Molecular weight chain end

Anionic and cationic polymerizations were some of the original techniques used for ring closure due to control over the molecular weight, chain-end functionality, and narrow molecular weight distribution (MWD). However, ionic polymerization requires strict experimental conditions (usually under anhydrous conditions). The... 

A series of structurally characterized di-yttrium(III) complexes bearing alkoxy-derivatized triazacyclononane ligands have been examined as initiators for lactone ROP.886 Both (296) and (297) are active for the polymerization of rac-LA at RT, but little control is afforded over molecular weights. Chain length distributions are broad, (Mw/Mn= 1.5-2.2) and attempts to identify the initiating group via end-group analysis have not been successful. 

It is however possible to find conditions, called unperturbed or theta conditions (because for each polymer-solvent pair they correspond to a well-defined temperature called d temperature) in which a tends to 1 and the mean-square distance reduces to Q. In 6 conditions well-separated chain segments experience neither attraction nor repulsion. In other words, there are no long-range interactions and the conformational statistics of the macromolecule may be derived from the energy of interaction between neighboring monomer units. For a high molecular weight chain in unperturbed conditions there is a simple relationship between the mean-square end-to-end distance < > and the mean-... 

When one looks at a polymer chain, one will soon realize that there are a number of structural factors that should or may be controlled in syntheses (Fig. 1). These factors may include (1) molecular weight (chain length) (2) molecular weight distribution (MWD) (3) end group (4) pendant group (5) sequence (the arrangement of constitutional repeat units... 

The effect of moldcular weight can be thought about in the same way—the end groups often have a different size and/or shape to the other chain segments and are usually excluded from the lattice. Low molecular weight chains will then tend to form crystals that are thinner than their high... 

As has been shown elsewhere in this book (Chapter 1), a flexible high molecular weight chain can be represented by a large number N of freely jointed segments of length b. According to Eqs. (1.13) and (1.14), the free energy associated with a freely jointed chain whose end-to-end distance is r can be written as (4)... 

Depolymerization to cyclic oligomers could be occurring concurrent with chain scission or may be initiated by the scission process itself at the ends of the lower molecular weight chains. The formation of depolymerization products (PFAP(II) cyclic oligomers) would not be detected as a change in the solution viscosity which is more dependent on high molecular weight PFAP (II). 

At 200°C the initial volatilization rate does decrease after 50 hr, but not to the extent observed at lower temperatures. The total weight loss after 764 hr is 40%. This weight loss is attributed to the formation of cyclic oligomers produced by depolymerization of PFAP(II). After 50 hr the chain scission degradation process has lowered the molecular weight to ca. 450 repeat units. Thereafter, the depolymerization mechanism is viewed as an unzipping reaction initiated at the ends of the low molecular weight chains. 

A series of low intensity peaks at 15-35,66,72 and 131 ppm in the spectra of polyAOTcp and copoly(AOTcp-styraie) are thought to result from the end groups of relatively low molecular weight chains. For the relatively higher molecular weight polystyrene (see Table 2) tb j a] are not observed. However, it is difficult to ascertain the chemical nature of such low concentrations of end groups purdy by CNMR spectroscopy. A similar CNMR... 

One key feature of the bis(NHC) organometallic polymers is their reversibility, or dynamicity. The dynamic nature of the copolymerization prompted us to investigate the use of chain-transfer agents to modulate both polymer molecular weight and end-group structure (Scheme 1.15). Copolymerization with Pd(OAc)2 in... 

The hydrogen atom may come from any species in the mixture. Chain-transfer agents, often thiols, are often used to control molecular weight and end-group structure in polymerization chemistry. 

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