高分子化学浙江大学polymer23Measurementofratecoefficients

1、18. measurement of rate coefficients2history up to the late 1980s termination and propagation rate coefficients were accessible only in their coupled form, kp/kt0.5 or individually via combination with pseudo flickering techniques like the rotating sector or spatially intermittent polymerization met

2、hods in combination with stationary polymerization measurements. the situation has dramatically improved with the invention of the pulsed laser polymerization (plp) technique in the late 1980s. since then, this technique has been extensively used to collate propagation and termination rate coefficie

3、nts for various homo- and copolymerizations.38.1 methods for the measurement of kdrd = fkd i the concentration can be measured via any quantity to which it is directly proportional, such as a spectroscopic infrared (红外光谱红外光谱) absorption that can be easily followed with reaction time. infrared spectr

4、oscopy has been used extensively in the past to study the decay of organic peroxides in various reaction media.48.2 methods for the measurement of kppulsed laser polymerizationsize-exclusion chromatography (plp-sec)l0l0+2l0l0+2l0+3l0irradiated by a pulsed laser beam (5-20ns)monomerphotoinitiator, (s

5、olvent & transfer agents)mw of polymer produced by plp: l0,n=nkpmt0568.2 methods for the measurement of kpelectron spin resonance spectroscopy-stationary polymerization (esr)(电子自旋共振电子自旋共振)mrkdtmdrpp1221lnttrkmmp78.2 methods for the measurement of kpquenched instationary polymerization systems (q

6、uips)a photopolymerizable mixture passes through a capillary system, is irradiated at a specific location, and polymerizes in the capillary during a well-defined dark period until it drops into a quenching bath with an inhibitor(nitroxyl radicals).measuring the chain length distribution by size-excl

7、usion chromatography that allows determination of kp in accordance with the equation.l=kpmt088.3 methods for the measurement of ktrmayo methodthe usual procedure for measuring the chain transfer constant ct involves determination of xn for a range of t/m values and plotting the data as 1/xn versus t

8、/m, that is, a mayo plot. the value of ct is then determined as the straight-line slope of this plot. mtccrmkkxtmpptn221198.4 methods for the measurement of ktsingle pulse pulsed laser polymerization (sp-plp)121tkrrt tpkkttrkmm2/012 0112 tkrrt22rkdtrdtmrkdtmdrpp10applying pulsed laser-polymerization

9、 (plp) in conjunction with infrared or near-infrared spectroscopic measurement of monomer conversion induced by a single laser pulse (sp-plp) allows for the determination of the ratio of termination to propagation rate coefficients, kt/kp, in wide ranges of temperature; pressure; and monomer convers

10、ion. tpkkttrkmm2/012118.5 determining the mode of termination: disproportionation versus combinationidentification and quantification of chain ends are not simple as they give only small signals (relative to the rest of the polymer chain) in a spectroscopic analysis. this can be overcome to some ext

11、ent by isotopic labeling of the initiator end groups by 14c or by using initiator fragments containing fluorine or phosphorus as nmr-sensitive molecules.the application of the matrix-assisted laser desorption ionization time-of-flight mass spectroscopy (maldi-tof-ms) to the problem of end-group anal

12、ysis of polymers brought some promising results to this field of polymerization kinetics. 12maldi-tof-ms1314some rate coefficients 159. molecular weight distribution分子量分布16how to know mwd? from the experimental measurement precipitation fractionation (沉淀分级) gel permeation chromatograph/size exclusio

13、n chromatograph (gpc/sec 凝胶渗透色谱/体积排斥色谱) maldi-tof form theoretical analysis statistical approach (统计方法) kinetic approach （动力学法）179.1. distribution for disproportionation or chain transfer consider only chains whose growth is terminated by either disproportionation or chain transfer. note that in bot

14、h the cases, the number of chains produced is equal to the number of chain terminated. 18let p = probability of adding a monomer to a grow chain.dttrpprrrrp,a polymer chain with the length n is formed by n-1 propagation steps and one chain stopping (termination or transfer) event of probability (1-p

15、).so the probability of finding a chain of length n is: ppnnnn11number of chains of size ntotal number of chainsinstantaneous number-fraction distributionnumber-fraction distribution19weight-fraction distributionweight of chains of length n: wn=mnnnweight of all chains : w=mn0initial number of monom

16、ersmw of monomerppnnnnnnmnmnnwwnnnn11000since one termination results in one chain, so pnn10211pnpwwnnppnnnn1120molecular weight distribution curve02000400060008000100000246810number fraction (10-4)polymerization degree p 0.999 0.9995 0.9997504000800012000160000.000.010.020.030.04weight fraction (10

17、-2)polymerization degree p 0.999 0.9995 0.9997521average molecular weightpnnxn110dttrpprrrrp,1111,kineticndttrpprobnxrrrpxaccording to definition:trdtptrdtptrdtipkineticnrrrrrrrrrrx,)22(212211rmiik22average molecular weightppnpnnnxnnn11)1 (1according to calculation:ppppnwwnxnnw11)1 (21221pxxnwpolydi

18、spersity index(多分散性指数), pdi232222)1 ()1 () 1(pnpppnnnnnn there are (n-1) different ways to form a chain of length n from the combination of two chains, so 9.2. distribution for combinationconsider a chain of length n formed by combining m monomers with (n-m) monomers. for chain of length m, here are

19、 (m-1) addition steps and 1 termination for chain of length (n-m), here are (n-m-1) addition steps and 1 terminationprobability of finding a chain of length n is:22211)1 (1pppppnmnm24322)1 (21ppnwwnnpnnnxnn12ppwwnxnw125 . 121pxxnw22)1 (pnpnnnn2504000800012000160000.000.010.020.030.04number fraction

20、(10-2)polymerization degree p 0.999 0.9995 0.9997504000800012000160000.000.010.020.03weight fraction (10-2)polymerization degree p 0.999 0.9995 0.99975molecular weight distribution curve26comparisonaverage molecular weight disproportionation vs combinationpxn12ppxw125 . 1nwxxpxn11ppxw112nwxx27020004

21、00060008000100000.000.010.020.030.040.050.06weight fraction (10-2)polymerization degreexn=1000 disproportionation, p=0.999 combination, p=0.998molecular weight distribution curvedisproportionation vs combinationpxn12pxn112810. polymerization thermodynamics 29thermodynamics polymerization possibility

22、 is the monomer polymerizable? polymerization equilibrium polymerization or depolymerization ?30 g0 from polymer to monomer g=0 equilibrium between monomer and polymer 1nkknrmrdppsthg31enthalpy contribution hentropy contribution s s is always negative, reflecting the loss of degrees of freedom of th

23、e monomer becoming a part of the polymer chain. s is around -105-125 j/(mol*k) for polymerization from r.t. to 100 oc, -ts is around 3042 kj all chain polymerizations are exothermic. most free-radical polymerizations are negative with typical values ranging from 30 to 80 kj/mol. sthgpolymerization i

24、s mainly determined by h32polymerization heatcccc350 kj/mol610 kj/mol2906103502 hsteric effect: -h resonance effect: -h styrene(69.9)butadiene(72.8) acrylonitrile (72.4) negative substitute: -h vinyl chloride(95.8) vinylidene fluoride(129.7)tetrafluoroethylene(154.8) h bond & solvent: -h acrylic

25、 acid(66.9) methacrylic acid(42.3) -33monomer - h (kj/mol) - s (j/mol.k)ethylene95.0100.4propylene85.8116.3butylene-179.5112.1isobutylene51.5119.7isoprene72.885.8butadiene7389.0styrene69.9104.6-methylstyrene35.1103.8tetrafluroethylene155.6112.1vinylchloride95.6-methly methacrylate56.5117.2acrylic ac

26、id66.9acrylamide82.0data of h and s for some monomers (25c)34ceiling temperature (聚合上限温度)mrtgmrrrtgkrtggnn1lnlnln01000sthgshtc1nkknrmrdppeccmrtsthg1ln00ccmrshtln0000shtcm=1.03536for example: h s m at 70 ockj/mol j/(molk)mma 56 1184.3610-3styrene73 104 2.0910-6-methylstyrene45 1487.60 001lnsthrmeemrt

27、sthg1ln00equil. monomer concentrationequilibrium monomer concentration3711. controlled/living radical polymerization38disadvantages of tradition radical polymerization slow and continuous initiation prevents synthesis of well-defined polymers with degrees of polymerization predetermined by the ratio

28、 of concentrations of the converted monomer to the introduced initiator, with controlled topologies (stars, combs) and compositions (blocks, grafts, gradients). the typical lifetime of a propagating chain is very short, in the range of 1 s. molecular weight distribution is broad. poor control on the

29、 regio- and stereoselectivity39features of living polymerization no termination reaction! linear kinetic plot in semilogarithmic coordinates (ln(m0/m ) vs. time), if the reaction is first-order with respect to the monomer concentration. linear evolution of mw with conversion. mw is predicted by m/i0

30、 polydispersity (mw/mn) should be close to unity end-functionality of polymer chain block polymer can be prepared by continuous polymerization of second monomer40controlled/living radical polymerization41nitroxide mediated polymerization (nmp)in this mechanism, the dormant chain is dissociated into polymer radical p and capping radical x, where x is assumed to be stable enough to undergo no reaction other than the combination with p42ni