Monomer reactivity ratio determination

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. 

The rates of addition to the unsubstituted terminus of monosubstituted and 1,1-disubstiluted olefins (this includes most polymerizable monomers) are thought to be determined largely by polar Factors.2 16 Polymer chemists were amongst the first to realize that polar factors were an important influence in determining the rate of addition. Such factors can account for the well-known tendency for monomer alternation in many radical copolymerizations and provide the basis for the Q-e, the Patterns of Reactivity, and many other schemes for estimating monomer reactivity ratios (Section 7.3.4). 

From this scheme it can be seen that the copolymer composition is determined by the values of four monomer reactivity ratios. 

The arrangement of monomer units in copolymer chains is determined by the monomer reactivity ratios which can be influenced by the reaction medium and various additives. The average sequence distribution to the triad level can often be measured by NMR (Section 7.3.3.2) and in special cases by other techniques.100 101 Longer sequences are usually difficult to determine experimentally, however, by assuming a model (terminal, penultimate, etc.) they can be predicted.7 102 Where sequence distributions can be accurately determined Lhey provide, in principle, a powerful method for determining monomer reactivity ratios. 

One final point should be made. The observation of significant solvent effects on kp in homopolymerization and on reactivity ratios in copolymerization (Section 8.3.1) calls into question the methods for reactivity ratio measurement which rely on evaluation of the polymer composition for various monomer feed ratios (Section 7.3.2). If solvent effects arc significant, it would seem to follow that reactivity ratios in bulk copolymerization should be a function of the feed composition.138 Moreover, since the reaction medium alters with conversion, the reactivity ratios may also vary with conversion. Thus the two most common sources of data used in reactivity ratio determination (i.e. low conversion composition measurements and composition conversion measurements) are potentially flawed. A corollary of this statement also provides one explanation for any failure of reactivity ratios to predict copolymer composition at high conversion. The effect of solvents on radical copolymerization remains an area in need of further research. 

The solvent in a bulk copolymerization comprises the monomers. The nature of the solvent will necessarily change with conversion from monomers to a mixture of monomers and polymers, and, in most cases, the ratio of monomers in the feed will also vary with conversion. For S-AN copolymerization, since the reactivity ratios are different in toluene and in acetonitrile, we should anticipate that the reactivity ratios are different in bulk copolymerizations when the monomer mix is either mostly AN or mostly S. This calls into question the usual method of measuring reactivity ratios by examining the copolymer composition for various monomer feed compositions at very low monomer conversion. We can note that reactivity ratios can be estimated for a single monomer feed composition by analyzing the monomer sequence distribution. Analysis of the dependence of reactivity ratios determined in this manner of monomer feed ratio should therefore provide evidence for solvent effects. These considerations should not be ignored in solution polymerization either. 

Monomer reactivity ratios are generally but not always independent of the reaction medium in radical copolymerization. There is a real problem here in that the accuracy of r values is often insufficient to allow one to reasonably conclude whether r or rx varies with changes in reaction media. The more recent determinations of r values by high-resolution NMR are much more reliable than previous data for this purpose. It has been observed that the... 

Some ROPs proceed with the simultaneous operation of two different mechanisms, for example, NCA copolymerizations initiated by some secondary amines proceed with both the amine and activated monomer mechanisms. The monomer reactivity ratios for any comonomer pair are unlikely to be the same for the two different propagations. Any experimentally determined r values are each composites of two different r values. 

Copolymer composition curves in the copolymerization of MMA (Mi) and TrMA ( 2) with BuLi at -78°C are shown in Figure 1. From these curves the monomer reactivity ratios were determined to be ri=6.28 and 2 = 0.13 in toluene and ri = 0.62 and 2 0.62 in THF. In THF both the monomers showed similar reactivity. The compositional and configurational analyses of the copolymers indicated that the copolymerization approximately follows the terminal model in this solvent. 

The method of determining monomer reactivity-ratios is well defined. A series of copolymerizations is conducted over a wide range of monomer ratios, and the polymerizations are terminated before the... 

W 1. Wiesser, M. Temperaturverlauf bei der thermischen Hartung von un-gesattigten Polyestem mit Styrol. Diplomarbeit, TH Stuttgart (1963/64). W 2. Wiley, R. H., and E. E. Sale Tracer techniques for the determination of monomer reactivity ratios. II. Monomer reactivity ratios in copolymerisations with divinyl monomers. J. Polymer Sci. 42, 491 (1960). 

Labelled monomers have been used in co-polymerizations for analysis of the resulting co-polymers and consequent determination of monomer reactivity ratios (15, 16). This technique is of particular value when the compositions of the different monomer units are rather similar or when the co-polymer contains only very small amounts of one of the monomers. These points can be appreciated by considering calculations on co-polymers of methyl methacrylate and methyl acrylate summarized in Table 1. The analyses have been calculated ignoring contributions of end-groups it assumed that the acrylate ester is labelled with carbon-14 and that specific activities are expressed in units such as curies/g of carbon. 

From the Finemann-Ross plot of the data in Table II, monomer reactivity ratios are determined. riK = 0.654, r2/K = 1.09. The copolymer composition diagram is shown in Figure 3, which includes both spontaneous and catalytic polymerization to prove the similarity of both mechanisms. 

Binary copolymerization of the CPT-AN system was carried out at 40°C using AIBN as initiator. From the Finemann-Ross plot of the copolymer composition and monomer feed ratios, the monomer reactivity ratios were determined. (CPT) = 0, r2 (AN) = 1.97. The increase of the relative reactivity of CPT by forming a complex with SC is calculated as follows. 

Therefore, the copolymer composition or the frequency of the branches is essentially determined by the monomer composition and the monomer reactivity ratio of the comonomer. 

The relative reactivity of the macromonomer in copolymerization with a common comonomer, A, can be assessed by l/rA=kAB/kAA> i-e-> the rate constant of propagation of macromonomer B relative to that of the monomer A toward a common poly-A radical. In summarizing a number of monomer reactivity ratios in solution copolymerization systems reported so far [3,31,40], it appears reasonable to say that the reactivities of macromonomers are similar to those of the corresponding small monomers, i.e., they are largely determined by the nature of their polymerizing end-group, i.e., essentially by their chemical reactivity. 

In the predominating reactions, the number of different types of monomer units and their sequences are determined by their relative molecular reactivities for the macrocellulosic radicals and the monomer reactivity ratios. These types of reactions are useful in that less reactive monomers can be included in copolymers to add selected organochemical and macromelecular properties to the modified cellulosic products. In cases where vinyl monomers have been reacted to form oligomers, these reactions are useful in increasing the reactivity of oligomers with macrocellulosic radicals (29, 30, 31). 

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.

I T-(isobutox.ymethyl)acrylamide (NIBa). MBA can be copolymer ised readily with many standard formulations, since it is scluble in most monomers. Reactivity ratios as determined in a benzene solution copolymerisation are ... 

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... 

Here [Pf ] is the concentration of growing centres ending in monomer x and kx y is the absolute rate coefficient of reaction of P with monomer y. Two difficulties arise in anionic polymerization. In hydrocarbon solvents with lithium and sodium based initiators, [Pf ] is not the total concentration of polymer units ending in unit x but, due to self-association phenomena, only that part in an active form. The reactivity ratios determined are, however, unaffected by the association phenomena. As each ratio refers to a common active centre, the effective concentration of active species is reduced equally to both monomers. In polar solvents such as tetrahydrofuran, this difficulty does not arise, but there will be two types of each reactive centre Pf, one an anion and the other an ion-pair. Application of eqn. (22) will give apparent rate coefficients as discussed in Section 4 if total concentrations of Pf are used. Reactivities can change with concentration if defined on this basis. 

Most kinetic studies on copolymerizations using coordination catalysts have been restricted to the determination of monomer reactivity ratios. There are problems both experimentally and in interpretation since the major simplification assumed to hold for most free radical initiated systems, namely that monomer incorporation is determined only by the monomer concentrations and the four rate coefficients, cannot be taken for granted. Further, catalyst activity and selectivity are influenced by the conditions of catalyst preparation including the manner and order of... 

In copolymerizations of cyclic ethers, too, basicity of the monomers seems to correlate well with the reactivity ratios determined from copolymer composition. Recently, Saegusa et al. [54] have pointed out that the apparent values of the monomer reactivity ratios in cyclic ether copolymerizations may be more influenced by the exchange... 

This is the fundamental equation, upon which various methods were suggested to determine the monomer reactivity ratios of Mi and M2. These methods will be discussed in a later section. 

Various methods have been used to determine monomer reactivity ratios for various copolymerization systems. According to Eq. (6), a series of monomer molar fractions (/i) and their corresponding instantaneous copolymer compositions (Fi) are required to obtain monomer reactivity ratios. The most common experimental procedure is to carry out a set of copolymerizations with the feed ratios of Mi to M2 ranging from 0.1 to 0.9, and the conversions are kept below 5%. It is noted that owing to the relatively low conversion, the instantaneous copolymer composition can be approximated by the composition of the final copolymer... 

To overcome the errors brought in by the approximation for instantaneous copolymer composition and the drift in monomer feed ratio, the integration equation for Eq. (5) was used to determine the monomer reactivity ratios, which was given as ° ... 

Fineman, M. Ross, S.D. Linear method for determining monomer reactivity ratios in copolymerization. J. Polym. Sci. 1950, 5, 259-262. Tiidos, F. Kelen, T. Foldes-Berezsnich, T. Turcsanyi, A. Evaluation of high conversion... 

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