Salt effects on vinyl polymerization - A boundary study of organic and inorganic chemistry
Vinyl polymerization is often affected by the addition of metal salts. A common example of metal salt effects is already seen in redox initiation systems. In this paper the salt effects on the propagation process of vinylpyridine and Nvinylimidazole,andthedirectredoxinitiation of vinylpyridine, N-vinylimidazole and N-vinylcarbazole are discussed. INTRODUCTION INTERMEDIAI13 fields of chemistry which are not included in the traditional territory of both organic and inorganic chemistry are rapidly coming to the fore. When one deals with organic reactions, it is immediately realized that numerous reactions are influenced by the presence of metal salts. The specific catalytic effects of metal salts should be attributed to interactions between organic compounds and metal salts. What kind of electronic as well as steric effects are brought about on the ligand when organic molecules are coordinated? The answer to this question is essential for an understanding of the catalytic action of metal salts in organic reactions. The present trend of coordination chemistry is mostly directed to understanding the reactivity of the central metal ion, stereochemistry, nature of coordination bond and so on. In general, no emphasis is placed on the reactivity of coordinated organic ligands. As a branch of organic reactions, vinyl polymerization is often affected by the addition of metal salts. For example, redox inert metal salts such as those of zinc1 and lithium2 are known to affect the rate of radical propagation in certain cases. Complex formation of metal salts with monomer and/or growing radical would be the cause of the effects due to salts. Further insight into the salt effect as to why complex formation is so influential remains unexplained. The problem should be dealt with from the viewpoint of reactivities of coordinated organic ligands. A more common example of metal salt effects is seen in redox initiated systems. The redox properties of transition metal salts are favourably utilized as promoters of redox initiator systems. The simplest redox pair of the peroxide—ferrous ion system is 49 SHIGEO TAZUKE AND SEIZO OKAMURA expressed as follows: ROOR + Fe(n) RO + 0R + Fe(m) (1) RO + Fe(n) -+ RO+ Fe(iii) (2) R0 + monomer -÷ROM (3) This simple reaction scheme involves many useful suggestions. First, reaction 1 seems to be primarily an electron transfer process. The counterpart of the metal salt is not necessarily the peroxide or so-called initiator since removal or donation of an electron would produce a cation radical or an anion radical on an organic molecule, respectively, and such ion radicals could induce polymerization either by radical or ionic processes. Secondly, the movement of electrons should have a close connection with photochemistry. In particular, irradiation at the charge transfer band brings about electronic transmission corresponding to electron transfer between metal ion and ligand. Many redox processes both in organic and inorganic reactions are known to be induced under photoirradiation. Thirdly, redox reaction between radical and metal ion (reaction 2) is responsible for the retarding effects of metal salts in radical polymerization. Kinetic mechanistic studies of reactions between growing polymer radical and metal salt have been made. Both electron transfer3 and ligand transfer4 mechanisms were suggested depending upon the nature of the growing polymer radical and the metal complex. In this report, the salt effects on the propagation process of vinylpyridine and vinylimidazole, and the direct redox initiation of vinylpyridine, Nvinylimidazole and N-vinylcarbazole are discussed. PROPAGATION Effects of zinc salts on homopolymerization of vinylpyridine The basic nitrogen in vinylpyridine has strong coordinating properties towards various metal salts. For example, complexes of4-vinylpyridine(4-VP), 2-vinylpyridine(2-VP) and 2-methyl-5-vinylpyridine(MVP) with Zn(Ir), Cd(ri) and Hg(iI) are separated as crystals and the mole ratios of amines to metal salts5 are always 2:1. In solution, however, the monomer complexes will exist as mixtures of 1:1 and 2:1 complexes depending upon the stepwise stability constants and the concentrations of amine and metal salt. The complexed vinyl compounds naturally have different reactivities and the rates of polymerization (Rn) of 4-VP, 2-VP and MVP are greatly affected by the presence of zinc acetate6 as depicted in Figure 1. The trend of the salt effect on R differs strikingly for 4-VP, 2-VP and MVP. With 4-VP, the value of R levels off when the concentration of Zn(u) is well over that of 4-VP, indicating the completion of complex formation. The approximately linear correlation between R and degree of polymerization as measured at different zinc acetate concentrations implies that the main effect of complex formation is to enhance the rate of propagation. The determination of absolute rate constants in somewhat different polymerization systems indicates that k is increased exclusively by the 50 Figure 2. Plots of polymer yield versus molecular weight of poiy constant; [Zn(CH3COO)2]: variable. 51 4-VP. [4-VP] and [AIBN]: SALT EFFECTS ON VINYL POLYMERIZATION