TRUE COPOLYMERIZATIONS

As opposed to the template polymerizations described above, and composites such as those of CPs with plastics (e.g. with poly(vinyl acetate), poly(ethylene tere-phthalate), poly(vinyl alcohol)), true copolymers of CPs are those in which the CP and the other polymer components are synthesized/rom their monomers and also have all components part of a polymer structure, i.e. they are structural copolymers in the polymer chemistry sense of the term. 

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If the enthalpies of the cross-propagation steps are more negative than those of the homo-propagation steps, then copolymerization will be enhanced (the overall —AH, is higher). When the opposite is true, copolymerization will be hampered. For identical enthalpies and entropies of homo- and cross-propagations, there still remains the term ASmix, which for ideally random copolymers (ASmix 8 J mol-1 xdeg-1) reduces the equilibrium concentration of each comonomer by a factor of two. In such cases a = (3 = 1/2. 

Brookhart and co-workers recently reported tantalizing results that were close to constituting true copolymerizations of ethylene and methyl acrylate. ° ° The catalyst employed was the palladium version of the diimine complexes that were previously reported for ethylene and a-olefin homopolymerizations (complexes IV). °° The close qualification... 

In ionic polymerizations, the polarity of the monomers or the ions is far more important than resonance stabilization, while in free radical copolymerizations the reverse is true. For example, if > r2 in cationic copolymerization, then, by contrast, in anionic copolymerization r2 (Table 22-13). If the polarities are very different, then it is no longer possible to have either cationic or anionic copolymerization. The styryl anion, for example, still adds butadiene, but the butadienyl anion does not add styrene. Only monomers with almost identical polarity can undergo true copolymerizations (with r r2 < 1) unless complexes are formed between the active growing end and the monomer. 

Polymerization, block This term has been applied to both bulk polymerization (casting of polymerizing syrup) and sequence copolymerization (block copolymerization). It is only this latter that is recognized by lUPAC as true copolymerization. Confusion may be avoided to some extent by the use of the prefix co which implies the polymerization of more than one monomer. 

For the foUwing estimation of the copolymerization parameters it is useful to discern between the ovmaU or mixed parameters and the true oopolymerization parameters. First we assume that there are only uniform active centres located on the catalyst sur ce, (i.e., one centre model), and use ethene and comonomer peaks in the NMR spectrum of the polymer mixture for the estimation of the oopolymerization parameters according to the Mayo Lewis equqtion This evaluation, via the r versus diagram, leads to the overall or mixed copolymerization parameters. However, for the estimation of the true copolymerization parameters we now use the following considerations. The Mayo-Lewis equation describes the composition of the copolymer as a function of the initial monomers mixture and the oopolymerization parameters. If we know these and the monomers mixture we can calculate not only the copolymer composition but also, by means of statistical considerations, the sequence length distribution of Mj and M2 sequences in the copolymer... 

The dependence of the mixed and the true copolymerization parameters, in ethene / 1-hexene copolymeiization, on the Ti loading and the specific surface area of the MgH2 / TiCl catalyst is shown in Fig. 1. It is observed that there are considerable differences in the values of the copolymeiization parameters evaluated, especially between the mixed and the true r parameter. This finding confirms that we have a polymer mixture between pure ethene homopolymer and ethene / 1-hexene copolymer. Consequently, this interesting result indicates that we have different types of active centres namely, centres for the homopolymerization of ethene and others for the copolymeiization of... 

The product of reactivity ratios rjTj can be related to other quantities for which more direa physical evidence is available, e.p. the parameter introduced by Chuj6 (48) to describe deviations from randomness in terms of the number and weight averages of isotactic (or syndiotactic) diads. The analogy with the true copolymerization is remarkable here a similar parameter, the index of sequential homogeneity (49), characterizes the distribution of sequence length of the two monomers. 

In cationic initiation, true copolymerization is difficult due to the wide range of reactivity of oxiranes with acid or cationic attack (Table 1). Generally, the copolymer formed is lower in molecular weight, with a significant amount of cyclic product formed, than is the case in homopolymerization. Except at very low temperatures, around -78°C, linear copolymers tend to terminate by proton transfer before significant molecular weights are achieved when cationic initiation is used. 

Give two examples of true copolymerizations of CPs, and clarify how they differ from CP composites and blends. Include as much detail of methodology as possible. 

The triad distribution allows the calculation of the true copolymerization parameters. For a given polymer, the triad distribution is deduced from experimental C-NMR spectra (7) (see below). 

We saw in the last chapter that the stationary-state approximation is apphc-able to free-radical homopolymerizations, and the same is true of copolymerizations. Of course, it takes a brief time for the stationary-state radical concentration to be reached, but this period is insignificant compared to the total duration of a polymerization reaction. If the total concentration of radicals is constant, this means that the rate of crossover between the different types of terminal units is also equal, or that R... 

Frequency factors for addition of small radicals to monomers are higher by more than an order of magnitude than those for propagation (Table 4.12). Activation energies are typically lower. However, trends in the data are very similar suggesting that the same factors are important in determining the relative reactivities for both small radicals and propagating species. The same appears to be true with respect to reactivities in copolymerization (Section 73.1.2)/88... 

The first of these assumptions, generally accepted in macromolecular chemistry [1,3], is correct enough when considering the propagation reaction under copolymerization of the majority of monomers. Simple estimates reported in paper [74] support the correctness of the second assumption. As for the third one, it is true, strictly speaking, only under 0-conditions. The conformational statistics of macromolecules in a thermodynamically good solvent is known [30] to differ from the Gaussian one. Nevertheless, this distinction may hardly influence the qualitative conclusions of the simplest theory of interphase copolymerization. To which extent the account of the excluded volume of macromolecules will affect quantitative results of this theory, may be revealed exclusively by computer simulations. 

There have also been several papers [61-63] on the importance of carefully establishing the reaction mechanism when attempting the copolymerization of olefins with polar monomers since many transition metal complexes can spawn active free radical species, especially in the presence of traces of moisture. The minimum controls that need to be carried out are to run the copolymerization in the presence of various radical traps (but this is not always sufficient) to attempt to exclude free radical pathways, and secondly to apply solvent extraction techniques to the polymer formed to determine if it is truly a copolymer or a blend of different polymers and copolymers. Indeed, even in the Drent paper [48], buried in the supplementary material, is described how the true transition metal-catalyzed random copolymer had to be freed of acrylate homopolymer (free radical-derived) by solvent extraction prior to analysis. 

Progress in Polystyrene Research. As so often happens, practical applications have a stimulating effect on investigations of the underlying fundamentals. This is true of polystyrene, too. Styrene is a particularly versatile monomer, as it is copolymerizable with numerous other monomers, may be copolymerized by any known method, and is also easy to handle. 

An interesting feature of the styrene-S02 system, —which indeed is true of all SO2 copolymerizations with comonomers capable of homopolymerizing—, is the existence of a ceiling temperature above which the formation of alternating units, SMS, is forbidden. The number fraction of M sequences of length n is... 

Our interest in PDMS as an epoxy modifier lies partly in its low Tg relative to the ATBN and CTBN modifiers. Up to this time, however, improvements in K,c through copolymerization of dimethyl siloxane with TFP and DP siloxane require raising the Tg of the siloxane modifier above that of PDMS, as shown by Table 1. It is hoped that increased understanding and control of the synthesis and morphologies of siloxane-modified epoxies will make it possible to retain the low Tg of the modifier while raising the fracture toughness of the resin. The true value of this objective could eventually be shown by measurement of Klc at temperatures below ambient. 

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. 

Copolymerizations initiated by lithium metal should give the same product as produced from lithium alkyls. Usually the radical ends produced by electron transfer initiation have so short a lifetime they can have no influence on the copolymerization. This is true for instance in the copolymerization of isoprene and styrene (50). The product is identical if initiated by lithium metal or by butyllithium. With the styrene-methylmethacrylate system, however, differences are observed (79,80,82). Whereas the butyllithium initiated copolymer contains no styrene at low conversions, the one initiated by lithium metal has a high styrene content if the reaction is carried out in bulk and a moderate one even in tetrahydrofuran. These facts led O Driscoll and Tobolsky (80) to suggest that initiation with lithium occurs by electron exchange and that in this case the radical ends are sufficiently long-lived to produce simultaneous radical and anionic reactions at opposite ends of the chain. Only in certain rather exceptional circumstances would the free radical reaction be of importance. Some of the conditions required have been discussed by Tobolsky and Hartley (111). The anionic reaction should be slow. This is normally true for lithium based catalysts in hydrocarbon solvents. No evidence of appreciable radical participation is observed for initiation by sodium and potassium. The monomers should show a fast radical reaction. If styrene is replaced by isoprene, no isoprene is found in the copolymer for isoprene polymerizes slowly by free radical initiation. Most important of all, initiation should be slow to produce a low steady concentration of radical-anions. An initiator which produces an almost instantaneous and complete electron transfer to monomer produces a high radical concentration which will ensure their rapid mutual termination. 

Furthermore, when r2 = 1 it is generally true that rt < 1 and all the necessary conditions for Eq. (V) to be valid are fulfilled. Thus, in order to have an ideal copolymerization it is only necessary to choose a comonomer for which, with the azo monomer to be used, r2 = 1. This argument was tested for the copolymerization of 7. With the simplifying assumption that this azo monomer behaves like unsubstituted benzyl methacrylate (Q, = 0.7 or 0.84 e, = 0.42 54)) it is possible to calculate the values of Q2 and e2 which the comonomer should have if it is to confirm to an ideal copolymerization with 7 using ... 

The general question of polymerization on or in the particle vs. polymerization in the true aqueous phase has been discussed for a number of years. Evidence other than that cited above suggests that both sites may be important depending on conditions. Polymer chains might be initiated in the aqueous phase and then grow further after transport to the particles. Unreported work in this Laboratory suggests that polymerization in the two sites concurrently, may account for some results on molecular weight distribution and on copolymerization. Mino (100) has shown from his own experiments and those of others that the parti-... 

In many copolymerizations compositional differences arise either intermolecularly or intramolecularly (or both) as a result of the kinetics of the copolymerization. This is true in the anionic batch copolymeriza-... 

The latter technique is more rapid than the former (27, 31). In 1930 Wagner-Jauregg showed that alternating copolymers are obtained when maleic anhydride is copolymerized with vinyl monomers (34). This is true for copolymerization in good solvents, but when the molar ratio of styrene to maleic anhydride is greater than 1, styrene may add to the alternating copolymer in poor solvents to produce block copolymers. 

Dispersion copolymerization of PEO-MA macromonomers (Cj-fEO -MA, C1-(EO)48-C6-MA, C1-(EO)48-C10-MA) with MMA was successful in producing very stable PMMA dispersions of micron size [81]. In this case, however, Cr (EO)48-MA was more effective in giving monodisperse particles than C1-(EO)48-C10-MA (the reverse is true with styrene, see above). The particles obtained were found to have uneven surfaces with a number of craters. These results suggest that some compatibility between PMMA and PEO chains and also between PMMA and the medium (methanol/water) may play a role in controlling the particle formation. 

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