FREE-RADICAL POLYMERIZATION. I. Basics A. Free-radical polymerization is a type of chain polymerization (like ionic polymerization).

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1 FREE-RADIAL PLYMERIZATIN I. Bascs A. Free-radcal olymerzao s a ye of cha olymerzao (lke oc olymerzao). B. Free radcal moomer aacks double bod of aoher moomer o form bod ad roagae he free radcal.. Mos vyl moomers are ameable o free-radcal olymerzao. II. Kecs A. Iao. Decomoso s frs-order aor. I R. Iaors are characerzed by T d ad ½ a. T d decomoso emeraure. Temeraure where decomoso of aor udergoes auoaccelerao.. Thermal eergy released from decomoso a aor molecule ads decomoso aoher aor molecule. b. ½ - half-lfe. Tme eeded for al amou of aor o decrease cocerao half-lfe.. Relaed o decomoso rae cosa. Very emeraure sesve ν d =kd I ν =k I 3. Iao s frs-order moomer ad aor radcal. d d M R M [ ] ν = k R M

2 4. Iaor effcecy f a. No all aor radcals ae olymerzao b. Possble faes of aor radcal afer decomoso. Recombao solve cage. Recombao ousde of solve cage. ombao of aor radcal wh olymer cha radcal v. Reaco wh aor (R I I R ) v. ydroge absraco from cha v. Reaco (hydroge absraco, ossbly) wh solve v. ha ao c. Effcecy s rao of umber of chas sared o oal umber of aor radcals d. Iaor effceces rage from. o.8 5. Iaors a. Peroxdes. Bezoyl eroxde (T d = 3 ) [T d s he self-acceleraed decomoso emeraure s he emeraure where he rae of ehaly geeraed from he decomoso of he eroxde serves o rovde suffce eergy for furher decomoso.]. Daceyl eroxde (T d = 35 ) 3 3

3 3. D -buyleroxde (T d = 8 ) v. umyl eroxde (T d = 5 ) 3 3 b. Azo comouds., -azobssobuylrle (AIBN) (T d = 5 ) N N N N N N Pheylazorheylmehae (T d = 4 ) The rheylmehylradcal s very sable. N N N

4 4 c. Redox aors. Persulfae S 8 - S 3 - S 4 - S 4 - S 3 -. Fe w/eroxde a.) Fe acs as a caalys o lower acvao eergy ad he decomoso emeraure. Fe - Fe 3 d. Phooaors (for room emeraure olymerzaos). Sulfde R S S R R S. Bezo λ = 36 Å. Bezl v. Bezoheoe

5 5 B. Proagao. Assumos abou roagao a. Proagao s deede of cha legh (cha legh s large) b. No cha rasfer (o be cosdered laer) c. ly oe free radcal er cha d. Seady sae aroxmao. ocerao of radcal cosa hroughou reaco. Iao rae equals ermao rae. Frs-order radcal ad moomer. M M M 3. ead-o-al or head-o-head roagao ossble, hough exclusve head-o-head ad al-o-al roagao s rare. 4. Ihbors a. Necessary for sorage of mos vyl moomers b. Uwaed radcal reacs wh hbor c. Ihbor radcal does o reac wh moomer d. Rearders are radcals ha have slower reaco rae wh moomer. e. Examles. robezee, rrobezee.,5-d -buyl--cresol [ ] ν = k M M

6 6. Termao. ombao (coulg) M M M M c c [ M ] ν = k. Dsrooroao R R R R [ M ] ν = k 3. Some olymerzaos refer combao (olysyree) whle ohers refer dsrooroao (PMMA). Termao referece of dffere olymers a 6. Dsrooroao ombao Polyvyl cyade % % Polysyree 3% 77% Polymehylmehacrylae 79% % Polyvylaceae % % 4. The oal rae of ermao s he sum of he combao ad he dsrooroao rae. k = k c k D. Degree of Polymerzao. Rae of roagao s frs-order moomer ad radcal. [ ] ν = k M M. Aly he seady sae aroxmao o fd he cocerao of cha radcal. [ ] f effcecy of aor decomoso [ ] dm k R M = k[ R][ M] k = = k dr [ ] kfi d = kdf [ I] k[ R][ M] = [ R] = k [ M] k[ R][ M] kkdf[ I][ M] kdf[ I] = = = k k k M k

7 7 3. alculae he roagao rae. kf d [ I] ν = k M M = k M = k M I k 4. alculae he ermao rae. kf d [ I] ν = k = k = kdf[] I k 5. alculae he degree of olymerzao (rao of roagao rae o ermao rae). DP d k M I ν k k = = = ν kf d [] I kkf d [ M] [] I E. Measurg he rae of olymerzao (dlaomery). Polymerzao rae s he rae of moomer cosumo. dm ν oly =. The average al rae ca be calculaed as: ν = kf [ M] [ M] oly 3. Drec measureme of he reaco rae (so reaco a dffere mes, measure amou of moomer) s ossble, bu me-cosumg. 4. Measurg a chage a hyscal roery would be much easer. 5. Measurg chage volume as reaco roceeds s dlaomery. a. Volume of reaco mxure s measured va he hegh of a callary ube. b. Toal volume s he volume of moomer, olymer ad solve. V= w v w v w v c. Assume he wegh of he olymer s cosumed moomer. V w mv wsvs w = wm wm wm = v v d. A me zero, volume s from moomer ad solve. e. Assume a fe me, volume s from olymer ad solve. Noe: w = wm V = w v w v m m s s V = w v w v m m s s m s s m

8 8 f. Subrac V - V. V V = w v w v w v w v = w v w v = w v v g. Yeld of reaco (Y) ca be wre erms of moomer wegh ad coseque volume (or hegh) dlaomeer. wm wm V V Δh() Y = = = wm V V Δh( ) h. Thus rae s [ M] Δh( ) [ M] ν oly = Y = Δh F. Average degree of olymerzao ad umber-average molecular wegh. The rae of olymer formao deeds o coug he olymer from dsrooroao ad combao correcly. η s he kec cha legh. The rae of olymer formao deeds o coug he olymer from dsrooroao ad combao correcly. a. If combao domaes,.e., k c >> k b. If dsrooroao domaes,.e., k >> k c m m s s m s s m m m m m w m ( m s s) V w v w v V w v w v V V = = = v v V V w V V m s s w m m m dp 3. Number-average molecular wegh deeds o degree of olymerzao. ( k k )[ M ] = c [ ] ( k k ) [ ] ν dm k M M k M DP = = = = ν dp k k M c c [ ] ν k M M k M η= = = ν k k k k ηk ηk [ ] η ( ) M k k DP = = = = k k M k k k k k k k c c c c c ( ) η k k η k = = η c c DP kc k kc DP ( ) η k k η k c = =η kc k k M = MDP

9 9 G. Dsrbuo of DP. To cosder he dsrbuo of olymer cha leghs, we mus beg makg a dsco bewee dffere olymer cha leghs. Thus he rae of formao of olymer cha wh us s dm = k[ M][ M ] k[ M][ M] k[ M] =. The rao of olymer wh us o - us s [ M ] k[ M] k[ M] k = = = [ M ] k[ M] k k[ M] k[ M ] k[ M] k[ M] 3. Recallg he defo of he kec cha legh, η=ν ν = k [ M] k [ M ] [ ] [ M ] M = η 4. Mullyg successve - raos ogeher yelds [ ] [ ] [ ] 5. The fraco of roagag radcals ha have a degree of olymerzao s 6. A alerave aroach volves cosderao of he seady-sae aroxmao. dm [ ] = ν k M = dm [ ] = ν k[ M] k = 7. Subsuo ad rearrageme yelds 8. Thus he fraco of roagag radcals wh DP of s M M M M M 3 M = = M M M M 3 M 4 M η [ ] [ ] M M = = η f [ ] M = η η [ ] M f = = η η ( ) ( )

10 9. Usg he above exresso we wll fd mole fracos of olymer havg DP of. dp [ ] dp [ ] X = = d P d P. To do he summaos, we mus be careful abou he dsco bewee ermao va combao versus ermao va dsrooroao. a. I combao, a smle summao would cou each ermao wce exce for wo radcals ha had / us. M M - P M M - P M 3 M -3 P ec M / M / P b. I dsrooroao, coug argumes (gorg rae cosas) ell us ha a olymer of cha legh s made wce as fas as combao. dp [ ] = k [ M ] c. Thus ogeher he rae of formao of olymer wh cha legh s dp = k c, M M kc,m M m M m k M M Σ m= d. The rae cosa for combao of ulke radcals s wce he rae of combao of decal radcals. (Aga: a smle coug argume.) k c, m = k c, dp [ ] m = kc [ M m ][ Mm ]() k [ M ]. To do he summaos, we mus be careful abou he dsco bewee ermao va combao versus ermao va dsrooroao.. Subsug for he fraco of radcals m= dp = k M M k M M dp k k [ ] M f = = η η c m = () η η m= η η c, c,m m m m= [ m] [ m] () m [ ] = c m= M M dp M M M k k M

11 m 3. The summao yelds () =. m= 4. The mole fraco for each olymer cha legh s 5. Termao s exclusvely combao,.e., k = 6. Termao s exclusvely dsrooroao,.e., k c = dp = kc k M η η η 7. Addoal maulao yelds he wegh fracos k X η k c W = = DP η η k k c. Effec of cha rasfer. The roduco of olymer wh cha legh ca also occur va cha rasfer. M X P X [ ] k kc k [ M ] dp [ ] η η η η k c X = = = dp ( kc k ) η η k k c X = η η X = η η. Thus he exresso for he rae of formao of olymer ad degree of olymerzao gve above mus be modfed. dp dp [ ] = = ( kx[ X] kc k ) k [ M] ( )[ ] DP = k X k k M x c

12 3. For mahemacal coveece, we ll cosder he verse of DP. ( DP) ( k k ) k [ M] k X k k = = k M η k k c x ν c c 4. ha rasfer ca occur wh moomer(m), olymer(p), solve(s), aor(i) or a secfcally added cha rasfer age(y). = k X k M k S k I k P k Y x M S I olymer Y 5. Defe cha rasfer cosas as k = k 6. Thus he verse of he degree of olymerzao ca be wre as a. ( DP) s he verse of DP whou cha rasfer. 7. The fraco of roagag radcals wh DP of s ow wre as R γη f = = γ [ R] η η a. where γ s he cha rasfer erm defed as 8. The mole fraco for each olymer cha legh s I S Y P DP = DP I S Y olymer M M M M ( DP) k [ M] kc k r = M I S Y γ= M S Y M M M γη k k ( ) γη dp [ ] γη η kc kc γη X = = dp k k γ η η γη kc kc γη

13 3 I. Kecs Summary k M I. Proagao rae: ν =. Termao rae: ν = kf[ I] 3. Number-average molecular wegh k [ M] f M = M k k f I d f comb fraco of combao (versus dsrooroao) 4. The frs cha aed radly forms a hgh molecular wegh cha. 5. Moomer cocerao seadly decreases ( coras o se olymerzao). 6. k, k, k follow Arrheus behavor 7. Icreasg emeraure creases ao hus decreases molecular wegh. 8. Above a celg emeraure, chas wll deolymerze. Ehales ad eroes of olymerzao a 5 d Δ (kj./mol) ΔS (J./mol) T c (K),3 -buadee Ehylee Isoree Mehyl Mehacrylae Sryee Molecular weghs ca be lmed by vscosy of reaco mx. [] comb.