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Some basics on free radical polymerization Xavier Coqueret Institut de Chimie Moléculaire de Reims, CNRS UMR 7312, Université de Reims Champagne-Ardenne, France E+ Intensive course – KTU – October 2016 Introduction to free radical polymerization ■ High industrial and economic importance – « high pressure »PE, PS, PVC – speciality polymers : acrylics, poly(vinylacetate) – hardening of unsaturated polyesters ■ Historical and scientific importance – mechanisms – kinetics ■ Large number of monomers polymerized wia free radical process à including with high energy radiation or photo-chemical initiation The free radical center ■ Generally a carbon species with a single e- in one AO – obtained by homolytic cleavage of a s or p bond X C C H X H ■ 4 basic reactions – combine R° + R ’° à R-R ’ – transfer R° + A-B à R-A + B° – eliminate A-B-C° à A° +B=C – add R° + A=B à R-A-B° Chain kinetics in free radical polymerization ■ General description – ~~~~(M)n ~~ ~~M° + M à ~~~~ (M)n+1~~~~M° – 4 main steps : ¤ initiation production of new free radicals ¤ propagation growth step, maintains [R°] tot constant ¤ termination consumes active centers ~~~M° + ° M~~ à ~~M-M~~ ¤ transfer stops polymer chain growth but does not decrease the number of R° Monomer insertion ■ Insertion of a vinyl monomer R° CH2=CH-R Tail---Head – propogation step ■ secondary free radical ~~~~~CH-C°HR ■ monomer approach for insertion – ~~~CH2-C°HR + CH2=CH-R ~~CH2-CHR-CH2-C°HR ~~~CH2-C°HR + CHR= CH2 ~~CH2-CHR-CHR-C°H2 Monomer insertion – ~~~CH2-C°HR + CH2=CH-R ~~CH2-CHR-CH2-C°HR T H T H ~~CH2-CHR-CH2-CHR ~~ – ~~~CH2-C°HR + CHR= CH2 ~~CH2-CHR-CHR-C°H2 T H H T ~~CH2-CHR-CHR-CH2~~ ■ 2 criteria for estimating the chance of each insertion mode – thermodynamics – kinetics ■ « Head to tail » the normal insertion mode - fHtoH <1% generally fHtoH about 5 % PVC, PVAc and 20% for PVDF Chain kinetics in solution ■ Initiation : 2 steps – initiator decomposition: In = R’-R’ à 2R’° slow step – monomer activation : R’° + CH2=CHR à R’-CH2-C°HR fast – RD = kD [In] initiation rate Rinit Rinit: production of new kinetic chains – Rinit = 2 f kD [In] ■ Propagation – ~~Mn~~CH2-C°HR + CH2=CHR à ~~Mn+1~~CH2-C°HR – rate constant kprop - up to thousands additions per chain Rprop = kprop [M][MnR°] Chain kinetics insolution ■ Transfer – ~~Mn~~CH2-C°HR + A-B à ~~Mn~~CH2-CHR-A + B° – many different compounds (A-B) may give rise to transfer: monomer, solvent, initiator, polymer – various mechanisms are possible AB AB – Rtrans = ktransf [MnR°][A-B] – if B° is an active free radical, it is a true transfer – if not, transfer can be considered as a termination Chain kinetics in solution ■ Termination 2 free radicals collide and are mutually consumed by forming stable molecules – Rate of termination : rate of free radical consumption – 2 Rterm = 2 kterm [MnR°] – 2 different mechanisms are possible : combination and disproportionation ~~Mn~~CH2-C°HR + C°HR-CH2~~Mp~~à ~~~~ Mn+p+2 ~~~~ ~~CH2-C°HR + ~~CH2-C°HR à ~~CH2-CH2R +~~CH=CHR Chain kinetics in solution ■ Kinetic treatment – growth time for a chain : a few seconds for 103 -104 repeat units – Steady state approximation – overall stationary concentration in R° « R init= R term » – termination is a diffusion controlled reaction 7 - 9 -1 -1 – k term-bimol. in the range 10 10 l.mol .s in low viscosity liquids – steady concentration [R°] is about 10-8 mol.l-1 – rate constant kprop is considered not to vary with the size of the macroradical Chain kinetics in solution ■ Steady concentration of R° – how does vary [R°] = f(t) ? – formation R (+R°) = Ram = 2f kD [In] 2 – disappearance R (-R°) = 2 kterm [R°] 2 ■ Differential equation : d[R°]/dt = Ram - 2 kterm [R°] • 0.5 æ v ö dC ç am ÷ + C 1 ç 2k ÷ = 2ktermdt è term ø 0.5 ln 0.5 = 2ktermt vam 2 - C æ vam ö æ vam ö 2k 2ç ÷ ç ÷ - C term è 2kterm ø è 2kterm ø Chain kinetics : steady state 0.5 0.5 ■ Solution : æ v ö exp 2(2k v ) t -1 C t = ç am ÷ [ term am ] ( ) ç ÷ 0.5 è 2kterm ø exp[2(2ktermvam ) t]+1 3 2 [R°]+7-5 [R°]+8-5 x -1 -1 kterm = 10 L.mol .s [R°]+8-6 [R°]+6-4 y -1 -1 V am = 10 mol.L .s 1 [R°] / µmol.l-1 0 0 0.2 0.4 0.6 0.8 1 t / s Instant polymerization rate – Steady state approximation gives [~Mn~°] or [Mn°] = Cte 2 – « R init= R term » 2 f kD [In] = 2 kterm [Mn°] = 0.5 – [Mn°] ( f kD [In]) / kterm) – the polymerization rate is defined as Rp = - d[M]/dt – monomer is consumed essentially during propagation – Rp » Rprop = kprop [M][Mn°] 0.5 0.5 – thus Rp = kp [M][Mn°] = kp ( f kD/k term ) [M] [In] Chain kinetics in solution – the kinetic chain length l – instant value of the average number of propagation steps per new chain – if no tranfers take place l = Rp / R init – if tranfer exists, the number of new chains generated per unit time is higher: l = R p / Rinit + Rtransf – R transf the sum of the rate for all transfer reactions – Chain kinetics in solution ■ Relation between DPn and l – l is the mean value of DP for ~~~~R° at instant of deactivation – terminaison without transfer ■ DPn = 2 l , recombination ■ DPn = l , disproportionation* ■ DPn = (1+a) l , if both exist and if precomb. = a – * disproportionation : ~~CH2-C°HR + ~~CH2-C°HR à ~~CH2-CH2R + ~~CH=CHR Chain kinetics ■ Relation between DPn and l – chain deactivation by termination and by transfer DPn = l = R p / (R init+ R transf) , if disproportionation only as termination mechanism ■ A more general relation, corresponding to termination by recombination competing with transfer: DPn = R p / (0.5 Rinit + R transf) Initiation ■ Initiators – molecules or salts with a weak covalent bond – activated thermally, by a redox reaction , photochemically ■ Thermal initiators – DBPO, AIBN, DCP …….. – structure, mechanisms, solubility – dissociation rate constants – recommended reaction temperature ■ Redox-initiators – hydroperoxide + reducing metal – 100% organic : POB + tertiary aniline Initiation ■ Benzoylperoxide O O T C D 2 C O O O C O ■ Azo-bis-isobutyronitrile DT CN AIBN N C N N C N N2 + 2 O O ■ Potassium persulfate, H hydrogen peroxide O S O O S O O O O O H 2 K 2+ 3+ - . ■ Fenton reagent H2O2 + Fe ® Fe + OH + OH° Monomers for free radical polymerization ■ ethylene ■ propylene, a-olefins ■ other unsaturated monomers CH CHCl CH2 CH COOR 2 CH2 CH CN CH2 CH OCOCH3 CH3 CH2 CH CH CH2 CH2 C COOR – Questions to discuss ■ efficient mechanism for a monomer : R°, +, - ? ■ compared reactivity of monomers Monomers for free radical polymerization ■ Factors controlling polymerizability – existence of the active center – reactivity of the active center – affinity of monomer for its active center – special behavior in copolymerization ■ Compared stability of the active centers (carried by a C atom) – free radica / carbocation / carbanion ■ Stabilization – substituants : nature, number – resonance stabilization Monomers for free radical polymerization Monomère Radicalaire Anionique Cationique Ethylène + + + 1,1-dialkyloléfine (isobutène) - - + Ethers de vinyle - - + Halogénures de vinyle + - - Esters de vinyle + - - Esters méthacryliques + + - Acrylonitrile + + - Styrène + + + Buta-1,3-diène + + + Monomers for free radical polymerization -1 -1 ■ k p (l.mol .s ) : VAc, VC, MA, AN, MMA, St, BD 12000 250022001900 700 150 100 ■ free radical reactivity VC MA AN ST BD ■ monomer reactivity VAC MMA ■ conclusion: reactivity of free radical is the dominant parameter ■ steric factors, polarity Thermodynamical aspects of propagation ■ n M à -[M]n- DS<0 ■ DG = DH -TDS must be < 0 Monomère -DH (kJ.mol-1) -DS (J.mol-1.K-1) éthylène 93 155 propène 84 116 1-butène 83.5 113 isobutène 48 121 a-méthylstyrène 35 110 Styrène 73 104 Tétrafluoroéthylène 163 112 Acrylonitrile 76.5 109 Méthacrylate de méthyle 56 117 Acétate de vinyle 88 110 Thermodynamical aspects of propagation ■ Mn° + M à Mn+1° DGprop = DG° + RT ln [Mn+1°]/[Mn° ][M] ■ critical conditions for DGprop = 0 – parameters : DH° et DS° – 2 variables : [M] et T DH° T = ■ Critical Temperature (ceiling) plafond DS° + Rln[M ] ■ Critical Concentration (ground) æ DH° DS° ö [M ]plancher = expç - ÷ è RT R ø ■ Ceiling T at reference concentration: soit M pur : [M]=1000r/m , soit [M]=1 mol.L-1 Stereochemical aspects of propagation ■ Vinyl polymers ■ Stereochemistry of active centers: ■ no chirality - free rotation ■ monomer approach – interactions H H R R H R R H H C C R CH CH + CH CH R CH CH CH 2 2 2 2 * 2 * 2 enchaînement tête-queue diade racémique ■ Conformation at instant of bonding – conformer 1,3 syndyad meso – conformèer 1,3 antidyad racemo Stereochemical aspects of propagation ■ Small energy difference between conformations ■ Low potential barrier ■ population anti is larger but syn population decreases with T (Boltzmann) ex : PMMA free radical process Température de diades racémiques N anti / N syn polymérisation en % -78 °C 88 7,4 0°C 79 3,8 50°C 77 3,3 100°C 73 2,7 Transfer reactions ■ Transfer to solvent ~~~~CH2-C°HR + Solv-Hà ~~~CH2-CH2R + Solv° ■ Transfer stops polymer growth ■ Large effect on molecular weight : DPn depends on l DPn = v p/ (0.5v am + Sv transf) ou DPn = v p/ (v am + S v transf), X 1 / DPn = 1 / (DPn)0 + SC [X]/[M] Transfer reactions ■ X 1 / DPn = 1 / (DPn)0 + C [X]/[M] ■ CX , transfer constant, no dimension ■ Graphical plot 1 / DPn = f( [X]/[M] ) ■ Slope is a measure of transfer eficiency CX ■ Comparison of CX values Transfer reactions ■ Transfer to polymer – bimolecular transfer – main chain activation followed by propagation – ~~CH2-CHR° + ~~CH2-CHR-CH2~~~ à ~~~CH2-CH2R + ~~~~CH2-C°R-CH2 ~~~~ – then growth of a side chain ■ PS, PVAc – long and short branches in « high pressure » PE Termination ■ 2 mechanisms : recombination / disproportionation – Normal mode is recombination – Eactivation recombination << Eactivation disproportionation ■ Polymers with sterically hindered active centre : competition – e.g.