic steps.

  • A. Initiation

  • 1. Radical Formation (Generation)

  • D

  • In

    In

    In

    In

    +

    h

    v

    , etc.

    2. Initiation

    In

    M

    In

    M

    +


    B. Propagation

    In-M1. + M2In-M1M2.

    In-M1M2. + M3In-M1M2M3.

    In-M1M2M3…MX. + MY In-M1M2M3…MXMY.


    C. Termination

    1) Radical Coupling (Combination)

    In-MX. + .MY-In In-MX-MY-In

    In

    In

    In

    In

    +

    2) Disproportionation (-hydrogen transfer)

    H

    H

    H

    H

    In

    M

    In

    M

    C

    C

    C

    C

    +

    y

    x

    H

    H

    H

    H

    CH

    In

    M

    M

    In

    H

    C

    CH

    CH

    +

    x

    y

    3

    2

    2


    D. Chain Transfer (sometimes)– An atom is transferred

    to the growing chain, terminating the chain growth

    and starting a new chain.

    Chain Transfer to Chain Transfer Agent:

    R

    P

    H

    R

    P

    +

    x

    +

    x

    Chain Transfer to Monomer:

    Px. + H2C=CH-(C=O)OR

    Chain Transfer to Polymer:

    Causes Branching

    H

    P

    P

    P

    P

    +

    x

    +

    y

    x

    y


    E. Inhibition and Retardation– a retarder is a substance

    that can react with a radical to form products incapable

    of reacting with monomer. An inhibitor is a retarder

    which completely stops or “inhibits” polymerization.

    2. Monomers that are susceptible to free radical addition

    A. Vinyl Monomers

    H

    C

    CHX

    H

    C

    CH

    Cl

    2

    2

    Vinyl chloride

    H

    F

    H

    X

    H

    F

    H

    Y

    Vinylidene fluoride


    B. Allyl Monomers

    Cl

    X

    Allyl Chloride

    C. Ester Monomers

    1) Acrylates

    OR

    OH

    O

    O

    Acrylic Acid

    Acrylate Esters


    2) Methacrylates

    O

    O

    OH

    OR

    Methacrylic Acid

    Methacrylate Esters

    3) Vinyl Esters

    O

    Vinyl Acetate

    O

    D. Amide Monomers

    O

    O

    NH

    NH

    2

    2

    Acrylamide Methacrylamide


    3. Monomers that are not susceptible to Free Radical

    Addition

    A. 1,2-a-olefins (Polymerize to oils only)

    x

    B. Vinyl ethers

    R

    O

    O

    methyl vinyl ether

    C. 1,2-disubstituted Ethylenes

    Cl

    Cl

    1,2-dichloroethylene

    H

    H


    • 4. Initiation – “Getting the thing started!”

    • A. Radical Generators (Initiators)

    1. Benzoyl Peroxide

    O

    O

    0

    80-90

    C

    C

    O

    O

    C

    O

    2 CO

    C

    O

    +

    2

    2

    (continued)


    +

    New Active Site

    Initiator End-Group

    Ph

    Ph

    2) t-Butyl Peroxide

    CH

    CH

    CH

    3

    3

    3

    0

    0

    120

    -140

    C

    H

    C

    C

    H

    C

    C

    O

    O

    C

    CH

    2

    3

    3

    3

    CH

    CH

    CH

    3

    3

    3

    (continued)


    CH

    3

    O

    H

    C

    C

    +

    3

    O

    CH

    3

    O

    O

    3) Azobisisobutyronitrile (AIBN)

    CH3CH3

    H3C – C – N=N – C – CH3

    CNCN

    ~60oC

    or hn

    (continued)


    CH

    CH

    3

    3

    Ph

    H

    C

    C

    H

    C

    C

    C

    CH

    N

    +

    2

    3

    3

    H

    2

    CN

    CN

    4) Cumyl Hydroperoxide

    CH

    3

    C

    O

    OH

    OH

    +

    Ph

    O

    CH

    3

    (continued)


    (continued)

    O

    +

    Ph

    O

    O

    O

    Ph

    O

    O


    Hydroperoxides can generate radicals by “induced

    decomposition” from growing polymer chains:

    +

    P

    H

    O

    O

    R

    PH

    +

    O

    O

    R

    R

    OO

    2

    2 RO

    O

    +

    R-OO-OO-R

    2

    What effect does this have on the polymerization process?

    Acting as a chain-transfer agent, it reduces the

    degree of polymerization and molecular mass.


    5) Redox Initiator Systems

    2+

    3+

    Fe

    HO + OH + Fe

    H

    O

    O

    H

    +

    OR

    2-

    -

    O

    S

    O

    O

    SO

    SO

    SO

    +

    +

    3

    3

    3

    4

    2-

    -

    SO

    +

    S-SO

    4

    3


    6) Photoinitiators(Photocleavage – Norrish I)

    O

    O

    h

    v

    C

    HO

    +

    OH

    benzoin

    C

    H

    H

    C

    +

    Ph

    OH

    OH

    Ph

    Ph

    H


    (continued)

    OR

    O

    O

    h

    v

    C

    C

    O

    benzil

    2

    C


    7) Photoinitiators (Photo-Abstraction)

    O

    Photosensitizer

    *

    O

    h

    v

    Ph

    Ph

    benzophenone

    excited state

    R

    R

    R

    R

    H

    C

    N

    OH

    +

    C

    N

    R

    R

    Coinitiator

    Ph

    Ph

    R

    R


    5. Propagation- “Keeping the thing going!”

    A. The addition of monomer to an active center (free radical)

    to generate a new active center.

    H

    X

    R

    C

    C

    CH

    C

    R

    C

    CH

    2

    H

    H

    H

    2

    2

    2

    X

    X

    X

    H

    X

    X

    R

    C

    C

    CH

    C

    H

    H

    etc.

    etc.

    2

    2

    n

    X

    X

    (continued)


    Examples:

    Polystyrene

    H

    Ph

    R

    C

    CH

    R

    C

    C

    CH

    C

    2

    H

    H

    H

    2

    2

    2

    n

    Ph

    Ph

    Ph

    O

    CH

    R

    C

    C

    CH

    3

    H

    H

    2

    2

    O

    C

    O

    Polymethyl

    Acrylate

    O

    CH

    3

    H

    R

    C

    C

    C

    CH

    C

    H

    H

    H

    2

    2

    2

    C

    O

    C

    O

    O

    O

    CH

    CH

    3

    3


    B. Configuration in Chain-Growth Polymerization

    1) Configuration Possibilities

    favored

    CH

    H

    C

    P

    C

    C

    -attack

    -attack

    H

    2

    H

    X

    2

    X

    P

    .

    H

    HC

    CH

    X

    P

    CH

    C

    2

    2

    X

    X

    sterically

    and electronically unfavored


    2) Radical Stability Considerations

    Which possible new active center will have the greatest

    stability?

    .

    P

    C

    CH

    P

    C

    CH

    2

    H

    H

    2

    2

    P

    C

    CH

    -attack produces resonance

    stabilized free radical

    H

    2


    H

    No resonance stabilization

    P

    CH

    C

    X

    2

    ______________________________________________

    CH2

    CH

    O

    2

    P

    CH

    HC

    C

    O

    CH

    X

    3

    O

    C

    O

    CH

    3

    P

    P

    C

    CH

    H

    C

    C

    C

    O

    CH

    2

    3

    H

    H

    H

    2

    C

    O

    O

    P

    C

    CH

    H

    H

    Secondary radical

    is resonance stabilized

    O

    CH

    2

    3

    C

    O

    O

    CH

    3


    (more examples)

    Cl

    Cl

    H

    X

    P

    C

    CH

    2

    Cl

    H

    Cl

    P

    Cl

    H

    Cl

    P

    C

    C

    H

    2

    Cl

    H

    Cl

    Cl

    Cl

    P

    C

    C

    P

    C

    C

    H

    H

    2

    2

    Cl

    Cl

    Tertiary radical is resonance stabilized


    3) Steric Hinderance Considerations

    HC

    CH

    X

    2

    X

    P

    • For large X, -substitution

    • is sterically favored

    CH

    H

    C

    2

    X

    4) Radical Stability

    3o > 2o > 1o


    • 5 ) “Bottom Line”

    • Resonance and steric hinderance considerations lead to the

    • conclusion that -substitution(head-to-tail) is strongly

    • preferred in chain-growth polymerization.

    H

    H

    H

    H

    C

    C

    C

    C

    C

    C

    C

    C

    H

    H

    H

    H

    2

    2

    2

    2

    X

    X

    X

    X

    Alternating configuration


    6. Termination -“Stopping the thing!”

    A. Coupling (most common)

    H

    H

    P

    C

    P

    C

    C

    C

    +

    y

    x

    H

    H

    2

    2

    X

    X

    H

    H

    P

    C

    P

    C

    C

    C

    y

    x

    H

    H

    2

    2

    X

    X

    • - occurs head-to-head

    • produces two initiator fragments (end-groups)

    • per chain.


    B. Disproportionation

    H

    H

    H

    H

    In

    M

    In

    M

    C

    C

    C

    C

    +

    y

    x

    H

    H

    H

    H

    CH

    In

    M

    M

    In

    H

    C

    CH

    CH

    +

    x

    y

    3

    2

    2

    - Production of saturated chain and 1 unsaturated chain

    per termination

    - Produce one initiator fragment (end-group) per chain


    C. Factors affecting the type of termination that will take

    place.

    1) Steric factors -large, bulky groups attached directly

    to the active center will hinder coupling

    2) Availability of labile -hydrogens

    3) Examples – PS and PMMA

    H

    H

    P

    C

    C

    P

    C

    C

    +

    x

    y

    H

    H

    2

    2

    Combination (coupling)

    Polystyrene

    (continued)


    H

    H

    P

    P

    C

    C

    C

    C

    y

    x

    H

    H

    2

    2

    Ph

    Ph

    Ph =

    CH3H3C

    ~~~PX – CH2-C. + . C-CH2- PY~~~

    C=O O=C

    O O

    CH3 CH3

    PMMA

    • Sterically

    • hindered

    • 5 b-Hydrogens

    • Disproportion-

    • ation dominates

    (continued)


    CH3H3C

    ~~~PX – CH2=C + HC-CH2- PY~~~

    C=O O=C

    O O

    CH3 CH3

    • Electrostatic Repulsion Between Polar Groups –

    • Esters, Amides, etc.


    Polyacrylonitrile (PAN)

    ~~~PX – CH2-CH. + . HC-CH2- PY~~~

    d+ CN d-d- NC d+

    One might assume electrostatic repulsion in this case.

    BUT, how about electrostatic attraction from the

    nitrogen to the carbon? Also, steric hindrance is

    limited.

    At 60oC, this terminates almost exclusively by

    coupling!


    D. Primary Radical Termination

    ~~~PX – CH2-CH.+ . In

    X

    ~~~PX – CH2-CH-In

    X

    More Likely at

    High [In.]

    So molecular mass can be controlled using chain-transfer

    agents, hydroperoxide initiators, OR higher levels of

    initiator!


    7. Chain-Transfer -“Rerouting the thing!”

    • Definition – The transfer of reactivity from the

    • growing polymer chain to another species. An

    • atom is transferred to the growing chain,

    • terminating the chain and starting a new one.

    ~~~PX – CH2-CH. + X-R  ~~~PX – CH2-CHX + R.

    Y Y

    B. Chain-transfer to solvent:

    ~~~PX – CH2-CH. + CCl4 ~~~PX – CH2-CHCl + Cl3C.

    Y Y


    C. Chain-transfer to monomer:

    ~~~PX – CH2-CH. + H2C =CH

    ~~~PX – CH2-CH2 + H2C =C.

    OR


    H H

    ~~~PX – CH - C. + H2C =CH

    ~~~PX – CH2=CH. + H3C - C.


    Propylene – Why won’t it polymerize with Free Radicals?

    ~~~PX – CH2-CH. + HCH=CH

    CH3 CH3

    ~~~PX – CH2-CH2-CH3 + CH2=CH-CH2.

    H2C-CH-CH2

    Chain-transfer occurs so readily that propylene won’t polymerize

    with free radicals.


    D. Chain-transfer to polymer:

    ~~~PX – CH2-CH2-CH2. +

    ~~~CH2-CH2-CH2~~~

    ~~~PX – CH2-CH2-CH3+ ~~~CH2-CH-CH2~~~

    Increases branching and broadens MWD!

    E.Chain-transfer to Initiator (Primary Radical

    Termination):

    ~~~PX – CH2.+ R-O-O-R  ~~~PX – CH2-OR + . OR


    F. Chain-transfer to Chain-transfer Agent:

    Definition – The transfer of reactivity from the

    growing polymer chain to another species. An

    atom is transferred to the growing chain,

    terminating the chain and starting a new one.

    Examples: R-OH; R-SH; R-Cl; R-Br

    ~~~PX – CH2-CH2.+ HS-(CH2)7CH3

    ~~~PX – CH2-CH3+ . S-(CH2)7CH3

    . CXH-CH2-S-(CH2)7CH3

    etc., etc., etc.


    • Inhibition and Retardation -“Preventing the thing

    • or slowing it down!”

    Definition – Compounds that slow down or stop poly-

    merization by forming radicals that are either too

    stable or too sterically hindered to initiate poly-

    merization OR they prefer coupling (termination)

    reactions to initiation reactions.

    ~~~PX – CH2-CH. + O= =O

    para-Benzoquinone

    Will Not

    Propagate

    ~~~PX – CH2-CH2-O- -O.

    ~~~PX – CH2-CH-O-O .

    ~~~PX – CH2-CH. + O=O


    Kinetics of Free Radical Polymerization

    1. Initiation

    kd

    (RDS)

    I 2 R.Radical Generation

    ki

    R. + M M1.Initiation

    Assuming that ki >>kd and accounting for the fact that two

    Radicals are formed during every initiator decomposition,

    The rate of initiation, Ri, is given by:

    Ri = d[Mi] = 2fkd[I]

    dt

    f = efficiency of the initiator and is usually 0.3< f >0.8


    2. Propagation

    kp

    M1. + M M2.

    kp

    We assume that the

    reactivity of the growing

    chain is independent of the

    length of the chain.

    M2. + M M3.

    kp

    M3. + M M4.

    .

    .

    .

    kp

    Mx. + M Mx+1.

    Rp = - d[M] = kp[M .][M]

    dt


    3. Termination

    ktc

    Mx. + . My Mx-My(Combination)

    ktd

    Mx. + . My Mx + My(Disproportionation)

    Since two radicals are consumed in every termination, then:

    Rt = 2kt [M .]2

    4. Steady State Assumption

    Very early in the polymerization, the concentration of radicals becomes constant because Ri = Rt

     2fkd[I] = 2kt [M .]2


    2fkd [I] = 2kt [M .]2

    Solve this equation for [M.]:

    [M.] = (fkd [I]/kt)1/2

    Substituting this into the propagation expression:

    Rp = kp[M.][M] = kp [M](fkd [I]/kt)1/2

    Since the rate of propagation, Rp, is essentially the

    rate of polymerization, the rate of polymerization is

    proportional to [I]1/2 and [M].


    5. Kinetic Chain Length, n

    Definition – The average number of monomer units

    polymerized per chain initiated. This is equal to the

    Rate of polymerization per rate of initiation:

    n = Rp/Ri = Rp/Rtunder steady state conditions.

    • = kp[M][M.] = kp[M]

      2kt[M.]2 2kt[M.]

    n will decrease with increases in

    initiator concentration or

    efficiency.

    n

    = __kp[M]___

    2(f ktkd[I])1/2

    DP = n if termination is exclusively by disproportionation.

    DP = 2n if termination is exclusively by coupling.


    6. When Chain-transfer is Involved

    When chain-transfer in involved, the kinetic chain

    length must be redefined.

    Bottom Line:

    1/ntr = 1/n + Cm[M] + Cs[S] + CI[I]

    [M]

    Where Cx = ktr, x /kp


    7. Qualitative Effects – a Summary

    FactorRate of RxnMW

    [M]IncreasesIncreases

    [I]IncreasesDecreases

    kpIncreasesIncreases

    kdIncreasesDecreases

    ktDecreasesDecreases

    CT agentNo EffectDecreases

    InhibitorDecreases (stops!)Decreases

    CT to PolyNo EffectIncreases

    TemperatureIncreasesDecreases


    Thermodynamics of Free Radical Polymerization

    DGp = DHp - TDSp

    DHp is favorable for all polymerizations and DSp

    is not! However, at normal temperatures, DHp

    more than compensates for the negative DSp term.

    The Ceiling Temperature, Tc, is the temperature above

    which the polymer “depolymerizes”.

    At Tc , DGp= 0.  DHp - TcDSp = 0

    DHp = TcDSp  Tc = DHp/ DSp


    Thiol-ene Polymerization: A Brief Introduction

    Thiols (mercaptans) can react with any “-ene”; any

    double bond. After all, they ARE chain-transfer

    agents!

    They serve as a “bridge” between step-growth

    and chain-growth polymerization processes because

    they use free radicals in a step-growth polymerization

    process.

    HS-R-SH + H2C=CH-R’-CH=CH2

    HS-R-S-CH2-CH-R’-CH=CH2

    UV


    If either thiol or ‘ene’ is only monofunctional, no

    polymerizations will take place. The thiol will serve

    as a chain-transfer agent and a standard free radical

    polymerization of the ‘ene’ will take place. If the

    If the mole ratio of thiol to ‘ene’ is close to one, no

    Effective polymerization will take place.

    If both are difunctional and in stoichiometric

    balance, a linear polymer will form.

    In order to get a crosslinked thiol-ene polymer, the

    thiol must be at least trifunctional.


    The process begins with a hydrogen abstraction from

    the thiol – a very rapid process – to form a ‘thiyl’

    radical:

    (HS)2-R-SH + . In  (HS)2-R-S . + H-In

    The thiyl radical attacks a double bond:

    (HS)2-R-S . + H2C=CX – R’ 

    (HS)2-R-S-CH2-CX – R’

    This radical then abstracts a hydrogen atom:

    (HS)2-R-SH +

    (HS)2-R-S-CH2-CX – R’  etc.


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