Polydisperse character

Humic substances are typical products of the humification process. They are the basic component of the humus, occurring in the humus material to the extent of 80 to 90%. They consist of a group of acid, yellow to dark brown polymeric substances of a heterogeneous aromatic and polydisperse character. Their acid nature allows them to react with mineral substances to form stable organomineral complexes sometimes referred to as humines. They differ from the remaining organic material in the soil by various typical features as follows ... 

An extension of all these equations given above to multi-eomponent mixtures is possible. Reviews of eontinuous thermodynamies whieh take into aceount the polydisperse character of polymers by distribution funetions ean be found elsewhere. 

After activation with MAO (molar ratios [Al] [Zr] = 1000) the polymerization of ethylene has been successfully carried out using the zirconocene functionalized dendrimer at 40 bar ethylene pressure and 70 °C. We obtained high activity and productivity values for the ethylene polymerization and polymers with very high molecular masses in the range of 2 x 10 g/mol. The polydispersity of the polymer is quite low (3.0) indicating the single site character of the catalytically active species. Optimization of this system and study of the mechanism are stiU under investigation. Nevertheless, these preliminary results reveal the suitability of polyphenylene dendrimers as supports for zirconocene catalysts. 

If ATRP is conducted in methanol, under the same conditions, the rate of polymerisation is slower (95% conversion required 2-5 h at 20 °C). However, for several hydrophilic monomers such as 2-hydroxyethyl methacrylate [HEMA] and glycerol monomethacrylate [GMA], methanolic ATRP is the preferred method for optimal living character since narrower polydispersities (typically 1.10 to 1.20) and better blocking efficiencies are generally obtained. 

Difficulties arise from the various possible interactions between the counter radical, the initiator and the monomer. The truly living character is only demonstrated when some requirements are fulfilled. Molecular weight must increase in a linear fashion with conversion. Polydispersity must be narrow and lower than that in classical process (theoretical value, M J = 1.1 —1.5) which supposes in... 

The Mw/Mn ratio is usually equal to 5-10 for polyethylene [49,64,66,67, 123,244-247], A much lower polydispersity is displayed by polymers obtained in polymerisation with homogeneous metallocene catalysts the Mw/Mn ratio usually does not significantly exceed a value of 2 [22,95,101,112,138,140], By polymerising propylene with soluble vanadium-based Ziegler-Natta catalysts at low temperature, a very narrow molecular weight distribution of the polypropylene has been found (the Mw/Mn ratio usually reaches values of 1.15-1.25) and a linear increase in its Mn with time has been observed, indicating a noticeable living character of the polymerisation [75,76,241],... 

The polymerization of norbornene, Eq. (19), is stopped by cooling the reaction mixture to room temperature. The active polymer 11 can be stored for long periods of time. Heating 11 to temperatures above 65 °C in the presence of monomer causes renewed chain propagation. The subsequent addition of different cyclic olefins, such as endo- and exo-dicyclopentadiene, benzonorbomadiene and 6-methylbenzonorbornadiene resulted in the formation of well-defined AB- and ABA-type block copolymers, Eq. (21) [38]. Triblock copolymers 13 with narrow molecular weight distributions (polydispersity = 1.14) were prepared. Thus, the living character enables the preparation of new uniform block copolymers of predictable composition, microstructure and molecular weight. 

In 1998, Chiefari et al. [10] attempted to combine the convenience of radical polymerization with the many benefits of living polymerization, e.g. control of the molecular weight and polydispersity and the possibility of synthesizing block copolymers of complex architecture. They used free-radical polymerization reagents of formula (I) to produce a sequence of reversible addition-fragmentation in which the transfer of the S=C (Z) S moiety between active and dormant chains serves to maintain the living character of the polymerization ... 

Living radical polymerizations in miniemulsions have also been conducted by de Brouwer et al. using reversible addition-fragmentation chain transfer (RAFT) and nonionic surfactants [98]. The polydispersity index was usually below 1.2. The living character is further exemplified by its transformation into block copolymers. 

An other interesting example of copolymer is given by Georges et al. [52,59] who first demonstrated the living character of the polymerization of styrene initiated by dibenzoyl peroxide in the presence of Tempo or Proxyl (2,2,5,5-tetramethyl-l-pyrrolydinyloxy). Polystyrene with a narrow polydispersity (Mw/Mn = 1.2) is obtained and block copolymers with butadiene, isoprene, acrylate and methacrylate sequences are prepared ... 

Another remarkable character of this class of Ni complexes is their tendency to promote living polymerization of a-oleftns at low temperatmes and with low concentrations of the monomer. Thus, the low-temperatme polymerization of propylene leads to a material whose number average molecular weight (Mn) increases almost linearly as a function of time and propylene consumption, reaching values ofM = 160 000 Daltons and polydispersities of ca. 1.13. This character allows these Ni catalysts to produce A-B-A type block copolymers composed of semicrystalline and amorphous segments, which is used to prepare thermoplastic elastomeric polymers. The Ni catalysts can also polymerize internal cyclic... 

The general character of the experimental dependence is similar to that of the theoretical dependence the value of G decreases with increasing chain length. However, in terms of absolute magnitude, the experimental values of G greatly exceed the theoretical ones. This is probably mainly due to the molecular we t polydispersity of fractions that was not taken into account in the theory. A high molecular... 

Bushell, G., Amal, R., and Raper, J., The effect of polydispersity in primary particle size on measurement of the fractal dimension of aggregates. Part. Syst. Charact., 15, 3, 1998. 

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