Chain copolymerization ideal

The variability and potential of the graft polymerization technique is best discussed in terms of the various parameters involved. The graft reaction is, to a large extent, controlled by the structure of the backbone prepolymer. The temperature at which grafting can take place and the number of grafted chains can be controlled via the type and concentration of the azo functions. Additionally, the molar mass of the backbone prepolymer has an influence on the number of azo groups per polymer chain and thus on the number of side chains. The comonomer for the backbone can be freely chosen unless quantitative conversions are required. In this case a comonomer should be used which copolymerizes ideally with the azo monomer. 

These monomers copolymerize ideally. Halogen substituents reduce the rate of polymerization while methyl or ethyl groups in the para position have a slight activating effect. This is in conformity with the view that monomer entry into the polymer chain is determined only by individual reaction rates (Table 24) [200]. 

Both and T2 are approximately unity ia an ideal copolymerization. In this case, k 2 22 21 growing chains show Htde preference for... 

Indeed, cumyl carbocations are known to be effective initiators of IB polymerization, while the p-substituted benzyl cation is expected to react effectively with IB (p-methylstyrene and IB form a nearly ideal copolymerization system ). Severe disparity between the reactivities of the vinyl and cumyl ether groups of the inimer would result in either linear polymers or branched polymers with much lower MW than predicted for an in/mcr-mediated living polymerization. Styrene was subsequently blocked from the tert-chloride chain ends of high-MW DIB, activated by excess TiCU (Scheme 7.2). 

The instantaneous composition of a copolymer X formed at a monomer mixture composition x coincides, provided the ideal model is applicable, with stationary vector ji of matrix Q with the elements (8). The mathematical apparatus of the theory of Markov chains permits immediately one to wright out of the expression for the probability of any sequence P Uk in macromolecules formed at given x. This provides an exhaustive solution to the problem of sequence distribution for copolymers synthesized at initial conversions p l when the monomer mixture composition x has had no time to deviate noticeably from its initial value x°. As for the high-conversion copolymerization products they evidently represent a mixture of Markovian copolymers prepared at different times, i.e. under different concentrations of monomers in the reaction system. Consequently, in order to calculate the probability of a certain sequence Uk, it is necessary to average its instantaneous value P Uk over all conversions p preceding the conversion p up to which the synthesis was conducted. 

Currently this model is one of the most commonly used in the theory of free-radical copolymerization. The formation of a donor-acceptor complex Ma... iVlbetween monomers Ma and in some systems is responsible for a number of peculiarities absent in the case of the ideal model. Such peculiarities are due to the fact that besides the single monomer addition to a propagating radical, a possibility also exists of monomer addition in pairs as a complex. Here the role of kinetically independent elements is played by ultimate units Ma of growing chains as well as by free (M ) and complex-bound (M ) monomers, whose constants of the rate of addition to the macroradical with a-th ultimate unit will be... 

This assumption is implicitly present not only in the traditional theory of the free-radical copolymerization [41,43,44], but in its subsequent extensions based on more complicated models than the ideal one. The best known are two types of such models. To the first of them the models belong wherein the reactivity of the active center of a macroradical is controlled not only by the type of its ultimate unit but also by the types of penultimate [45] and even penpenultimate [46] monomeric units. The kinetic models of the second type describe systems in which the formation of complexes occurs between the components of a reaction system that results in the alteration of their reactivity [47-50]. Essentially, all the refinements of the theory of radical copolymerization connected with the models mentioned above are used to reduce exclusively to a more sophisticated account of the kinetics and mechanism of a macroradical propagation, leaving out of consideration accompanying physical factors. The most important among them is the phenomenon of preferential sorption of monomers to the active center of a growing polymer chain. A quantitative theory taking into consideration this physical factor was advanced in paper [51]. 

These polymer networks have commercial applications. A number of experimental studies suggest that the network formation by this method proceeds in a highly non-ideal fashion. During copolymerization, a high fraction of pendant vinyls of a primary chain are consumed by intramolecular reactions, causing practically no increase in the molar mass of the system (see Chapter 7) ... 

As previously noticed, butyl rubber (HR), poly(methylpropene-co-2-methyl-1,3-butadiene), is a random copolymer of isobutene and 0.7-2.2 mol% of isoprene. The industrial slurry process used all over the world consists in a low-temperature copolymerization initiated by A1C13 in meth-ylchloride. In contrast to 1,3-butadiene, isoprene copolymerizes readily with the more reactive isobutene. Reactivity ratios of the pair isobutene-isoprene, ri = 2.5 0.5 and r2 = 0.4 0.1, measured at the conditions of industrial process [10], show that the copolymerization behaves ideally (ri-r2 = 1), and, at the used low concentration of isoprene, isolated units of this latter comonomer are randomly distributed along the chain with 90% M-p-aiw-enchainment [52,53] ... 

Sections of polymer chains must be capable of packing together in ordered periodic arrays for crystallization to occur. This requires that the macromolecules be fairly regular in structure. Random copolymerization will prevent crystallization. Thus, polyethylene would be an ideal elastomer except for the fact that its very regular and symmetrical geometry permits the chains to pack together closely and crystallize very quickly. To inhibit crystallization and confer elastomeric properties on this polymer, ethylene is commonly copolymerized with substantial proportions of another olefin or with vinyl acetate. 

The relative amounts of the two monomer units along the copolymer chain are thus determined by the relative concentrations of the monomer units in the feed and the relative reactivities of the two monomers. Thus a very important practical consequence of ideal copolymerizations is that it becomes progressively more difficult to produce copolymers containing appreciable amounts of both monomers as the difference in reactivities of the two monomers increases. It should be noted that the term ideal copofymerization does not in any sense connote a desirable type of copolymerization. 

The condition when c1xr2 = l has been called ideal copolymerization. In this case, the relative reactivity erf the two monomers is the same for either an ethylene terminated or propylene terminated chain... 

MS insertion. The values of ij x T2 are less than but near to unity in both temperatures, which suggests the nearly ideal random copolymerization reactions and very small probability to find two adjacent p-MS units in the polymer chain. In other words, the p-MS units shall be homogeneously distributed in the polymer chain. In catalyst II cases, the copolymerization reactions exhibit even higher rj (rj > 60), very strongly favorable for ethylene incorporation, and almost no possibility of p-MS consecutive insertion (r2 0). The less opened active site in Et(Ind)2ZrCl2 catalyst may sterically prohibit p-MS consecutive insertion. 

To give an example of reduced activity of PVGs, papers [45, 46] can be mentioned where hydrodynamic properties of the products of bulk copolymerization of styrene with 0.4% DVB (without removing the inhibitor ieri-butylcatechol) have been examined. If the copolymerization was ideal, one DVB molecule, surrounded by 250 styrene units, would form a tetrafunctional branching point in the macromolecule and the molecular weight of a chain between two branching points should be as high as... 

---