Peculiarities of Chain Growth

The cationic polymerization of lactones is not yet sufficiently well understood. The structure of the active centers is not known with certainty and the direction of the opening of the lactone ring (O-alkyl (a) vs. O-acyl (b)) has been established only recently for a few representative monomers 16,17)  

In the cationic polymerization of p-propiolactone ( 3PL), initiated with dimethyl-iodonium hexafluoroantimonate (a source of CH cations), the initiator induces rapid and quantitative formation of the active centers (by transfer of the methyl cation). The structure of the end-groups has been determined, the terminal one by cation trapping with triphenyl phosphine. These end-groups indicate O-alkyl ringopening 16) (route a in Equ. 9.1.)  

O-acyl cleavage would lead to the formation of the following structure, not detected 

In the polymerization of e-caprolactone (eCL) initiated with CH coming from (CH3)2I SbFf, NMR spectroscopy of the end-groups (3.605 by 4H-NMR and 23.25 by 31P-NMR) also indicates O-alkyl opening of the seven membered lactone17). 

The following mechanism is proposed to account for the findings of the polymerization of PPL and eCL initiated with CH  

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The above listed monomers polymerize exclusively by cationic ring-opening polymerization. Polymerization of cyclic imino ethers has been reviewed by Saegusa1,4,). In this section we follow mostly the conclusions of the Kyoto group summarizing the applied synthetic methods. Subsequently we describe initiating systems, peculiarities of chain growth, side reactions, copolymerization systems and possible applications, in this order. 

From these observations some aspects of chain growth peculiar to zwitterionic propagating species can be deduced. It seems that the proportion of cyclics to long zwitterionic chains is strongly influenced by the dielectric properties of the medium. The most logical explanation is a shift in the inter-intramolecular ion pair equilibrium constant. 

A historical perspective on the development of hydrophobe-modified, water-soluble polymers is presented. The various synthetic procedures used to obtain different associative thickeners are discussed in terms of the complexities in ionogenic monomer polymerizations. This discussion serves two purposes. The first is to present the peculiarities in anionic and cationic polymer synthesis in contiguity with previous work on water-soluble polymers that related only to their use. The second purpose is to draw parallels between the discontinuities in the classical chain-growth polymerization of nonionic with ionogenic monomers and those that should be expected to occur with hydrophobe-modified monomers, but for which there are insufficient data in associative thickener technology to define properly. 

When vernonia oil reacts with difunctional sebacic acid, a three dimensional esterification reaction occurs. Chain growth is not restricted to one direction and the polymer produced forms a network. This kind of system has the peculiarity that at some... 

The role of reactive centers is performed here by free radicals or ions whose reaction with double bonds in monomer molecules leads to the growth of a polymer chain. The time of its formation may be either essentially less than that of monomer consumption or comparable with it. The first case takes place in the processes of free-radical polymerization whereas the second one is peculiar to the processes of living anionic polymerization. The distinction between these two cases is the most greatly pronounced under copolymerization of two and more monomers when the change in their concentrations over the course of the synthesis induces chemical inhomogeneity of the products formed not only for size but for composition as well. 

A greater degree of certainty exists with zwitterion monomers that the growing chains are also zwitterions. Hence, they seem to be the most suitable class of compounds with which to study ionic reactions of this type. The concentration of growing zwitterions is high and any peculiarities associated with their growth are likely to be readily apparent. A closer examination of the polymerization of these monomers is warranted. 

Crystallization of main-chain LCPs is considerably different from that of conventional polymers, such as polyethylene or PET. LCPs have reduced flexibility compared to the latter, which implies that large translations of their molecules are required for recrystallization. Thus the crystallization process in an LC phase may present its own peculiarities. The orientational order associated with mesophase may act as a precursor for further crystal growth, especially in monotropic LCPs where the metastability of mesophase generally leads to the formation of a more stable crystal phase. 

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