Polydispersity effects, polymer chains

In situ growth via covalent binding of a hybridizing component to a nanocarbon can be achieved in the case of polymers, dendrons and various other macromolecules which are synthesized in a stepwise manner. The in situ synthesis of such macromolecules potentially increases binding site density while steric effects of the nanocarbon can lead to increased variation in average polymer chain length (polydispersity) [101 103]. 

Partition coefficients of NPE surfactants were determined for hexadecane-water mixtures (Table 1). Similar results were obtained by Crook et al. (2) for octyl analogs in an isooctane-water system. However, Log K is not a linear function of the mol% ethylene oxide in the surfactant as predicted by Eq. 2. Nonlinearity in the octyl analogs was due to polydispersity in the polymer chain length (2), and similar effects presumably operate here. 

It should be mentioned that donor substitution of the phenylene backbone of the salphen ligand was shown to have a decreasing effect on activity [103], which explains the overall lower productivity compared with halogen-substituted chromium salphens. However, experiments clearly proved an increased activity upon dimerization. Whereas the monomeric complex m = 4) converts about 30% of p-BL in 24 h, producing a molecular weight of 25,000 g/mol, the corresponding dimer yields up to 99% conversion with > 100,000 g/mol. Moreover, the smaller polydispersity (PD < 2) shows the better polymerization control, which is attributed to the decreased rate of polymer chain termination. This behavior is caused by the stabilization of the coordinated chain end by the neighboring metal center, as recently reported for dual-site copolymerizations of CO2 with epoxides [104-106]. The polymeric products feature an atactic microstructure since the... 

A ternary system consisting of two polymer species of the same kind having different molecular weights and a solvent is the simplest case of polydisperse polymer solutions. Therefore, it is a prototype for investigating polydispersity effects on polymer solution properties. In 1978, Abe and Flory [74] studied theoretically the phase behavior in ternary solutions of rodlike polymers using the Flory lattice theory [3], Subsequently, ternary phase diagrams have been measured for several stiff-chain polymer solution systems, and work [6,17] has been done to improve the Abe-Flory theory. 

The discovery that Ti complex (4) was an effective catalyst for living ROMP chemistry resulted in the synthesis of several new types of polymers that were inaccessible with conventional catalysts. Narrow polydispersity polymer and di- and triblock copolymers were synthesized soon after living ROMP was discovered. Because polymer chains that are formed in a living ROMP reaction are terminated with metal carbene groups, fimctionalization of the chain ends is possible. Reaction of the carbene with an aldehyde occurs in a Wittiglike fashion, making a metal oxo complex that is inert to finther metathesis chemistry and terminates the chain with the alkylidene group of the aldehyde. 

Let us assume that the experiment is carried out with asymptotically swollen polymers then polydispersion effects and a lack of resolution remain. To be sure, the samples are prepared with extreme care. The polystyrene chains are obtained by emulsion polymerization, which gives the highest possible molecular masses. Moreover, the samples are fractionated by precipitation and this fractionation is repeated several times. The final result is very homogeneous (see the characteristic values Mz/Mw, Mw/Mn in Table 15.1, teams A and C). Nevertheless, there are fluctuations which increase the uncertainty on the unknown quantity v, because of the corrective term e(v) [see (15.3.38) and (15.3.36)]. 

There is practically no difference in the polydispersity between PMMA samples prepared in the presence of the diamagnetic (18-electron) and paramagnetic (17-electron) closo complexes 4 and 5, respectively. On the contrary, steric factors have a quite noticeable effect on the chain propagation step and the macromolecular characteristics of the samples. Thus, if the phosphine groups at the metal center are linked via methylene bridges (complexes 4 and 5), favorable steric conditions for controlling the polymer chain growth probably occur that lead to the formation of polymers with MWDs much narrow than in the case of complex 2. 

As mentioned above, most polymers are characterized by a distribution of molar masses of the individual polymer chains that is, almost every polymer sample is a mixture of polymers with different molar masses, an effect which is referred to as polydispersity. In the past, significant attempts have been made to produce polymers with a narrow molar mass distribution, and to prepare polymers with precisely identical molar masses. This is a consequence of the inherent desire of the synthetic chemist to produce a compound that is as well defined as possible - in just the way that Nature teaches us. Yet, only natural polymers such as DNA are really 100% monodisperse. In the following case study, it should be noted that even the absolute counterpoint to these longlasting attempts can open the way to a successful polymer in a highly competitive market. The subject here is probably the most competitive landscape in polymer chemistry over all, the polyolefins. 

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