高分子化学浙江大学polymer22PolymerizationRate

1、15. polymerization rate25.1 macro kinetics (宏观动力学宏观动力学)1. monomer conversion and rp%1001%100(%)000mmmmmcdtmdrprpc%t31. direct sampling (直接取样直接取样)weight the polymer at different intervals.2. dilatometer method (膨胀计法膨胀计法) v c3. refraction index (折光指数法折光指数法) e.g. n pvac-nvac=0.0714. spectroscopy (光谱法光谱

2、法) mma at 1630 cm-1。5. calorimetry (量热法量热法) h cmeasurement of rp4typical polymerization rate curvesrpc%tinduction：decomposition of initiatorinitial ：c% = 0 5-10，a low steady ratemiddle ：auto-acceleration c%=5-10 70-90）end：drop of rate (c% =70-90 100)55.2 micro kinetics (微观动力学微观动力学)ri in the range of

3、 10-8 10-10 mol/l.s, where f is initiation efficiencykinetic equation: rp vs concentration of reactantsridk21rmrik ikdtiddtrdd22dtrddtrdri12ifkrdiinitiation6propagation,.)3 , 2 , 1( 1,irmrikiiprp = 10-4-10-6 ms; kp = 102-104 m-1s; m = 10-7-10-8 m; m=1-10 mol/lmrkmrkmrkrpipiipp,r=riassuming identical

4、 activity of radicals with various length, so kp=kp,i .722,2,222rkrkrkdtrdrtdtcttterminationyxkyxprrct,yxkyxpprrdt,kt = kt,c + kt,d2 represents two radicals were consumed by one reaction (us)disproportionation combination8mrkrrrdtmdrpppitotal polymerization ratemrkrpthe polymerization degree is so l

5、arge, that the monomer consumed in the initiation reaction can be neglected, or rirp.r=?9steady state (稳态稳态)a steady-state is established early in the reaction.the concentration of radicals is very small (ca. 10-8 m) and nearly constant throughout.steady-state the concentration of radicals is consta

6、nt or initiation rate is equal to termination speed.2ifkrdi22rkrtt 2121212ikfkkrrtdtir determined by initiation mode10polymerization rate with different initiations initiator mikmikfkkdtmdrtdp212121 tikmm210ln21tdpkfkkkself-initiation of styreneln(m0/m) t:low conversion;i and f are constant 3mkrii 2

7、5212521212mikmikkkrtip212tipkkkk 221221212mikmikkkrtip2mkrii11actual polymerization rate nmmikr different initiation and termination reactionsthe specific mechanism can be deduced by the parameters m and n 125.3 influence of temperature arrhenius equationrteaek21tdpkfkkk21tdpaaaadtpeeee2121for norma

8、l polymerization initiated by initiator: ed 125 kj/mol, ep29 kj/mol, et 17 kj/mol, so e=83 kj/molthis corresponds to a two or three fold rate increase for a 10 oc temperature increase.135.4 autoacceleration(自动加速效应自动加速效应)pmma polymerization in benzene solution at various concentrations.at low concent

9、rations (40%), the polymerization proceeds smoothly with no unusual effects. at higher concentrations, a pronounced rate acceleration after partial conversion of monomer to polymer. the more concentrated the solution, the earlier the acceleration occurs.14autoacceleration only occurs during polymeri

10、zations with high concentrations of monomer (i.e., little or no solvent). the reaction proceeds normally for a while, then suddenly the rate of polymerization goes up dramatically. the molecular weight of chains that grow during the accelerated period is substantially higher than that of chains that

11、 grew earlier.rpt15reasons and explanationtermination involves the reaction between two chain ends. the high viscosity hinders the diffusion of chains because of entanglements, so the rate of termination slows considerably. the initiation and diffusion of small molecular monomers is hardly affected

12、by viscosity, so propagation proceeds as before. once a low, steady state concentration of radicals gives way to increasing concentration, chains grow without termination, so the conversion is rapid and the mw is high. for neat monomer, often in cases where the polymer formed is a high tg material,

13、there can come a point at which even the diffusion of monomer is slow. the mixture has become a hard glass, and unreacted radicals become trapped inside. the reaction shuts down at less than 100% conversion.note that autoacceleration can be dangerous because the exotherm of polymerization can be rel

14、eased suddenly, leading to a runaway reaction. 16conditions affecting autoacceleration temperaturethe higher the tp, the lower the viscosity monomer concentrationthe content of polymer solvent good solvent in favor of the diffusion of polymer chain tg of polymer the homogeneity of polymerization med

15、iahomogeneous or heterogeneous175.5 typical polymerization rate curvestconversion %a: slow initiation-acceleration-drop (“s” shape) b: uniform ratec: fast initiation-dropabc18bulk polymerization of mma monitored by esr spectroscopyshen j, tian y, wang g. makromol chem 1991;192:2669.196. average mole

16、cular weight20average molecular weight polymer consists of chains with a variable number of monomer units. of the key parameters influencing the physical properties of polymers, the average molecular weight usually is the dominant factor, often influencing the other parameters. control on the molecu

17、lar weight is one of the most important topics of polymer synthesis 216.1 kinetic chain length (动力学链长动力学链长)kinetic chain length: how many propagation steps occur before the chain mechanism is stopped?the amount of monomer consumed by one active species from initiation to terminationtpiprrrrspecies a

18、ctive totalofnumber themonomer dpolymerize ofnumber thesteady stateno chain transfer22kinetic chain length: rkmkrkmrkrrtptptp222mrkrpmkrrppptprkmk222 22rkrrtti212tikrr212itprkmk2ifkrdi 21212ikfkmktip mi-0.523 - termination mode-polymerization degree+2vvvvvcombinationxn=2disproportionationxn=ctdtdtkk

19、k,any mixture of these both mechanisms can be described by:12nxxn: polymerization degree, or the number of monomer per chain24variation of with temperaturefor initiator system, decreases with the increment of temperature. 21212ikfkmktipdtpeeee2121kj/mol 41 e256.2 chain transferkinetic chain length:

20、monomer still consumed after chain transfer polymerization degree: fixed after chain transferthe propagating macroradical abstracts a weakly bonded atom (e.g., x) from the transfer agent, xy. a dead polymer with a saturated end group is generated as well as a new free-radical y, which in turn might

21、react with monomer units. the transfer agent may be the monomer itself, the initiator, the solvent, or any other deliberately added transfer agent.26 and xnin this case,=280 and xn=(100+80+100)/3r199mr100p10079mr80r1r1p8099mr100p1002nd chain transferto m1st chain transferto mimmm27average polymeriza

22、tion degreegroups end formed ofnumber thehalfchainpolymer ofnumber themonomer dpolymerize ofnumber thepnrxxn: the number of monomer per chain28the various reactions within the polymerization process generate different amounts of end groupstransfer 2 end groupstptrikitriidk221rmiikinitiation1 end gro

23、up,.)3 ,2, 1( 1,irmrikiippropagation0 end groupsyxkyxpprrdt,termination by disproportionation1 end groupyxkyxprrct,termination by combination 0 end groups29trdtipnrrrrx221,mrkrpp2,2rkrdtdt2,)(22rkkifkrctdtdi ittrtrtrkri mtkkrmkkrmkkkxipttrppdtppctdtni22,22,1 mtkkrmkkkxipttrppctdtni22,21trtpnrrrx30ch

24、ain transfer constant pttrikkcipmtrmkkcmonomerpstrskkc solventpitrikkc initiatorpttrtkkc chain transferagentpptrpkkc polymer .mtcmpcmicmsccmtkktpismipttri31variation of xn with chain transfer mtcmpcmicmsccrmkkkxtpismppctdtn22,21 mtcmpcmicmsccrmkkxtpismpptn2211ctdtdtkkk,pnrx11maxmncx32for bulk polyme

25、rization at low conversionmpptncrmkkx2211a plot of the xn-1against the rate of polymerization rp (the rate of polymerization can easily varied by the concentration of the initiator), yields:221mkkptmc33termination by disproportionationfor =1, mtcmpcmicmsccrmkkxtpismpptn2221ptpnrkmkx2220,for dispropo

26、rtionation without chain transfer mtcmpcmicmsccxxtpismnn0 ,11so we got346.2.1 transfer to monomermnncxx0 ,11for vinyl chloride, cm is around 10-3, sopmtrmnkkcx1(etr-ep) is around 1763 kj/mol, so cm increases with temperature it is important to notice that the transfer to monomer reaction cannot be d

27、ecreased via a decrease in m, because xn is independent on the m for the transfer to monomer step.the only transfer reaction that cannot be avoided is transfer to monomer. thus, the maximum upper limit of the mw that can be reached under a given set of reaction conditions is given by the transfer to

28、 monomer reaction, assuming the absence of all other transfer events35366.2.2 transfer to initiator miccxximnn0 ,11for bulk polymerizationin addition, the effects on the overall polymerization kinetics and on the molecular weight distribution are small, if low concentrations of initiator are employe

29、d.also unavoidable is the transfer to the initiator molecule used to induce polymerization.ptpnrkmkx2220 ,37386.2.3 transfer to solventfor solution polymerizationthere are a few solvents that show significantly higher transfer constants, such as ccl4.the transfer to a solvent molecule is of consider

30、able importance, because solvents are used in high concentrations in most industrial polymerization processes. msccxxsmnn0 ,11396.2.4 transfer to polymerat higher monomer conversion, transfer processes to the formed polymer are becoming significant. interestingly, the transfer to polymer rate consta

31、nts are considerably higher (by a factor of 10) than those observed for the corresponding monomer. h.+.h.back biteintermolecular transfer to polymer resulting in so-called long-chain branching.intramolecular transfer to polymer, (backbiting) leading to short-chain branching (in ethylene polymerizati

32、ons).406.2.5 transfer to additivestypical agents with very high transfer constants are thiols and halogenated compounds such as cbr4. chain transfer agents with chain transfer constants greater than one are very useful, as they can be employed in low concentrations. these agents are important for in

33、dustrial processes, because they allow for the regulation of the molecular weight of the generated polymer and thus significantly reducing the viscosity of the reaction medium and allowing for an optimum heat transfer.41427. inhibition and retardation43inhibition (阻聚)if chain transfer occurs, but th

34、e new radical is incapable of reinitiation, this is a kind of termination reaction. the agent responsible is called an inhibitor.if the generated radical is less reactive than the propagating radical, retardation takes place, which is characterized by a decrease in the rate of polymerization.retardation (缓聚) 44 most commercial monomers are packaged with traces of inhibito