Homo-propagation

Occasionally, unusual effects are observed. For example, addition of styrene to living poly-vinyl-naphthalene results in complexing of the first polystyryl-anion to the preceding naphthalene moiety501. Consequently, three constants describe the process the rate constant of addition of the first styrene molecule, the very low rate constant of addition of the second styrene molecule, and a large rate constant of homo-propagation ensuing from addition of the third, forth, etc., styrene molecules. 

In a separate chapter we treat copolymerizations. The importance of reversibility is stressed and it is shown that a large number of what were assumed to be the valid reactivity ratios (r2 and r2), were determined by the oversimplified two parameter scheme (four reactions two homo-propagations, and two cross-propagations). Reversibility of some steps requires a consideration of depropagation the corresponding methods are described. The chemistry of alternating, block, and graft copolymers is also presented. 

If the enthalpies of the cross-propagation steps are more negative than those of the homo-propagation steps, then copolymerization will be enhanced (the overall —AH, is higher). When the opposite is true, copolymerization will be hampered. For identical enthalpies and entropies of homo- and cross-propagations, there still remains the term ASmix, which for ideally random copolymers (ASmix 8 J mol-1 xdeg-1) reduces the equilibrium concentration of each comonomer by a factor of two. In such cases a = (3 = 1/2. 

When the processes of Eqs 1-3 occur exclusively throughout the course of polymerization, alternating copolymer —is produced. In many combinations of and Mg, the so-called spontaneous alternating copolymerizations have been realized. In some cases, however, one of homo-propagations of Eqs 4 and 5 takes place to result in copolymers of biased compositions. 

Several boundary combinations of reactivity ratios are illustrated in Figure 4. For the case in which neither active center shows any preference for any monomer species, i.e., ri = f2 = 1, copolymer composition is determined at all times solely by the concentration of the respective monomer species in the feed composition. This defines an azeotropic composition over the entire range of monomer feed compositions. For the other extreme case, the one in which each active center adds exclusively the monomer other than that which represents the terminus on the active chain, i.e., ri = T2 = 0, an alternating copolymer is formed in which half of the polymer chain consists of homopolymer of Mi and the second half of homopolymer of M2. In the case in which either active center shows preference for one, the more reactive, monomer, i.e., ri > 1 and T2 < 1, the copolymer is always richer in that monomer and the feed composition is conversely richer in the less reactive monomer. In the final case (Fig. 4), in which each active center prefers cross-propagation to homo-propagation, i.e., ri < 1 and T2 < 1, a tendency exists toward alternation. Copolymerizations with preferred alternation... 

In these equations I is the initiator and I- is the radical intermediate, M is a vinyl monomer, I—M- is an initial monomer radical, I—M M- is a propagating polymer radical, and and are polymer end groups that result from termination by disproportionation. Common vinyl monomers that can be homo-or copolymeri2ed by radical initiation include ethylene, butadiene, styrene, vinyl chloride, vinyl acetate, acrylic and methacrylic acid esters, acrylonitrile, A/-vinylirnida2ole, A/-vinyl-2-pyrrohdinone, and others (2). 

The localization of the HOMO is also important for another reason. Since it describes the distribution of a hole in a radical cation, it relates to the hindrance that a positive charge will encounter as it propagates along the chain. There is indeed experimental evidence (9) that the hole states of the polysilane chain are localized and that they move by a hopping mechanism. 

By studying mixtures of L- and D-LA of varying composition, Spassky et al. have demonstrated that (256) yields PLA containing long isotactic sequences, with a ratio of homo cross propagation of 2.8.796 Hence, an 80/20 L D mixture when polymerized to 70% conversion displayed an optical purity of 87%. Even at the relatively low optical purity of 65/35 L D, isotactic block lengths of >12 repeat units were reported. Achiral (259) converts rac-LA into highly isotactic PLA a Tm of 192 °C indicates that the chains of P(L-LA) and P(D-LA) form a stereocomplex.792 797-799... 

Co-polymerizations and homo-polymerizations of monomers such as dienes or 4-methylene dioxolan, in which two or more types of ion may propagate simultaneously, are further examples of enieidic polymerizations. These dienes, of course, also provide examples of eniedic radical and anionic polymerizations. Indeed the idea of dieidic polymerization has been suggested by several authors in relation to anionic polymerizations it arose from the aggregation in solution of the lithium alkyls [135], and similar phenomena. 

RH solvent or impurities with active hydrogen atoms Suffixes "H" and "G" represent homo and graft propagating chains, respectively. 

Once chirality is induced and amplified by some mechanism, the excess must first persist and then propagate in order to survive. A distinctive characteristic of homo-chiral protein and nucleic acid biopolymers is that they function within the enclosed environment of cells, which provide a membranous boundary structure that separates the intracellular components from the external environment. It has accordingly been postulated frequently that analogous but simpler enclosed environments must have been available and operative on the primitive Earth. 

Fig. 3 HOMO / LUMO scheme for a modified molecular shift register in which bits are encoded and propagate as free electrons instead of the system based on hole transfer described in Figure 2. Data is encoded by electrochemical reduction of the donor moiety adjacent to the cathode, resulting in the electronic configuration shown in (a). Subsequent stepwise thermal electron transfer...

In the course of these studies key features such as olefin coordination [19,20], olefin insertion (propagation) [16,21,22], /(-hydrogen elimination [16,21,22], and /3-alkyl elimination [23] could be spectroscopically and structurally proven. In particular, yttrium aluminates have been proposed to model isoelectronic cationic homo- and heterobridged Zr/Al heterobimetallic complexes as dormant species and potential polymer chain transfer candidates [24]. Such zirconium aluminate complexes seem to be elusive [25], while the first structurally characterized titanium alumi-... 

Polymerization of captodative olefins is seen to be a potentially wide area of interest in polymer chemistry. Contrary to impressions in the literature, such olefins can homo- and copolymerize to high molecular weight polymers. Results to the contrary can be explained in terms of excessive steric hindrance by large electron-donating substituents, which should lead to slow propagation rates and low ceiling temperatures. 

These concepts are important for an understanding of the roles played by metal ions and their complexes in the catalysis of oxidation reactions via homo-lytic mechanisms. Thus, metal complexes may function as catalysts by interfering with any of the various initiation, propagation, and termination steps outlined earlier. 

From a frontier orbital point of view the most important interaction in this coupling reaction is between the HOMO of the nucleophile and the SOMO of the radical126. Thus, in the propagation cycle the reactivity of the carbanions would depend on their HOMO (pATa) as well as on the SOMO of the radical anion formed, the HOMO-SOMO energy difference... 

The polymerization described so far is homo-polymerization based on single monomers. Some polymers used in pharmaceutical applications are copolymers. They have properties that each homo-polymer does not exhibit. For example, the copolymer of hydroxyethyl methacrylate and methyl methacrylate is synthesized in order to obtain a polymer exhibiting a hydrophilic/hydrophobic balance. A variety of copolymers (alternating, block, random) can be formed from two different monomers. Special processes produce alternating and block copolymers, while random copolymers are produced by free-radical copolymerization of two monomers. The polymerization steps, such as initiation, propagation, and termination, are the same as in free-radical homo-polymerization. Copolymerization kinetics are depicted as follows ... 

The resonances in scattering of electrons off diatomic molecules have a long history /16/ and the initial application of the dilated electron propagator to the 2ng shape resonance in e-N2 and 2II shape resonance in e-CO scattering is due to their appeal as representative homo and hetero-nuclear diatomic systems which are also isoelectronic and therefore lend themselves to a comparative study /18,19/. 

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