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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. PMMA

■ k dismut / k recomb = 5 (60°C) , 2.2 (25°C) , 1.2 (0°C) 100 50

80 40

60 30

40 20 k term / u.a. k term / u.a.

20 10

0 0 200 300 400 200 300 400 température (K) température (K) Autoacceleration : Trommsdorff effect

■ Changes in the physical characteristics of the medium – gelation – vitrification

gel effect 1 Rp æ R ö 2 ç init ÷ RP = kP ç ÷ [M ] è 2kterm ø

0,1 1 10 Conversion in % Conclusions

■ The specificities of free-radical free radical polymerization kinetics depending on initiation process and physical parameters ■ The prime influence of microstructure on properties controlled trough synthetic macromolecular chmistry ■ More to read on: ■ Radiation-initiated free radical polymerization ■ Free-radical copolymerization ■ Controlled free radical polymerization In This project has been funded with support from the European Commission. This publication reflects the views only of the author. Polish National Agency for the Erasmus+ Programme and the European Commission cannot be held responsible for any use which may be made of the information contained therein.

Date: Oct. 2017