Polymerization Practice

Highly strained 3- and 4-membered rings polymerize practically irreversibly but polymerization of 5-, 6-, 7-, and higher member rings, important from both a basic and practical point of view, is highly reversible. Thus, in these systems, reversibility of propagation step introduces an additional factor which should be taken into account in kinetic and mechanistic studies and which has practical consequences discussed in more detail in Section II.B.l. 

The enthalpy of polymerization of 3- and 4-membered rings is so much higher than the entropy factor that substitution does not significantly reduce their polymerizability. Disubstituted oxiranes (e.g., isobutylene oxide) or oxetanes (3,3-dimethyloxetane) still polymerize practically irreversibly. Substitution may prohibit polymerization of 5-membered monomers, however. 

Due to the high ring strain of 3- and 4-membered rings, oxiranes and oxetanes polymerize practically irreversibly. Thermodynamic polymeri-zability of these groups of monomers is not significantly affected by substi-... 

It is well Icnown in emulsion polymerization practice that the efficiency of a surfactant depends on its hydrophilic-lipophilic... 

Ziegler-Natta Catalysts Kinetics of Ziegler-Natta Polymerizations Practical Features of Ziegler-Natta Polymerizations Comparisons of Cis-1,4-Polydienes Metallocene Catalysts... 

In the examples described above, the transition is shown from ideal (n = /2) to nonideal (n > /2) behavior. There are, however, systems for which ideal emulsion polymerization practically cannot be achieved. It is nevertheless possible to describe the kinetics of such systems quantitatively. Recently, Gerrens has obtained values of the propagation and termination rate constants at diflFerent temperatures for vinyltoluene and vinylxylene (28). The termination rate of polymer radicals of these monomers is so low that even at small rates of initiation in small particles, n is larger than /2. From measurements of the reaction rate before and after injection of additional initiator in the polymerizing system it was possible to calculate n both at the original and at the boosted initiation rate with the aid of Equation 5. Consistent results were obtained when the additional amount of initiator was varied. From these rate data, the termination rate constant was found to be 10 and 17 liters mole- sec. at 45° C. for vinyltoluene and vinylxylene, respectively. These values are to be compared with 10 for styrene (Table IV). 

Covalent type MCMs with a metal-nitrogen bond (for instance, the products of the interaction of metal halides and alkoxides with allyl- and diallylamine, alkaline metal amides, etc.), a metal-sulfur bond and others have no wide uses in polymerization practice. 

While the types of MCMs described earlier have already received comparatively wide popularity in polymerization practice, polymers based on metal-containing monomers of the chelate type have only been prepared more recently. The methods of assembly of such MCMs, i.e. the simultaneous formation of the ligand and the corresponding complex, have been substantially developed. The synthesis of MCMs from /7-aminostyrene, 2-formylpyrrole and Cu(II) or Co(III) salts is an example of such a method. The last approach is especially characteristic of the preparation of MCMs with macrocyclic chelate nodes, in particular, from porphyrins, phthalocyanines and other macrocycles with exocyclic multiple bonds. It is worth noting that traditional methods of chelation are used for preparing MCMs when scientists want to ensure strong multicenter fixation of metals into monomer molecules, and, thus, into (co)polymers. 

These are the same chelate-type MCMs, but with tetradentate coordination (Table 4-1). Moreover, MCMs with a small strained structure (vinylimine, epoxide) have already been considered above. By considering MCMs with macrocycles in a separate section, we wish to focus attention on monomers of this type because of their widespread use in polymerization practice, especially, porphyrin and phthalocyanine derivatives. As we know, such macrocycles are n-conjugated planar tetradentate ligands and capable of forming rather stable chelates with almost all metals. A basic method of MCM synthesis is based on the incorporation of metal ions into a window of macrocycles completed by an exocyclic multiple bond [45 8]. Examples are 74-76. 

Can you surest an explanation that accounts for the fact that the radical polymerization PRACTICE PROBLEM 10.15 of styrene (C6H6CH = CH2) to produce polystyrene occurs in a head-to-tail fashion,... 

Effects ofglass transition have been discussed occasionally in a qualitative manner But a very dear quantitative explanation that polymerization practically stops when the overall polymerizatitm astern is vitrified was presented for the first time by the present authOTS. Kelley and Bueche expressed the glass transition temp ture T of a polymer-monomer or plastisizer) mixture by Eq. (1), assuming the adiditivity of the free volumes of the two components ... 

Higher olefins polymerize practically with the same rate as Ce- The comparison was carried out at deep temperatures to allow the gas phase olefins to be present in solution in the same starting concentration as the liquid members. This offers the chance of copolymerizing different olefins with n > 6 without discrimination [60], As to the products, the 1-olefins form comb-like polymer molecules with a practically unbranched backbone. The broad distribution of the molecular weight (MW) around 10 shows at low polymerization temperatures a three-modal profile that collapses at higher reaction temperatures to a mono-modal distribution with a lower MW value for its centre [61]. The chains contain one double bond per molecule [61]. 

Equation 10.13 is the classical rate expression for a homogeneous, free-radical polymerization. It has been an extremely useful approximation over the years, particularly in fairly dilute solutions, but deviations from it are sometimes observed. It is now evident that many deviations are due to the fact that the termination reaction is diffusion controlled, so k, is not really a constant. In certain cases of practical interest, these deviations can be of major significance. They are discussed in Chapter XIII on polymerization practice. 

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