Ionic Homopolymerization

Like radical polymerizations, ionic polymerizations also occur by a chain mechanism. In contrast to radical polymerizations the chain carriers are macroions  

in contrast to radical polymerization, there is no chain termination by combination, since the growing chains (macroions) repel each other electrostatically because of their like charges. Chain termination occurs only by reaction of the growing chain ends with substances such as water, alcohols, acids, and amines. The ions produced by reaction of these substances can sometimes initiate new chains (chain transfer). Under certain conditions the ionic propagation species retain their ability to grow over extended periods of time, even after complete consumption of monomer ( living polymers , see Sect. 3.2.1). 

In contrast to radical polymerizations, ionic polymerizations proceed at high rates even at low temperatures, since the initiation and propagation reactions have only small activation energies. For example, isobutylene is polymerized commercially with boron trifluoride in liquid propane at — 100°C (see Example 3.16). The polymerization temperature often has a considerable influence on the structure of the resulting polymer. 

At regular intervals, samples are taken of the emulsion for GC (0.3 ml each) and SEC (0.5 ml each) analysis using degassed syringes.The following reaction times proved to be appropriate for taking samples for GC and SEC 1 immediately after addition of the initiator 2 after 15 min 3 after 30 min 4 after 60 min 5 after 90 min 6 after 120 min 7 after 150 min 8 after 160 min 9 after 180 min 10 after full reaction time. 

For GC analysis, the emulsion samples are diluted in THF or acetone (1.5 ml). For SEC samples, the emulsions are dissolved in THF (3-5 ml, containing 0.06% toluene as an internal SEC standard).The solution SEC is filtered over aluminum oxide (to remove the copper residues) and then through a syringe filter prior to the injection into the SEC. 

For the evaluation of the obtained GC data, the decrease of MMA concentration is plotted vs. time. From the plot the apparent rate of conversion can be determined. Also, the degree of conversion can be calculated for each data pointThe resulting plot shows until what time the process occurs in a controlled manner and where the uncontrolled free radical polymerization sets in. 

For the evaluation of the SEC measurements, a second plot can be created where the obtained values o M (left coordinate, should increase linearly with conversion) and the polydispersity indices (PDI, right coordinate, should be lower than 1.3) are plotted vs. the conversion.This plot again can be used to discuss the degree of control, and the time period where control was achieved during the chain growth process.This time is in general approx. 2 h. 

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There are no essential differences in experimental technique required for ionic copolymerizations, as compared with ionic homopolymerizations. However, the type of initiator and the solvent have a potential influence on the course of ionic copolymerizations as well as on the composition of the copolymers so that the optimum conditions for each monomer pair must be individually determined. 

The outcome of charge-transfer polymerizations has been systematized by Iwatsuki and Yamashita in their penetrating early review [130]. They arrived at a correlation of polymerization behavior with the value of the EDA complex equilibrium constant, Keq, With weak donor and acceptor olefins, no spontaneous polymerization takes place, while the addition of a radical initiator results in a random or an alternating copolymer depending on the value of Keq. As the donor and acceptor strength of the olefins increases, spontaneous initiation rates for radical copolymerization increase and with even stronger donor and acceptor olefins, ionic homopolymerization takes place (cationic and/or anionic). 

These homodimerizations of the donor olefins are well accounted for by the ion-radical chain mechanism, and provide powerful evidence for the occurrence of single electron transfer. Were it not for these cyclodimers, ionic homopolymerizations of D or A could be interpreted as initiation by zwitterionic tetramethylenes formed from D and A. 

A zwitterionic tetramethylene initiates ionic homopolymerization, while a diradical tetramethylene initiates free radical copolymerization. As initiating species, zwitterions are likely to remain in the coiled gauche-conformation and collapse to small molecules. Diradicals, on the other hand, are easily transferred to the trans-conformation. Accordingly, diradicals are more effective initiators and more radical copolymerizations occur than ionic homopolymerizations. Addition of solvent will also influence the reaction of polar tetramethylene. A polar non-donor solvent may permit carbenium ion polymerization, while a polar donor solvent impedes it. 

The most important competing process to the bond-formation is the complete electron transfer to form ion-radicals, which occurs where no bond formation is possible, for example, for aromatic donor-acceptor pairs. For vinyl copolymerizable pairs, the bond will form between the components to give a diradical tetramethylene. For the ionic homopolymerization system, on the other hand, it is difficult to distinguish the ion-radicals from zwitterionic tetramethylenes by the kinetic analysis. In this case, the accompanying cycloaddition reaction offers powerful evidence for the zwitterion formation, i.e., the bond-formation. 

Hall has introduced an empirical test to estimate the relative importance of diradical and zwitterionic forms in tetramethylene intermediates rrans-1,4-tetramethylene diradical intermediates may initiate alternating radical copolymerizations if they add to another alkene faster than they undergo conformational isomerization to the gauche form and give a cyclobutane product through carbon-carbon bond formation, while zwitterionic 1,4-tetramethylene intermediates may initiate ionic homopolymerizations. 

While this empirical approach has undeniable merits, its application through experimentation poses some potential limitations, since any extraneous initiation of an ionic homopolymerization could be misinterpreted as indicating the presence of a zwitterionic intermediate. This concern is underscored by reported instances of polymerization initiated by TCNE or by impurities in or by acidic... 

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