Comonomer complexes

The derivation of the terminal (or hrst-order Markov) copolymer composition equation (Eq. 6-12 or 6-15) rests on two important assumptions—one of a kinetic nature and the other of a thermodynamic nature. The Erst is that the reactivity of the propagating species is independent of the identity of the monomer unit, which precedes the terminal unit. The second is the irreversibility of the various propagation reactions. Deviations from the quantitative behavior predicted by the copolymer composition equation under certain reaction conditions have been ascribed to the failure of one or the other of these two assumptions or the presence of a comonomer complex which undergoes propagation. 

Another model used to describe deviations from the terminal model involves the participation of a comonomer complex (Sec. 6-3b-3) [Cais et al., 1979 Coote and Davis, 2002 Coote et al., 1998 Seiner and Litt, 1971]. The comonomer complex competes with each of the individual monomers in propagation. The monomer complex participation model involves eight... 

The complex participation model, like the depropagation model, predicts a variation of the copolymer composition with temperature and monomer concentration. The effect of temperature comes from the change in K, resulting in a decrease in the concentration of the comonomer complex with increasing temperature. Increasing monomer concentration at a constant/i increases the comonomer complex concentration. 

Although initiation by monomer ion-radicals may also be operative in this case, alternatively, homopolymerization of excited comonomer complexes may occur to a limited extent, due to the low concentration of such complexes, followed by ion pair coupling or dissociation to initiate radical copolymerization. 

Complexation of a relatively poor electron accepting monomer with a Lewis acid or organoaluminum compound converts the acceptor monomer to a stronger electron acceptor, thus promoting the formation of ground state comonomer complexes. The latter undergo photoexcitation and homopolymerization. 

Spontaneous polymerization occurs when the equilibrium concentration of the complex is high enough. Lower temperatures favor complex formation, while polymerization may require thermal or catalytic activation. Since the polymerization with and without the radical catalyst follows the same course, the comonomer complex is considered to be the polymerizable species in either case. 

One proposal (I) holds that the propagating chain end in the homopolymerization of a comonomer complex is a complex, and that... 

Cellulose-water may act as a matrix and promote the development of arrays of comonomer charge transfer complexes (19). The cellulose acts not only as a substrate for such alignment but also as a complexing agent. The matrix of complexes may be represented as shown in I (styrene-methyl methacrylate) and II (butadiene-acrylonitrile). The radical-, thermal-, and radiation-induced graft polymerizations involve homopolymerization of comonomer complexes rather than copolymerization of uncomplexed monomers. 

The comonomer complexes may be anchored on the cellulose, analogous to structures I and II, or through the interaction of zinc chloride and the cellulosic hydroxyl groups aqueous solutions of the metal halide are known to break the hydrogen bonds in cellulose and reduce crystallinity. Although radicals generated on the cellulose as the result of reaction with the catalyst may initiate polymerization of the comonomer... 

Other models sometimes invoked include two-component models based on combinations of the models discussed above [5] and the complex participation model [6]. Examples of the use of these are given in section 2.3. The complex participation model is a modification of the first-order Markov model to take account of the formation of A-B comonomer complexes which compete with monomer during polymerisation. Thus, four propagation steps in addition to those shown for the first-order Markov model are required to describe addition of A-B and B-A complexes (i.e. it can add either way round) to the two types of growing chain and (either A or B). The monomer and comonomer complex addition probabilities are then related to the equilibrium constant for complex formation. As might be expected, this model has been applied particularly to systems that show a marked tendency towards alternation of their comonomers [7]. A probabilistic description of the complex participation model has been given by Cais et al [6]. 

The polymerization proceeds spontaneously at room temperature or elevated temperatures. The proposed matrix of the comonomer complexes is described in Reference 96. Examples of alternating acrylonitrile copolymerizations involve vinyl cyclohexanes with AlEC2H5tCl2 (97), vinyl acetate with ZnCl2 (98) or Ziegler-Natta catalyst (99), and styrene. 

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