Macroradical, defined

Mechanical synthesis by cold mastication of rubber and monomers depends on the reaction condition (monomer concentration, temperature, solvent concentration, atmosphere, presence of transfer agents, or catalyst) and on the physical and chemical properties of the rubbers, the monomers and the product interpolymers. A critical factor is the shear stress developed in the system rather than instrumentally-defined shear rates. The degree of reaction of polymer and consequently also the concentration of free macroradicals depends on stress. As a consequence, the influence of the above parameters may be connected to their influence on the viscosity of the reaction medium since an increase in viscosity causes an increase in stress at constant shear rate. 

Generally speaking, the values vhp and vhh depend on the conformation of the macroradical as a whole. In the first approximation, however, they can be considered as constant, vhp and vhh- hi this case, the polymerization process is described on the basis of the first-order reaction kinetic equations. In particular, one can define the following parameter characterizing the composition change along the chain [86]... 

The counter radical method has been studied with various monomers more or less successfully. However, the synthesis of only few block or grafted copolymers is effectively described. This is a strong indication that a true control of the polymerization is still not achieved with all monomers although progress is constant. Nevertheless, it is clear that the possibility of reversibly controlling the termination step offers a tool of choice for the synthesis of well-defined and pure block copolymers and many studies are still necessary to understand properly the precise mechanism of macroradical end capping in order to control the reversible character and possible secondary reactions. 

New approaches based on the introduction of reactive species into reaction mixtures that tend to cap the growing chains reversibly allow, in many cases, production of well-defined polymers and copolymers with narrow polydispersi-ties. Up to few years ago, such a possibility was unobtainable by a classical free radical process. The proposed principle of control of macroradical reactivity is very interesting conceptually, and represents a very powerful tool to prepare block copolymers with well-controlled structures. However, it is clear that the true living character as demonstrated by some anionic polymerizations is still not obtained and much more work needs to be done to understand and control this new process better. 

According to Cardenas and O Driscoll [39], there exist two populations of macroradicals, of a polymerization degree greater and smaller than a certain critical size Pc. Pc is defined by a bend in the plot of log tj (system viscosity) vs. log DP (degree of polymerization of the dissolved polymer). With increasing concentration and chain length of the polymer, the importance of poly-... 

The kinetics, as defined by the average number of macroradicals per particle (n) must not be significantly larger than one. This implies that if n is low (Smith-Ewart case I or II kinetics) the system is considered an emulsion . Similarly, if n is very large, on the order of 102 6, the polymerization is kinetically a suspension. The categorization of Smith-Ewart case III kinetics will be discussed in the following section of this paper. 

Macromolecular dispersion in HDPE with patch—like transfer is defined by polymer—metal and polymer—polymer adhesive interactions. The major contribution to macromolecular dispersion is from the alternating areas of polymer—polymer and metal-polymer contacts. Macroradicals generated within polymer—polymer contact may recombine on the metallic surface to form chemisorption and coordination complexes with an oxide film. Under the dynamic contact this process may increase the effect of mechanical actions on the macromolecular dispersion of polyolefine. 

These equations contain two parameters, q and s.To define their meanings, Schwan and Price [19] considered the reaction of addition of a monomer molecule to the growing macroradical ... 

A last, yet much less clearly defined technique of block copolymer preparation, will also be reviewed. Specifically, block copolymers may be prepared by addition of a second vinyl monomer to occluded "living" macroradicals. Thus by use of the proper technique block copolymers have been reported prepared by free radical polymerization. 

In order to find the partition function and statistical properties of the growing macroradical, Berezkin et aJ introduced a variable a (r) that was defined as follows a (r) = 1 if the nth unit located at the point r = x,y,z is adsorbed, and a (r) = 0 if the unit is not adsorbed. If the macroradical of length n has in some conformation k adsorption contacts with the surface, and its active center is located at the point r, the possible number of such conformations is denoted as f (fe,r). The following recurrence equation determines the value of P (fe,r) ... 

Attempts to quantitatively determine the extent of ionic dissociation of all relevant species including macroradicals and polymer molecules and to correlate such speciation with the variations observed for kp is difficult, if not impossible, in view of the complex acid-base properties and polyelectrolyte behavior as well as the coupled electrochemical equilibria. Studies into polyelectrolyte behavior in aqueous solution carried out so far, have been performed at conditions precisely defined with respect to solvent composition, ionic strength, concentration regime, and molecular weight. These conditions differ from the ones met in the actual free-radical polymerization experiments presented in Figure 3 and in Reference Despite this complexity, it has been reaUzed that with... 

The rate of polymerization R Rp is thus directly proportional to the monomer concentration, under ideal kinetics conditions. Larger initiator concentrations lead to higher polymerization rates and to lower kinetic chain length, v, defined as the number of monomer molecules that are added to an initiator radical before the resulting macroradical is terminated. The kinetic chain length is given by the ratio of the rate of propagation, Rp, to the sum of the rates of all terminations the type of termination does not matter (Eq. 14). 

The Full Chain Length Distribution. So far, only the average degree of polymerization has been considered. To calculate the distribution function itself for a steady-state polymerization it is convenient to choose a statistical approach based on kinetic parameters. A probability factor a of propagation is defined as the probability that a radical will propagate rather than terminate. The factor a is the ratio of the rate of propagation over the sum of the rates of all possible reactions the macroradical can undergo. 

The first method has serious limitations because well-defined end groups can be observed only if not more than one type of primary radical is formed that does not cause side reactions such as transfer. Moreover, a propagating radical will readily react with another radical, primary or macroradical, either through disproportionation or through coupling reactions (termination) (Fig. 2). The former will produce monofunctional telechehcs with both a saturated and an imsaturated chain end, while only the latter will yield bifunctional telechelics. 

Aspects of the sono-chemical degradation mechanism After the establish of the ultrasound waves physical and chemical components, the mechanisms of sono-chemical destruction have dominantly been interpreted on this basis. Thus, in the cavitation bubble collapse moment, N.Sata and H. Okuyama defined four types of possible reactions, namely 1) collision of the polymer - polymer molecules, occurring by a bimolecular reaction 2) collision of the polymer-solvent molecules, which proceeds by a pseudo-monomolecular reaction 3) intramolecular collision that occurs by a monomolecular reaction and 4) tearing resulting from the entanglement points that gives a monomolecular reaction. The authors accepted that the active centres of this type of reaction are free radicals and that about 30% from the efficiency of this process is assured by the subsequent reactions of the formed macroradicals with the polymer chains [1130, 1139]. 

It has also been demonstrated [52] that interaction of NO with radiation-produced macroradicals XFV at 150-200 Cleads to their decay without the production of ARs. The absence of ARs in irradiated PTFE was explained by the hindrance of spatial migration of the free valence within the rigid matrix of the fluorinated polymer in which the nitroso compounds formed by recombination of XIV with NO cannot serve as spin traps. It has been noted [52] that PTFE samples irradiated in air and exposed to NO at room temperature exhibit an ESR spectrum tentatively assigned to aminoxyl macroradicals, but the conditions under which these radicals are formed have not been clearly defined. Furthermore, to reliably identify the radicals by their solid-phase spectra, one should assess isotropic values with the appropriate parameters of low-molecular perfluoroalkylaminoxyl radicals studied thoroughly in the liquid phase [53, 54]. 

Mechanochemical degradation creates free macroradicals in pairs, practically without any side reactions, and most potential applications of this technique are centered around the formation and subsequent reactions of these reactive species. Elongational flow-induced degradation breaks polymer chains exactly at their center [42, 46]. This remarkable propensity is being explored in the author s laboratory as a simple means of obtaining well-defined block copolymers. Polymerization and... 

A long life time of the triplet state tends to lead to a reduced overall quantum yield, because chances are higher that alternative deactivation routes may successfully compete with radical formation. 2,2-Dimethoxy-2-phenylacetophenone (DMPA) is an example for an initiator with a rather short-lived first triplet state (t<0.1 ns)," causing a high quantum yield The quantum yield rm for the formation of macroradicals from the initiator fragment radicals is also known as the initiator efficiency, /, which is defined analogously to the efficiency of thermal initiators. 

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