Molar mass distribution living polymerization

The number-average molar mass of living polymerizations via macroions is completely determined by the monomer and initiator concentrations initially and at equilibrium. However, the molar mass distribution additionally depends on whether all the initiator anions or monomer anions do actually initiate polymerization simultaneously. If, for example, the initiator... 

Polymerization by ionic initiation is much more limited than that by free-radical initiation with vinyl monomers, but there are monomers such as carbonyl compounds that may be polymerized ionically but not through free radicals because of the high polarity. The polymerization is much more sensitive to trace impurities, especially water, and proceeds rapidly at low temperature to give polymers of narrow molar-mass distribution. The chain grows in a living way and, unlike in the case of free-radical polymerization, is generally terminated not by recombination but rather by trace impurities, solvent or, rarely, the initiator s counter-ion (Fontanille, 1989). 

It has been noted (Odian, 1991) that the steady-state approximation cannot be applied in many cationic polymerizations because of the extreme rate of reaction preventing the attainment of a steady-state concentration of the reactive intermediates. This places limitations on the usefulness of the rate expressions but those for the degree of polymerization rely on ratios of reaction rates and should be generally applicable. The molar-mass distribution would be expected to be very narrow and approach that for a living polymerization (with a poly-dispersity index of unity), but this is rarely achieved, due to the chain-transfer and termination reactions discussed above. Values closer to 2 are more likely. 

The particular features of anionic polymerization that made the polymer chains living were discussed above. The main requirement for a living polymerization is the absence of any process for spontaneous termination so that the degree of polymerization is controlled by the ratio of monomer to initiator concentrations. The molar-mass of the polymer therefore increases linearly with monomer conversion. On exhaustion of the monomer, the initiation centres remain, so chains may be re-initiated by addition of further monomer. Termination or chain transfer is controlled by the delibemte addition of a reagent to remove the living end. The resulting polymers will also have very narrow molar-mass distributions since rapid initiation ensures that all chains are initiated at the same time. 

Since the discovery of living polymerizations by Swarc in 1956 [1], the area of synthesis and application of well-defined polymer structures has been developed. The livingness of a polymerization is defined as the absence of termination and transfer reactions during the course of the polymerization. If there is also fast initiation and chain-end fidelity, which are prerequisites for the so-called controlled polymerization, well-defined polymers are obtained that have a narrow molar mass distribution as well as defined end groups. Such well-defined polymers can be prepared by various types of living and controlled polymerization techniques, including anionic polymerization [2], controlled radical polymerization [3-5], and cationic polymerization [6, 7]. 

Already before reporting this combined inifer and living polymerization approach, Kennedy and coworkers developed a controlled isobutene polymerization method based on cumyl ester initiators (Scheme 8.6) with boron trichloride as activator and incremental monomer addition [28], The livingness of the polymerization was demonstrated by the linear increase of number-average molar mass and the constant number of polymer chains (A) with the amount of PIB obtained (wp, as measure for conversion) as well as the narrowing of the molar mass distribution with conversion (Fig. 8.1) [28]. 

Some other distribution functions have been proposed for living systems. For example, Muller et al. [112, 113] have reported an analytical solution for the molar mass distribution in a polymerization with degenerative transfer between active and dormant chain ends. Goto and Fukuda... 

A variety of important monomers are polymerized via anionic polymerization including cyclic ones like ethylene oxide as base for poly(ethylene oxide) (PEO) or poly(ethylene glycol) (PEG) and A-carboxyanhydrides (NCAs) used for making polypeptides and caprolactam, the monomer of nylon 6 (Eig. 3.9). Whereas in all cases rather stringent conditions have to be used to allow at least an efficient polymerization, for technical products often not fully living conditions are achieved, which leads to broader molar mass distributions. 

New nucleotide units are added stepwise to an already existing polynucleotide chain under the influence of PN-pase and/or addase in theprimer-dependent synthesis. This method is especially useful for making block copolymers. For example, the polynucleotide rrr-d(ddd) is formed under the influence of PN-pase and addase, whereas the polymer chain ddd-r(ddd)n can be made under the influence of addase. The process corresponds to that of a living polymerization, that is, a narrow molar mass distribution is produced. 

Recently, typical step-reaction polymerizations, as in polyesters, polyethers, and polyamides, have been forced into chain-reaction mechanisms by designing complex chain ends that react fast with the monomer only. Under the proper conditions, the step reaction can be suppressed almost completely. Such chain-growth polycondensation may even yield living polymers with narrow molar-mass distribution. A link to the initial literature is given in the General References for this section. 

Brookhart, M. DeSimone, J. M. Grant, B. E. Tanner, M. J. Cobalt(III)-catalyzed living polymerization of ethylene Routes to end-capped polyethylene with a narrow molar mass distribution. Macromolecules 1995, 28, 5378-5380. 

Dispersions of molar mass distributions depend on polymerization reactions (see Table 4.1.6). Values of Dm close to 1 can be achieved by cmitrolled or living techniques whereas free radical polymerization leads to broad distributions. 

When compared to the abovementioned highly associated multinuclear coordination catalysts of very low initiation efficiency, metalloporphyrins with mononuclear Mt-X species have been reported to be excellent catatysts for the living polymerization of epoxides. The molar mass distribution of... 

The E-N copolymerization with PI catalysts is living. Particularly, the cylcohexyl substituent (VI-1) produces E-N copolymers with a narrow molar mass distribution = 1.07-1.16) and with value that increases linearly with the polymerization time. An value up to... 

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