Chain copolymerization monomer reactivity ratio

Copolymerization involves the reaction of at least two different monomers A and B. In the case of chain copolymerization, the reactivity ratios and are important, aiid rg = / bb BA di re /cy die... 

In this copolymerization, the reactivity ratios are such that there is a tendency for S and the acrylic monomers to alternate in the chain. This, in combination with the above-mentioned specificity in the initiation and termination steps, causes chains with an odd number of units to dominate over those with an even number of units. 

For any specific type of initiation (i.e., radical, cationic, or anionic) the monomer reactivity ratios and therefore the copolymer composition equation are independent of many reaction parameters. Since termination and initiation rate constants are not involved, the copolymer composition is independent of differences in the rates of initiation and termination or of the absence or presence of inhibitors or chain-transfer agents. Under a wide range of conditions the copolymer composition is independent of the degree of polymerization. The only limitation on this generalization is that the copolymer be a high polymer. Further, the particular initiation system used in a radical copolymerization has no effect on copolymer composition. The same copolymer composition is obtained irrespective of whether initiation occurs by the thermal homolysis of initiators such as AIBN or peroxides, redox, photolysis, or radiolysis. Solvent effects on copolymer composition are found in some radical copolymerizations (Sec. 6-3a). Ionic copolymerizations usually show significant effects of solvent as well as counterion on copolymer composition (Sec. 6-4). 

Counterion effects similar to those in ionic chain copolymerizations of alkenes (Secs. 6-4a-2, 6-4b-2) are present. Thus, copolymerizations of cyclopentene and norbomene with rhenium- and ruthenium-based initiators yield copolymers very rich in norbomene, while a more reactive (less discriminating) tungsten-based initiator yields a copolymer with comparable amounts of the two comonomers [Ivin, 1987]. Monomer reactivity ratios are also sensitive to solvent and temperature. Polymer conformational effects on reactivity have been observed in NCA copolymerizations where the particular polymer chain conformation, which is usually solvent-dependent, results in different interactions with each monomer [Imanishi, 1984]. 

In THF, however, no difference in the monomer reactivity ratios was observed between the (S)-MBMA-TrMA and (RS)-MBMA-TrMA systems, and the ratios (r =0.39 and 2- . ) showed similar reactivity of MBMA (Mi) and TrMA ( 2). The copolymerization seemed to proceed without termination and chain transfer reactions. An abnormal optical property was observed in some of the copolymers of (S)-MBMA and TrMA. Table shows the tacticity and optical data of the copolymers which were obtained in various polymer yields from the monomer mixtures of a constant molar ratio, [Mllo/[M2]o = The (S)-MBMA content in the copolymers decreased... 

We condude this section by stating that the field-accelerating effect on copolymerizations and the change of the monomer reactivity ratio with the field can be accounted for in terms of the interpretation proposed for cationic homopolymerizations, namely the field-facilitated dissociation of the growing chain ends. We should note that the observed field influence on the copolymerization excludes the possibility of the electroinitiated polymerization mechanism. 

In the alkyllithium initiated polymerizations of vinyl monomers, Lewis bases such as ethers and amines alter the kinetics, stereochemistry, and monomer reactivity ratios for copolymerization. In general, the magnitude of these effects has been directly or indirectly attributed to the extent or nature of the interaction of the Lewis base with the organolithium initiator or with the organolithium chain end of the growing polymer. Unfortunately, all of these observed effects are kinetic in nature, and therefore the observed effects of solvent represent a composite effect on the transition-state versus the ground state as shown below in Eq. (6), where 5 represents the differential... 

Table III shows the increase of molecular weight of BCMO polymerization with conversion, although the polymer tends to precipitate. The monomer reactivity ratios of DOL-BCMO copolymerization were previously determined as rx (DOL) = 0.65 0.05, r2 (BCMO) = 1.5 0.1 at 0°C. by BF3 Et20 (8). Table IV shows a preparation of block copolymer of DOL, St, and BCMO. In the first step we polymerized DOL and St in the second step we added BCMO to this living system. The copolymer obtained showed an increase of molecular weight, and considerable BCMO was incorporated in the copolymer still remaining soluble in ethylene dichloride. The solubility behavior together with the increase of molecular weight with addition of BCMO shows that this polymer consists of block sequences of DOL-St and (St)-DOL-BCMO. This we call block and random copolymer of DOL-St—BCMO. We can deny the presence of BCMO, St, or DOL homopolymers in this system, but some chain-breaking reactions are unavoidable, leading to copolymer mixtures. Thus, the principle of formation of block copolymers by cationic system is partly substantiated.

While the majority of SBC products possess discrete styrene and diene blocks, some discussion of the copolymerization of styrene and diene monomers is warranted. While the rate of homopolymerization of styrene in hydrocarbon solvents is known to be substantially faster that of butadiene, when a mixture of butadiene and styrene is polymerized the butadiene is consumed first [21]. Once the cross-propagation rates were determined (k and in Figure 21.1) the cause of this counterintuitive result became apparent [22]. The rate of addition of butadiene to a growing polystyryllithium chain (ksd) was found to be fairly fast, faster in fact than the rate of addition of another styrene monomer. On the other hand, the rate of addition of styrene to a growing polybutadienyllithium chain (k s) was found to be rather slow, comparable to the rate of butadiene homopolymerization. Thus, until the concentration of butadiene becomes low, whenever a chain adds styrene it is converted back to a butadienyllithium chain before it can add more styrene. Similar results were found for the copolymerization of styrene and isoprene. Monomer reactivity ratios have been measured under a variety of conditions [23]. Values for rs are typically <0.2, while values for dienes (rd) typically range from 7 to 15. Since... 

A mixture of two monomers that can be homopo-lymerized by a metal catalyst can be copolymerized as in conventional radical systems. In fact, various pairs of methacrylates, acrylates, and styrenes have been copolymerized by the metal catalysts in random or statistical fashion, and the copolymerizations appear to also have the characteristics of a living process. The monomer reactivity ratio and sequence distributions of the comonomer units, as discussed already, seem very similar to those in the conventional free radical systems, although the detailed analysis should be awaited as described above. Apart from the mechanistic study (section II.F.3), the metal-catalyzed systems afford random or statistical copolymers of controlled molecular weights and sharp MWDs, where, because of the living nature, there are almost no differences in composition distribution in each copolymer chain in a single sample, in sharp contrast to conventional random copolymers, in which there is a considerable compositional distribution from chain to chain. Figure 26 shows the random copolymers thus prepared by the metal-catalyzed living radical polymerizations. 

A value of unity (or nearly unity) for the monomer reactivity ratio signifies that the rate of reaction of the growing chain radicals towards each of the monomers is the same, i.e. kn ki2 and 22 — A 2i and the copolymerization is entirely random. In other words, both propagating species and M2 have little or no preference for adding either monomer. The copolymer composition is the same as the comonomer feed with a completely random placement of the two monomers along the copolymer chain. Such behavior is referred to as Bemoullian. Free-radical copolymerization of ethylene and vinyl acetate and that of isoprene and butadiene are examples of such a system, but this is not a common case. Random monomer distributions are obtained more generally in a situation where both types of radicals have exactly the same preference for the same type of monomer as represented by the relationship... 

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... 

Recently, the Research Group on NMR, SPSJ, assessed reliability of copolymer analysis by NMR using three samples of radically prepared copolymers of MMA and acrylonitrile with different compositions. 1H and 13C NMR spectra of the copolymers were collected from 46 NMR spectrometers (90 500 MHz) and the composition and sequence distribution were determined.232 Table 14 summarizes the monomer reactivity ratios determined by 13C NMR analysis. The large difference between rxx and r2X indicates the presence of a penultimate effect in this radical copolymerization, as previously reported.233 The values of riy, especially rxx, depended on the comonomer feed ratio, suggesting higher order of neighbouring unit effect on the reactivity of chain-end radicals. 

A value of unity (or nearly unity) for the monomer reactivity ratio signifies that the rate of reaction of the growing chain radicals towards each of the monomers is the same, i.e., A ii ki2 and 22 - 21 and the copolymerization is entirely... 

Random copolymers of styrene/isoprene and styrene/acrylonitrile have been prepared by stable free radical polymerization. By varying the comonomer mole fractions over the range 0.1-0.9 in low conversion SFRP reactions it has been demonstrated that the incorporation of the two monomers in the copolymer is analogous to that found in conventional free radical copolymerizations. The composition and microstructure of random copolymers prepared by SFRP are not significantly different from those of copolymers synthesized conventionally. These two observations support the conclusion that the presence of nitroxide in the SFR process does not influence the monomer reactivity ratios or the stereoselectivity of the propagating radical chain. Rather, the SFR propagation mechanism is essentially the same as that of the conventional free radical copolymerization process. 

Obviously, because of the difference in the reactivity of styrene and DVB, the networks prepared by free radical copolymerization do not relate to such an ideal system with uniform distribution of DVB units and constant chain lengths between the junction points. Also, it was not possible to eliminate this serious defect by an anionic copolymerization of the comonomers. The anionic copolymerization has often been initiated by n- or sec-hutyl lithium [110-112]. Under such conditions, styrene is consumed faster than p-DVB, the monomer reactivity ratios being ri = 1.5S and r2 = 0.32. Therefore, DVB-enriched domains wUl form toward the end of the anionic process. On the other hand, the styrene—m-isomer reactivity ratio (r = 0.65 and r2 = 1.20) points to the local incorporation of m-DVB crosslinks into the initially formed copolymer [113, 114]. In addition, the anionic process is also accompanied by intramolecular cycUzation, similar to radical styrene DVB copolymerization [115,116]. 

It should be noted, too, that the r values for this system do not permit an azeotropic polymerization, as predicted by Eq. (2.39). With respect to the distribution of styrene monomer units in the copolymer, the monomer reactivity ratio product, rers = 0.8, is close to a value of 1.0, which would correspond to an ideal copolymerization (Odian, 2004b) which would correspond to a random distribution of styrene units along the chain. For an ideal copolymerization, the relative rates of incorporation of the two monomers are independent of the chain end unit as predicted by Eq. (2.42). 

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