Consecutive homopolymerization

A special situation arises when one of the monomer reactivity ratios is much larger than the other. For the case of r >> r2 (i.e., r S> 1 and ri propagating species preferentially add monomer M,. There is a tendency toward consecutive homopolymerization of the two monomers. Monomer Mj tends to homopolymerize until it is consumed monomer M2 will subsequently homopolymerize. An extreme example of this type of behavior is shown by the radical polymerization of styrene-vinyl acetate with monomer reactivity ratios of 55 and 0.01. (See Sec. 6-3b-l for a further discussion of this comonomer system.)... 

When one reactivity ratio is greater than unity and the other is less than unity, either propagating species will prefer to add monomers of the first type. Relatively long sequences of this monomer will thus be formed if the reactivity ratios differ sufficiently. A special situation arises when ri 1 and T2 1 or vice versa. In this case, the product composition will tend toward that of the homopolymer of the more reactive monomer. Such reactivity ratios refiect the existence of an impractical copolymerization. An example of this type of behavior is the radical chain polymerization of styrene-vinyl acetate system, where monomer reactivity ratios of 55 and 0.01 are observed. The large differences between the monomer reactivity ratios imparts a tendency toward consecutive homopolymerization of the two monomers. For example, when ri 1 and T2 1, both and... 

The higher the ratio Ta/ b for a a>1, r repeat units and the shorter are the continuous sequences of B-type repeat units in the copolymer molecules formed. When Ta 1 with r 1 there is a tendency towards consecutive homopolymerization of the two monomers. The molecules formed early in the reaction have very long sequences of A-type repeat units with the occasional B-type repeat unit (i.e. they are essentially molecules of homopolymer A). Later in the reaction, when monomer A has been consumed completely, molecules of homopolymer B are formed by polymerization of the residual monomer B. 

M2 preferentially add monomer Mi till the monomer is consumed, which is then followed by homopolymerization of M2- This is therefore a case of consecutive horaopolymerizations. 

Infrared spectra of a copolymer and of a polyethylene are shown in Figure 6. The spectrum of the copolymer is quite different from that of polyethylene. Since the homopolymerization of hexafluoroacetone is difficult to achieve, the product obtained should be a copolymer. Since the absorption arising from consecutive ethylene units is not observed,... 

A similar compartmentalization resulted in a successful one-pot process for the combination of ATRP and eROP, i.e., in a one-pot, two-step procedure (17). For this process CL, t-butyl methacrylate (t-BMA), Novozym 435 and 2 were heated to 60 °C to initiate the eROP and obtain the PCL block end-capped with 2. After 120 min. CuBr/dNbipy was added in order to activate the ATRP and thus the block copolymer formation. Figure 4 shows that during the first step of the consecutive process, i.e. the eROP, CL conversion reached 95 % while only negligible conversion of t-BMA was detected. Only upon addition of the ATRP catalyst, the radical polymerization started and reached ca. 43 % conversion within 180 min (300 min. total). Both reaction kinetics are comparable with the kinetics observed from the homopolymerization under similar conditions, suggesting that both reactions run undisturbed by each other (Figure 4). A clear... 

Figure 4. Conversion of CL (d) and t-BMA (o) in a consecutive one-pot cascade polymerization (arrow marks addition of ATRP catalyst) in comparison with the conversion of CL (m) and t-BMA ( ) in a homopolymerization. Lines are added to guide the eye for data set of the cascade polymerization. (Reproduced with permission from reference 17. Copyright 2006 John Wiley and Sons, Ltd.)...

Alginic acid is a linear copolymer with homopolymeric blocks of (l-4)-linked p-D-mannuronate (M) and its C-5 epi-mer a-L-glucuronate (G) residues, respectively, covalently linked together in different sequences or blocks. The different forms of alginic acid are represented in Figure 1.29. The monomers can appear in homopolymeric blocks of consecutive G-residues (G-blocks), consecutive M-residues (M-blocks), or alternating M- and G-residues (MG-blocks). 

It is also worth remembering that, in contrast to the cyclic monomers, the large majority of unsaturated compounds polymerize, leaving a relatively low monomer concentration at the point of equiUbrium. Indeed, when considering the nonsolvent, bulk polymerization of monomers such as ethylene, methyl acrylate or styrene, then at 25 °C the situation is that [Mjeq = 10 , 10or 10 moll", respectively. When unsaturated monomers providing steric hindrance in the polymer units are considered, then homopolymerization may be hampered here, 1,1-diphenylethylene is the best example, as the joining of two consecutive units is prohibited. The introduction of a second (even small) substituent causes a considerable increase in [Mjeq. For example, in the case of methyl methacrylate or a-methylstyrene, [Mjeq = 10 or 2.2 molT at 25 °C, respectively, have been determined [50, 51]. 

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