Lecture 4 & 5 Polymerization Reactions Polymer Science and Engineering

Polymerization Reactions Step (aka: Condensation) Chain (aka: Addition) Free Radical Ionic Coordination Group Transfer

Gives off a small molecule (often H 2 O) as a byproduct Stepwise Stepwise polymerization  long reaction times Bifunctional monomers  linear polymers Trifunctional monomers  cross-linked (network) polymers Condensation Polymerization Characteristics

Polymerization mechanisms - Step-growth polymerization

 Examples:  Polyesters  Nylons  Polycarbonates

Polyesters from a hydroxy-acid  Acid and base functionality on one monomer: e.g., n. HO-CH 2 CH 2 CH 2 CH 2 COOH  -(-CH 2 CH 2 CH 2 CH 2 COO-) n- + nH 2 O  General reaction: n. HO-R-COOH  -(-R-COO-) n - + nH 2 O

Polyester from a di-alcohol and a di-acid  Example Condensation Polymerization — Esterification

Another polyester: PETE  Terephthalic acid + Ethylene glycol n. HOOC-C 6 H 4 -COOH + n. HO-CH 2 CH 2 -OH  -(-OOC-C 6 H 4 -COO-CH 2 CH 2 -) n + 2nH 2 O

Nylon 6 from polymerization of an amino acid  acid and base groups on one monomer H 2 N-(CH 2 ) 5 -COOH n H 2 N-(CH 2 ) 5 -COOH  (-(CH 2 ) 5 -CONH) n + nH 2 O 6-carbon monomer  6-carbon mer

Nylon 6,6 — 2 monomers: 6-carbon diamine + 6-carbon diacid ( hexamethylene diamine + adipic acid )

Calculate the masses of hexamethylene diamine and adipic acid needed to produce 1000 kg of nylon 6,6. Solution:  The chemical reaction is: nH 2 N-(CH 2 ) 6 -NH 2 + n HOOC-(CH 2 ) 4 -COOH  -(-HNOC-(CH 2 ) 4 -CONH-(CH 2 ) 6 -) n + 2nH 2 O The masses are in proportion to molecular weights. Per mer of nylon 6,6: C 6 H 16 N 2 + C 6 H 10 O 4  C 12 H 22 O 2 N 2 + 2H 2 O  x  x10 3 kg 146x10 3 kg 10 3 kg kg HMDA kg adipic acid Note: 159 kg of H 2 O byproduct Problem — Nylon 6,6

Polymerization mechanisms - Chain-growth polymerization

Characteristics of Chain Reaction Each polymer chain grows fast. Once growth stops a chain is no longer reactive. Growth of a polymer chain is caused by a kinetic chain of reactions. Chain reactions always comprise the addition of monomer to an active center (radical, ion, polymer-catalyst bond). The chain reaction is initiated by an external source (thermal energy, reactive compound, or catalyst).

Stages of Free Radical Polymerization Initiation(start) Propagation(growth) Chain transfer (stop/start) Termination(stop)  During initiation active centers are being formed.  During termination active centers disappear.  Concentration of active centers is very low ( mol/L).  Growth rate of a chain is very high ( units/s).  Chains with a degree of polymerization of 10 3 to 10 4 are being formed in 0.1 to 10 s.

Benzoyl Peroxide Azobisisobutyronitrile (AIBN)

Initiation Kinetics The rate of radical production is then given by:

The actual rate of initiation R i is expressed in terms of the rate of radical production that leads to actual polymer chains growing: where f is the efficiency factor: the fraction of radicals that really leads to initiation. The rate constant k i is NOT used in the mathematical description of the polymerization.

Propagation This reaction is responsible for the growth of the polymer chain. It is the reaction in which monomer is added at the active center : The rate of this reaction R p can be expressed as:

Termination Chain growth stops by bimolecular reaction of two growing chain radicals: termination by combination (k tc ) termination by disproportionation (k td ) The general kinetic equation reads:

Termination Every reaction consists of two steps: 1) approach of both reactants 2) chemical reaction The second step in the termination reaction is very fast. This means that the rate of approach (significantly) determines the overall termination rate.

at 5 % conversion Termination: which is faster? 1. + or or + in a viscous mediumin a non-viscous medium 3. + or + at 85 % conversion

Polymerization Kinetics The rate of polymerization in a chain growth polymerization is defined as the rate at which monomer is consumed. Since for the production of high molar mass material R p » R i this equation can be re-written as: From the beginning of the polymerization: increasing number of radicals due to decomposition of the initiator increasing termination due to increasing radical concentration (R t µ [M·] 2 ) eventually a steady state in radical concentration:

This steady state assumption leads to: From which the differential rate equation is derived: At low conversion this means: log(R p ) vs log[M] yields a slope = 1 log(R p ) vs log[I] yields a slope = 0.5

The number average degree of polymerization P n of chains formed at a certain moment is dependent on the termination mechanism: *combination: P n = 2  *disproportionation: P n =  Chemistry:

 low conversion polymer chains are in dilute solution (no contact among chains)  “intermediate” conversion  High conversion chains are getting highly entangled; kp decreases.

Trommsdorff effect Somewhere in the “intermediate” conversion regime: *Polymer chains loose mobility; *Termination rate decreases; *Radical concentration increases; *Rate of polymerization increases; *Molar mass increases; This effect is called: gel effect, Trommsdorff effect, or auto-acceleration

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.  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.  Chain Transfer agents are added to control molecular weight and branching

Branching: Chain Transfer to Polymer

10/24/ Qualitative Kinetic Effects Factor Rate of RxnMW [M]IncreasesIncreases [I]IncreasesDecreases k p IncreasesIncreases k d IncreasesDecreases k t DecreasesDecreases CT agentNo EffectDecreases InhibitorDecreases (stops!)Decreases CT to PolyNo EffectIncreases TemperatureIncreasesDecreases

10/24/ Thermodynamics of Free Radical Polymerization  G p =  H p - T  S p  H p is favorable for all polymerizations and  S p is not! However, at normal temperatures,  H p more than compensates for the negative  S p term.

The Ceiling Temperature, Tc, is the temperature above which the polymer “depolymerizes”:

 Gp =  Hp - T  Sp   Hp is favorable for all polymerizations and  Sp is not!  At operational temperatures,  Hp exceeds the negative T  Sp term.

 At T c,  G p = 0    Hp - Tc  S p = 0   H p = T c  S p   T c =  H p /  S p

Ceiling Temperature ● Depropagation has larger  S ● ∵ T  S term increases with T ∴ T  increase; k dp  increase ● At T = T c (i.e. ceiling temperature)  R p = R dp

COMPARISON

39 Two or more monomers polymerized together  random – A and B randomly positioned along chain  alternating – A and B alternate in polymer chain  block – large blocks of A units alternate with large blocks of B units  graft – chains of B units grafted onto A backbone A – B – random block graft alternating

Homopolymer Alternating Copolymer Random Copolymer Block Copolymer Homopolymer

k 11 k 12 k 21 k 22 —M 1 + M 1  —M 1 —M 1 + M 1  —M 1 —M 1 + M 2  —M 2 —M 1 + M 2  —M 2 —M 2 + M 1  —M 1 —M 2 + M 1  —M 1 —M 2 + M 2  —M 2 —M 2 + M 2  —M 2 }}

Case 1: r 1 =0 and r 2 =0 Each comonomers prefers to react with the other. Perfectly alternating copolymer. Case 2: r 1 > 1 and r 2 > 1 Each comonomers prefers to react with others of its own kind. Tendency to form block copolymers. Case 3: r 1 * r 2 =1 There is no preference due to the chain ends. Random incorporation of comonomers. "Ideal" copolymerization.

r 1 and r 2 for pairs of monomers. r1r1 r2r2 Styrene0.80Isoprene1.68 "0.52Methyl methacrylate0.46 "55Vinyl Acetate0.01 "0.04Acrylonitrile0.4 "0.04Maleic anhydride0.015 Note: Data are for free radical copolymerization under standard condition

then homopolymerization growth is preferred then only reaction with 2 will occur f 1, f 2 : mole fractions of monomers in feed F 1, F 2 : mole fractions of monomers in polymer …… ④ Monomer Reactivity and Composition Reactivity Ration Composition From ③, ④ …… ③ …… ⑤ characterizes the reactivity of the 1 radical with respect to the two monomers, 1 and 2

Ideal Copolymerization The two monomers have equal reactivity toward both propagating species random copolymer where