Kinetic Chain ordering

Eqs. (26) and (27) apply irrespective of the nature of the initiation process it is required merely that the propagation and termination processes be of the second order. They emphasize the very general inverse dependence of the kinetic chain length on the radical concentration and therefore on the rate of polymerization. The kinetic chain length may be calculated from the ratios k /kt as given in Table XI and the rate of polymerization. Thus, for pure styrene at 60°C... 

Thus, if m = 10 2 mol l"1, and if all the monomer is polymerised, and if all kinetic chains are started by addition of AlX+2 to monomer, there will be 10"4 mole of Al-C bonds per base-mole of monomer. For reactions in a hydrocarbon solvent, where kp is of the order of 10s 1 mol 1 s 1, and tV2 appreciably greater, [Pn+] is correspondingly smaller and the whole task much harder. Further, before the C-Al bonds can be identified, they must be converted into a stable, analysable species by unambiguous reactions and these must be such that the products formed from the unreacted AlBr3 during the conversion do not interfere with the subsequent analysis. It is evident that radioactive tracer methods offer the best hope of solving this problem. 

The allylic radicals that are formed are too stable to initiate polymerization, and the kinetic chain also terminates when the transfer occurs. The allylic radicals undergo termination by reaction with each other or, more likely, with propagating radicals [Litt and Eirich, I960], Reaction 3-155 is equivalent to termination by an inhibitor, which is the monomer itself in this case. In this polymerization the propagation and termination reactions will have the same general kinetic expression with first-order dependencies on initiator and monomer concentrations, since the same reactants and stoichiometry are involved. The degree of polymerization is simply the ratio of the rate constants for propagation and termination and is independent of the initiator concentration. 

Tetralin hydroperoxide (1,2,3,4-tetrahydro-l-naphthyl hydroperoxide) and 9,10-dihydroanthracyl-9-hydroperoxide were prepared by oxidizing the two hydrocarbons and purified by recrystallization. Commercial cumene hydroperoxide was purified by successive conversions to its sodium salt until it no longer increased the rate of oxidation of cumene at 56°C. All three hydroperoxides were 100% pure by iodometric titration. They all initiated oxidations both thermally (possibly by the bi-molecular reaction, R OOH + RH — R O + H20 + R (33)) and photochemically. The experimental conditions were chosen so that the rate of the thermally initiated reaction was less than 10% of the rate of the photoreaction. The rates of chain initiation were measured with the inhibitors 2,6-di-ter -butyl-4-methylphenol and 2,6-di-fer -butyl-4-meth-oxyphenol. None of the hydroperoxides introduced any kinetically first-order chain termination process into the over-all reaction. 

The cyclotrimerization of isocyanates is initiated by anionic type of catalysts and proceeds via propagation, transfer and termination steps ( ). It was found that in the case where the cyclotrimerization reaction proceeds with a long kinetic chain length, the kinetics of the reaction followed second order with respect to the isocyanate as measured by the disappearance of the isocyanate groups and was first order with respect to the initial concentration of the catalyst ( ,5). ... 

In the section on anionic polymerization, we mentioned that it was time for you to do some work So here is the homework question you have been dreading. Show that in an anionic polymerization of an appropriate monomer in a protic solvent, NH3, initiated using NaNHj, the rate of polymerization is second-order in monomer concentration. Also, obtain an expression for the kinetic chain length. Try not to throw up oh your answer sheet ... 

The rate of monomer consumption contains an additional contribution from initiation (see eqn 10.62). If the kinetic chains are long, this contribution is negligible, and the rate, then given by eqn 10.69, is first order in potassium amide, second order in styrene monomer, and of orders minus one in potassium ion and water. 

The change in slope in the retardation region of the CL curves (Figure 3) reflects the decrease in the rate of formation of POOH when the nitroxide is present. Considering initiation by n-th order decomposition of POOH with rate coefficient ka and noting that the kinetic chain length, u, is ... 

The kinetic chain length was evaluated from quantum yield measurements and found to be on the order of 1000 epoxy groups polymerized per initiating proton. 

Elucidation of the mechanism of propagation for iso-prene and butadiene in hydrocarbon solution with lithium as counterion in the past has been complicated by disagreement in the literature regarding both the kinetic order dependence on chain-end concentration and the degree of association of the chain ends, as well as by apparent changes in kinetic reaction orders with chain-end concentration [3, 56], Eor butadiene and isoprene propagation, reported reaction order dependencies on the concentration of poly(dienyl)lithium chain ends include 0.5, 0.33, 0.25, and 0.167. Kinetic smdies of isoprene propagation with lithium as counterion in hydrocarbon solvents showed... 

Recently the photoinitiated polymerization of multifunctional monomers in condensed phase (usually called UV curing) was characterized as an extremely fast reaction with a kinetic chain length in the order of 100 000 acrylate functions per initiating radical(1 ) ... 

Additives of the stearates of iron (IS), copper (CpS), cobalt (CbS), zinc (ZS), and lead (LS) within a certain concentration range were found to increase the polymerization rate of styrene and methylmethacrylate (MMA) in comparison with thermal polymerization. By initiating activity, they can be arranged as LS < CbS < ZS < IS < CpS. The decreases in the effective activation energy, the activation energy of the initiating reaction, and the kinetic reaction order with respect to monomer point to the monomer s active participation in chain initiation. IR spectroscopy data show that an intermediate monomer-stearate complex is formed and then decomposed into active radicals to initiate polymerization. The benzoyl peroxide (BP)-IS (or CpS) systems can be nsed for effective polymerization initiation. Concentration inversion of the catalytic properties of... 

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