高分子化学（浙江大学） polymer2-2 Polymerization Rate.ppt

1,5. Polymerization Rate,2,5.1 Macro kinetics (宏观动力学),1. Monomer Conversion and Rp,3,1. Direct sampling (直接取样) weight the polymer at different intervals. 2. Dilatometer method (膨胀计法) V C 3. Refraction index (折光指数法) e.g. n PVAc-nVAc=0.071 4. Spectroscopy (光谱法) MMA at 1630 cm-1。 5. Calorimetry (量热法) H C,Measurement of Rp,4,Typical Polymerization Rate Curves,Rp,C%,t,Induction：decomposition of initiator,Initial ：C% = 0 5-10，a low steady rate,Middle ：auto-acceleration C%= 5-10 70-90）,End：drop of rate (C% =70-90 100),5,5.2 Micro Kinetics (微观动力学),Ri in the range of 10-8 10-10 mol/L.s, where f is initiation efficiency,Kinetic equation: Rp vs concentration of reactants,Initiation,6,Propagation,Rp = 10-4-10-6 Ms; kp = 102-104 M-1s; M = 10-7-10-8 M; M=1-10 mol/L,R=Ri,Assuming identical activity of radicals with various length, so kp=kp,i .,7,Termination,kt = kt,c + kt,d,2 represents two radicals were consumed by one reaction (US),disproportionation,combination,8,Total polymerization Rate,The polymerization degree is so large, that the monomer consumed in the initiation reaction can be neglected, or RiRp.,R=?,9,Steady State (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 constant or initiation rate is equal to termination speed.,R determined by initiation mode,10,Polymerization rate with different initiations,Initiator,Self-initiation of Styrene,ln(M0/M) t: low conversion; I and f are constant,11,Actual Polymerization rate,different initiation and termination reactions,The specific mechanism can be deduced by the parameters m and n,12,5.3 Influence of Temperature,Arrhenius Equation,For normal polymerization initiated by initiator: Ed 125 KJ/mol, Ep29 KJ/mol, Et 17 KJ/mol, so E=83 KJ/mol This corresponds to a two or three fold rate increase for a 10 oC temperature increase.,13,5.4 Autoacceleration(自动加速效应),PMMA polymerization in benzene solution at various concentrations.,at low concentrations (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.,14,Autoacceleration only occurs during polymerizations 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 grew earlier.,15,Reasons and Explanation,Termination 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 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, 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 released suddenly, leading to a runaway reaction.,16,Conditions affecting autoacceleration,Temperature the higher the Tp, the lower the viscosity Monomer concentration the content of polymer Solvent good solvent in favor of the diffusion of polymer chain Tg of polymer The homogeneity of polymerization media homogeneous or heterogeneous,17,5.5 Typical Polymerization Rate Curves,A: Slow initiation-acceleration-drop (“S” shape) B: uniform rate C: fast initiation-drop,A,B,C,18,Bulk polymerization of MMA monitored by ESR spectroscopy,Shen J, Tian Y, Wang G. Makromol Chem 1991;192:2669.,19,6. Average Molecular Weight,20,Average 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 molecular weight is one of the most important topics of polymer synthesis,21,6.1 Kinetic Chain Length (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 termination,Steady State,no chain transfer,22,kinetic chain length: , MI-0.5,23, - termination mode-polymerization degree,Combination Xn=2,Disproportionation Xn=,Any mixture of these both mechanisms can be described by:,Xn: Polymerization degree, or the number of monomer per chain,24,Variation of with temperature,For initiator system, decreases with the increment of temperature.,25,6.2 Chain Transfer,kinetic chain length: monomer still consumed after chain transfer polymerization degree: fixed after chain transfer,The 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 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 Xn,In this case, =280 and Xn=(100+80+100)/3,27,Average Polymerization Degree,Xn: The number of monomer per chain,28,The various reactions within the polymerization process generate different amounts of end groups,Initiation1 end group,29,30,Chain transfer constant,31,Variation of Xn with chain transfer,32,For bulk polymerization at low conversion,a plot of the Xn-1against the rate of polymerization Rp (the rate of polymerization can easily varied by the concentration of the initiator), yields:,33,Termination by disproportionation,For =1,For disproportionation without chain transfer,So we got,34,6.2.1 Transfer to monomer,For vinyl chloride, CM is around 10-3, so,(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 decreased 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 monomer reaction, assuming the absence of all other transfer events,35,36,6.2.2 Transfer to initiator,For bulk polymerization,In addition, the effects on the overall polymerization kinetics and on the molecular weight distribution are small, if low concentrations of initiator are employed.,Also unavoidable is the transfer to the initiator molecule used to induce polymerization.,37,38,6.2.3 Transfer to solvent,For solution polymerization,There are a few solvents that show significantly higher transfer constants, such as CCl4.,The transfer to a solvent molecule is of considerable importance, because solvents are used in high concentrations in most industrial polymerization processes.,39,6.2.4 Transfer to polymer,At higher monomer conversion, transfer processes to the formed polymer are becoming significant. Interestingly, the transfer to polymer rate constants are considerably higher (by a factor of 10) than those observed for the corresponding monomer.,Intermolecular transfer to polymer resulting in so-called long-chain branching.,Intramolecular transfer to polymer, (backbiting) leading to short-chain branching (in ethylene polymerizations).,40,6.2.5 Transfer to additives,Typical 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 industrial 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.,41,42,7. Inhibition and Retardation,43,Inhibition (阻聚),If chain transfer occurs, but the 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 inhibitor to prevent premature