Polymerization Reactions Polymer Science and Engineering I Lecture 5 Polymerization Reactions

Condensation Polymerization

Condensation Polymerization Characteristics Gives off a small molecule (often H2O) as a byproduct Stepwise polymerization long reaction times Bifunctional monomers  linear polymers Trifunctional monomers  cross-linked (network) polymers

Polymerization mechanisms - Step-growth polymerization

Condensation Polymerization Examples: Polyesters Nylons Polycarbonates

Polyesters from a hydroxy-acid Acid and base functionality on one monomer: e.g., n . HO-CH2CH2CH2CH2COOH  -(-CH2CH2CH2CH2COO-)n- + nH2O General reaction: n . HO-R-COOH  -(-R-COO-)n- + nH2O

Condensation Polymerization — Esterification Polyester from a di-alcohol and a di-acid Example (Callister eq. 16.8):

Another polyester: PETE terephthalic acid + ethylene glycol n.HOOC-C6H4-COOH + n.HO-CH2CH2-OH  -(-OOC-C6H4-COO-CH2CH2-)n + 2nH2O

Nylon 6 from polymerization of an amino acid acid and base groups on one monomer H2N-(CH2)5-COOH n H2N-(CH2)5-COOH  (-(CH2)5-CONH)n + nH2O 6-carbon monomer  6-carbon mer

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

Problem — Nylon 6,6 Calculate the masses of hexamethylene diamine and adipic acid needed to produce 1000 kg of nylon 6,6. Solution: The chemical reaction is: nH2N-(CH2)6-NH2 + nHOOC-(CH2)4-COOH  -(-HNOC-(CH2)4-CONH-(CH2)6-)n + 2nH2O The masses are in proportion to molecular weights. Per mer of nylon 6,6: C6H16N2 + C6H10O4  C12H22O2N2 + 2H2O 72+16+28 + 72+10+64  144+22+32+28 + 2x18 116 + 146  226 + 36 116x103kg 146x103kg 103 kg 226 226 513 kg HMDA + 646 kg adipic acid Note: 159 kg of H2O byproduct

Free Radical Polymerization

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 (10-9 - 10-7 mol/L). Growth rate of a chain is very high (103 - 104 units/s). Chains with a degree of polymerization of 103 to 104 are being formed in 0.1 to 10 s.

Example Initiators Benzoyl Peroxide Azobisisobutyronitrile (AIBN)

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

The actual rate of initiation Ri 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 ki 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 Rp can be expressed as:

Termination Chain growth stops by bimolecular reaction of two growing chain radicals: termination by combination (ktc) termination by disproportionation (ktd) 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.

Termination: which is faster? 1. + or + 2. + or + in a viscous medium in a non-viscous medium 3. + or + at 5 % conversion 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 Rp » Ri 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 (Rt 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(Rp) vs log[M] yields a slope = 1 log(Rp) vs log[I] yields a slope = 0.5

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

Conversion Regimes 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

Chain Transfer 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 branchning

Branching: Chain Transfer to Polymer

Qualitative Kinetic Effects Factor Rate of Rxn MW [M] Increases Increases [I] Increases Decreases kp Increases Increases kd Increases Decreases kt Decreases Decreases CT agent No Effect Decreases Inhibitor Decreases (stops!) Decreases CT to Poly No Effect Increases Temperature Increases Decreases 4/22/2017 32

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. 4/22/2017 33

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

. . . Thermodynamic rational 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 and above Tc At Tc , Gp= 0  Hp - Tc Sp = 0 Hp = Tc Sp  Tc = Hp/ Sp

Ceiling Temperature Depropagation has larger S ∵ TS term increases with T ∴ T  increase; kdp  increase At T = Tc (i.e. ceiling temperature)  Rp = Rdp

Comparison

Coplymerization

Copolymers 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 alternating block graft 40

Copolymers: Types Homopolymer Alternating Copolymer Random Copolymer Block Copolymer Homopolymer

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Example Copolymer

} Reactivity Ratios —M1• + M1  —M1• —M1• + M2  —M2• —M2• + M1  —M1• k11 k12 k21 k22 —M1• + M1  —M1• —M1• + M2  —M2• —M2• + M1  —M1• —M2• + M2  —M2• }

Reactivity and Type of Copolymers Case 1: r1=0 and r2=0 Each comonomer prefers to react with the other. Perfectly alternating copolymer. Case 2: r1 > 1 and r2 > 1 Each comonomer prefers to react with others of its own kind. Tendency to form block copolymers. Case 3: r1 * r2=1 There is no preference due to the chain ends. Random incorporation of comonomers. "Ideal" copolymerization.

Typical Reactivity Ratios r1 and r2 for pairs of monomers. r1 r2 Styrene 0.80 Isoprene 1.68 " 0.52 Methyl methacrylate 0.46 55 Vinyl Acetate 0.01 0.04 Acrylonitrile 0.4 Maleic anhydride 0.015 Note: Data are for free radical copolymerization under standard condition

Monomer Reactivity and Compostion Reactivity Ration characterizes the reactivity of the 1 radical with respect to the two monomers, 1 and 2 then homopolymerization growth is preferred then only reaction with 2 will occur Composition f1, f2 : mole fractions of monomers in feed F1, F2 : mole fractions of monomers in polymer …… ③ ……④ From ③, ④ ……⑤

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

Ideal Copolymerization 1 F1 1 f1

End