Copolymerization mechanisms

Fig. 11. Proposed copolymerization mechanism for bismaleimides with dicyanates.

Free-radical copolymerizations have been performed ia bulb (comonomers without solvent), solution (comonomers with solvent), suspension (comonomer droplets suspended ia water), and emulsion (comonomer emulsified ia water). On the other hand, most ionic and coordination copolymerizations have been carried out either ia bulb or solution, because water acts as a poison for many ionic and coordination catalysts. Similarly, few condensation copolymerizations iavolve emulsion or suspension processes. The foUowiag reactions exemplify the various copolymerization mechanisms. 

The SB-copolymers may be classified, according to the copolymerization mechanism and technology, into several groups ... 

In hydrocarbon solvents it is known that most of the growing chains are associated and it is necessary to enquire what effect this has on the copolymerization mechanism. The reactivity ratios measured from copolymer composition are unaffected because they refer to a common ion-pair. The equilibrium constants for association cancel and the reactivity ratios measured give a true measure of the relative propagation constants of the two monomers. No assessment can be made of the real reactivity of two types of active chain with the same monomer, however. In this case the observed rates are a function of the relative reactivities of the free ion-pairs and also of the relative extents of association. For example in hydrocarbon solvents polystyryllithium reacts with butadiene much more rapidly than does polybutadienyllithium. Until we know the two equilibrium constants for self-association we cannot find out if the increased rate is due to greater intrinsic reactivity or to a higher concentration of free polystyryllithium. In polar solvents or in hydrocarbon solvents in the presence of small amounts of ethers, these difficulties do not arise as self-association is no longer important. 

Several mechanism have been proposed for the copolymerization of epoxides with cyclic anhydrides, initiated by tertiary amines. In one of the first papers on copolymerization mechanisms Fischer 39,40) suggested the following scheme ... 

Clear indications of the induction period and of an increase in the reaction rate after copolymerization has started were found for isothermal runs by DSC measurements by Peyser and Bascom 941 even for melt copolymerization. According to the copolymerization mechanism, the induction period is interpreted as a gradual increase in the concentration of active centres45,52 and is identical with the time for reaching the maximum on the conductivity curves57). An induction period has also been established by other measurements 39,40>73.90.95), where it is often considered as an imprecision in the determination of the monomer concentration, mixing effect, temperature establishement, or it is not considered at all. 

The copolymerization mechanisms show that the propagation reactions are bimolecular in two alternating steps and that the rate-determining step is the slower propagation reaction. In our case, the slower reaction is the addition of the epoxide (Eqs. (38), (43), (47), (52), (59), (67), and (71) in the respective schemes) 99) as has also been found for the initiation by ammonium salts56). Since the tertiary amine does not directly take part in growth reactions (cf. 3.3.3), a more suitable expression for the copolymerization rate is Eq. (84) where the tertiary amine should be replaced by an active centre. 

Fig. 26 Salen-Co(III) complex 30 with unusual bidentate ligand coordination mode and proposed copolymerization mechanism...

Schoonbrood, H.A.S., Unzue, MJ., Beck, O. and Asua, J.M. (1997) Reactive surfactants in heterophase polymerization. 7. Emulsion copolymerization mechanism involving three anionic polymerizable surfactants (surfmers) with styrene-butyl acrylate acrylic acid. Macromolecules, 30, 6024-33. 

Figure 4-23. The steps in the copolymerization mechanism following the acrylate insertion...

C with two feed compositions are shown in Tables I and II. Surface tension has been measured as a function of time, and the viscosity of the solutions are shown along with surface tension. The data clearly show that as the viscosity increases with time, surface tension increases, and the higher the rate of increase of viscosity, the higher the rate of increase of surface tension. It has been shown for silicone polymers that as the viscosity increases from an increase in molecular weight, the surface tension increases (27). A step growth copolymerization mechanism, as mentioned earlier for the sulfur-DCP solutions, will have an increase of molecular weight with time, and the surface tension behavior appears to support this mechanism. 

This reasoning predicts that a reactivity ratio or an r V2 product greater than unity will decrease with increasing temperature and vice versa. The tendency for random polymerization will increase and the tendency for monomer alternation will decrease with increasing reaction temperature, so long as the same copolymerization mechanism predominates over the experimental temperature range. 

The examination of monomer sequence distributions by NMR is one of the most extensively used applications in materials science. When two (or more) dissimilar monomers A and B are copolymerized, a polymer is obtained with varying placements of A and B units along the backbone as shown in Fig. 10. It is important to know the relative distribution of monomer sequences, as these have an influence on the polymer s properties, and information about the distributions is valuable for studies of copolymerization mechanism. Initially, NMR was the only technique available to determine monomer sequences. 

However, these observations are not proof of the role of a donor-acceptor complex in the copolymerization mechanism. Even with the availability of sequence information it is often not possible to discriminate between the complex model, the penultimate model (Seetion 7.3.1.2) and other, higher order, models. A further problem in analyzing the kinetics of these copolymerizations is that many donor-acceptor systems also give spontaneous initiation (Section 3.3.6.3). 

Let us start to explore some implications of Stockmayer s distribution it is another fundamental equation in polymer science and can be derived from the analytical solution of the copolymerization mechanism described in Table 2.11. Its derivation is long and tedious and not really required here it is enough to realize that it reflects the MWD and CCD of polymer made according to the copolymerization mechanism shown in Table 2.11 at a given time instant. The same considerations made for Flory s distribution apply to Stockmayer s distribution they will not be repeated here. 

Scheme 3.11 Basic free-radical copolymerization mechanism, assuming terminal radical kinetics.

A few spontaneous copolymerizations between exceptionally reactive donor acceptor olefinic pairs have been observed. Miller and Gilbert [37] observed that vinylidene cyanide spontaneously copolymerized with vinyl ethers when the two monomers were mixed at room temperature. Yang and Gaoni [38] observed that 2,4,6-trinitrostyrene as the acceptor monomer spontaneously copolymerized with 4-vinylpyridine as the donor monomer when the two were mixed at room temperature. Butler and Sharpe [39] reported that divinyl ether and divinyl sulfone spontaneously copolymerized upon monomer mixing. Thus, the participation of the charge-transfer complex in the copolymerization mechanism of such strong electron donor electron acceptor monomer pairs appears to have considerable support. 

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