Polymerization, degree termination

Because low-valent transition metals such Ni(0) and Pd(0) are air and/or moisture sensitive,34 the exclusion of oxygen and/or moisture is also crucial for the polymerization. Failure to exclude oxygen will deactivate the catalysts, thus causing the termination of the polymerization and influencing the polymerization degree. 

A thiol-terminated PS was used as a sample in the experiment. It was based on a living polymerized carboxyl-terminated PS with M = 93,800 and Mu = 100,400. The polydispersity was Mp( /M = 1.07. The degree of polymerization was about 900 and, thus, its contour length was about 220 nm. The thiol groups were substimted for the carboxylic ends using 1,10-decanedithiol by means of thiolester bonding, anticipating the preferential interaction between... 

Narrowly distributed Pl-ft-PS-i-PI triblock copolymer chains with both of their ends capped with bromobutyl groups were prepared by sequential addition of living anionic polymerization and terminated by excess of 1,4-dibromobutane (PS block Mw = 3.5 x 103 g/mol PI blocks Mw = 3.1 x 103 g/mol Mw/Mn = 1.12 The degree of end-functionalization was 92% characterized by HNMR). Figure 6 shows the SEC profile of such prepared triblock copolymer chains. The small but a detectable amount ( 5%) of Pl-i-PS-i-PI dimers, PI-Z>-PS-Z>-PI-c-PI-Z>-PS-Z>-PI, is presumably formed via the Wurtz-type coupling reaction. 

All of the observations above indicate the presence of living systems however, attempts to extend these well-defined systems above a limit of M = 100,000 have been unsuccessful, except at very low temperatures (< -70° C) [270]. Thus, polymers with predetermined polymerization degrees, low polydispersities, and with desired end groups can be obtained only for a sufficiently low molecular weight range. This indicates that contribution of transfer increases with temperature and with chain length [cf. Eq. (2) in Section II.C]. In the presence of transfer and termination polydispersities increase with the chain length and with conversion. 

Cationic polymerization of THF fulfills all the requirements of living polymerization. With several initiators, the initiation is relatively fast and quantitative, and propagation proceeds without transfer or termination. The dependence DP = ([M]0 - [M)e)/[I)0 holds up to high polymerization degrees. The only limitation is that, due to the reversibility of propagation, the molecular weight distribution is broadened and reaches the value of MjMn = 2 in equilibrium. Polymers with narrower MWD were obtained by terminating the polymerization at lower conversion [56]. 

For anionic polymerizations in protic media, you get the same expressions as those obtained for free radical polymerization with termination by disproportionation (p is still the probability of chain growth, but now 1 -p is the probability of chain transfer). Again, the averages and distributions you can obtain are only valid for low degrees of conversion. For cationic polymerization, there are several types of transfer and termination reactions that occur m most reactions, so you... 

The number-average degree of polymerization can be obtained from the rates of propagation (eqn 10.65) and chain breaking (sum of eqns 10.66 and 10.67) as in free-radical polymerization with termination by chain transfer to a transfer agent (see eqn 10.42) ... 

In the case of addition polymerization without termination, the number fraction distribution function (the probability that a given chain has degree of polymerization N) is given by the Poisson distribution function ... 

Termination reactions cannot be eliminated in radical polymerizations because termination reactions involve the same active radical species as propagation therefore, eliminating the species that participates in termination would also result in no polymerization. Termination between active propagating species in cationic or anionic processes does not occur to the same extent because of electrostatic repulsions. Equation (1) represents the rate of polymerization, Rp, which is first order with respect to the concentration of monomer, M, and radicals, P, while Eq. (2) defines the rate of termination, Rt, which is second order with respect to the concentration of radicals. To grow polymer chains with a degree of polymerization of 1000, the rate of propagation must be at least 1000 times faster than the rate of termination (which under steady state condition is equal to the rate of initiation). This requires a very low concentration of radicals to minimize the influence of termination. However, termination eventually prevails and all the polymer chains produced in a conventional free radical process will be dead chains. Therefore they cannot be used in further reactions unless they contain some functional unit from the initiator or a chain transfer agent. 

Problem 6.44 Consider a case of free-radical polymerization where termination involves both disproportionation and coupling of chain radicals but chain transfer reactions can be neglected. Derive an expression for the distribution function for the degree of polymerization of polymer in terms of the kinetic chain length and the ratio of termination by disproportionation to that by coupling [65]. Simplify the expression for two limiting cases where (a) termination is solely by coupling and (b) termination is solely by disproportionation. 

Figure 8.2 Number fraction and weight fraction distribution of the degree of polymerization with i/ = 50 for anionic "living" polymers and for free-radical polymerization (with termination by disproportionation). (Problem 8.8)...

AIBN solution in chloroform (0.1% w/v) was taken in polymerization tube. The chloroform was completely removed under vacuum, and predetermined amounts of monomers and THF (1 1 v/v) were charged into polymerization tubes. The reaction mixtures were made oxygen free by repeated freeze-thaw-pump cycles from liquid nitrogen temperature. Polymerization reactions were carried out at 50 C. After desired degree of conversion (<10%), polymerization was terminated and polymers were precipitated in a suitable non-solvent system. The monomer TCA (M ) was copolymerized with styrene (M ), methyl methacrylate (M ) and acrylonitrile (M designated at TCST, TCMA and TCAN respect-... 

According to Pravednikov s conception [69] in a case, when the average degree of a macromolecule s chain polymerization is small in comparison with some characteristic value P, the process can be described by the usual equation of classic kinetics. Radicals, the length / of which is more than this value of the polymerization degree P, sharply lose their mobility in comparison with the radicals of shorter chain length and in this case the kinetics are described by a dependence of the constant chain termination rate on the chain length. 

Ito s model [68] bears resemblances to the model of Ref. [35], but is different by two aspects. Firstly, it assumes that the constant rate of the chain termination depends on the number of monomeric units (so-called polymerization degree) of tn and n radical chains taking part in the termination reaction and represents the sum of the independent contributions of m and n. Secondly, the dependence of the chain termination constant on the length of chains under two types of conditions is described the first condition is < n, controlled by segmental diffusion, and the second one is m > controlled by the reptation diffusion. In the reptation chemical mechanism of diffusion in the deep states of conversion the macroradicals move snake-like between the network joints. De Gennes connected a reptative moving of macroradicals with the dynamic properties of the medium with the use of scaling ratios [37-40] as applied in Refs. [41-46] for the description of constant chain termination in the late conversion state. 

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