Monomers relative reactivity

When the product of monomer relative reactivity ratios is approximately one r x r2 = 1), the last inserted monomeric unit in the chain does not influence the next monomer incorporation and Bernoullian statistics govern the formation of a random copolymer. When this product tends to zero (r xr2 = 0), there is some influence from the last inserted monomeric unit (when first-order Markovian statistics operate), or from the penultimate inserted monomeric units (when second-order Markovian statistics operate), and an alternating copolymer formation is favoured in this case. Finally, when the product of the reactivity ratios is greater than one (r x r2 > 1), there is a tendency for the comonomers to form long segments and block copolymer formation predominates (or even homopolymer formation can take place) [448],... 

Table 3.5 Monomer relative reactivity ratios, r (ethylene) and r2 (propylene), for ethylene/propylene copolymerisations with various Ziegler-Natta catalysts 1...

Monomer Relative reactivity Monomer Relative reactivity 1... 

The conversion of aromatic monomers relative to C-5—C-6 linear diolefins and olefins in cationic polymerizations may not be proportional to the feedblend composition, resulting in higher resin aromaticity as determined by nmr and ir measurements (43). This can be attributed to the differing reactivity ratios of aromatic and aHphatic monomers under specific Lewis acid catalysis. Intentional blocking of hydrocarbon resins into aromatic and aHphatic regions may be accomplished by sequential cationic polymerization employing multiple reactors and standard polymerization conditions (45). 

I itro-DisplacementPolymerization. The facile nucleophilic displacement of a nitro group on a phthalimide by an oxyanion has been used to prepare polyetherimides by heating bisphenoxides with bisnitrophthalimides (91). For example with 4,4 -dinitro monomers, a polymer with the Ultem backbone is prepared as follows (92). Because of the high reactivity of the nitro phthalimides, the polymerkation can be carried out at temperatures below 75°C. Relative reactivities are nitro compounds over halogens, Ai-aryl imides over A/-alkyl imides, and 3-substituents over 4-substituents. Solvents are usually dipolar aprotic Hquids such as dimethyl sulfoxide, and sometimes an aromatic Hquid is used, in addition. 

Monomer CAS Registry Number Relative reactivity Formula... 

Chain-Growth Gopolymerization Theory. The theory of chain-growth (eg, radical, anionic, etc) copolymerisation has received more attention than that of step-growth or other copolymerisations. In the case of chain-growth copolymerisation, growing polymer chains must choose between more than one monomer. Such a choice or relative reactivity has been quantitatively treated by the reactivity ratio (6,7) and the Q-e schemes (8). 

The ratio describes the relative reactivity of polymer chain M toward monomer M and monomer M2. Likewise, describes the relative reactivity of polymer chain M2 toward M2 and M. With a steady-state assumption, the copolymerisation equation can be derived from the propagation steps in equations 3—6. 

Epichlorohydrin Elastomers without AGE. Polymerization on a commercial scale is done as either a solution or slurry process at 40—130°C in an aromatic, ahphatic, or ether solvent. Typical solvents are toluene, benzene, heptane, and diethyl ether. Trialkylaluniinum-water and triaLkylaluminum—water—acetylacetone catalysts are employed. A cationic, coordination mechanism is proposed for chain propagation. The product is isolated by steam coagulation. Polymerization is done as a continuous process in which the solvent, catalyst, and monomer are fed to a back-mixed reactor. Pinal product composition of ECH—EO is determined by careful control of the unreacted, or background, monomer in the reactor. In the manufacture of copolymers, the relative reactivity ratios must be considered. The reactivity ratio of EO to ECH has been estimated to be approximately 7 (35—37). 

The exact distribution of monomer units depends on the initial proportions of the two reactant monomers and their relative reactivities. In practice, neither perfectly random nor perfectly alternating copolymers are usually found. Most copolymers have many random imperfections. 

The reactions of cyanoisopropyl radicals with monomers have been widely studied. Methods used include time resolved EPR spectroscopy,352 radical trappingj53 355 and oligomer00 356 and polymer end group determination. 1 Absolute341 and relative reactivity data obtained using the various methods (Table 3.6) are in broad general agreement. 

Absolute rate constants for the attack of aryl radicals on a variety of substrates have been reported by Scaiano and Stewart (Ph ) 7 and Citterio at al. (/j-CIPh-).379,384 The reactions are extremely facile in comparison with additions of other carbon-centered radicals [e.g. jfc(S) = 1.1x10s M"1 s"1 at 25 °C].3,7 Relative reactivities are available for a wider range of monomers and other substrates (Tabic 3.b). Phenyl radicals do not show clear cut electrophilic or... 

The rate constant for p-scisskm is dependent on ring substituents. Rate constants for radicals X-Q.H. CCh are reported to increase in the series where X is / -Fi There is qualitative evidence that the relative rales for p-scission and addition are insensitive to solvent changes. For benzoyloxy radicals, similar relative reactivities are obtained from direct competition experiments10 as from studies on individual monomers when p-scission is used as a clock reaction.399,401... 

The transient radicals produced in reactions of hydroxy radicals with vinyl monomers in aqueous solution have been detected directly by EPR43 439 or UV spectroscopy,440-441 These studies indicate that hydroxy radicals react with monomers and other species at or near the diffusion-controlled limit ( Table 3.7). However, high reactivity does not mean a complete lack of specificity. Hydroxy radicals are electrophilic and trends in the relative reactivity of the hydroxy radicals toward monomers can be explained on this basis/97... 

Most studies have concerned the kinetics of arenethiyl radicals with monomers including S and its derivatives468 47" and MMA.469,473 The radicals have electrophilic character and add more rapidly to electron-rich systems (Tabic 3.10). Relative reactivities of the monomers towards the ben/.oy Ithiyl radical have also been examined.453... 

Labeled initiators have been used in evaluating the relative reactivity of a wide range of monomers towards initiating radicals.159 The method involves determination of the relative concentrations of the end groups fanned by addition to two monomers (e.g. 119 and 120) in a binary copolymer formed with use of a labeled initiator. For example, when AlBMe-a-13C is used to initiate copolymerization of MMA and VAc (Scheme 3.99),157 the simple relationship (eq. 14) gives the relative rate constants for addition to the two monomers. Copolymerizations studied in this way arc summarized in Tabic 3.13. 

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... 

It is possible to exercise control over this form of compositional heterogeneity (i. e. the functionality distribution) by careful selection of the functional monomer and/or the transfer agent taking into account the reactivities of the radical species, monomers, and transfer agents, and their functionality.11 250 Relative reactivities of initiator and transfer agent-derived radicals towards monomers are summarized in Section 3.4. Some values for transfer constants are provided in Chapter 6. 

Waters61 have measured relative rates of p-toluenesulfonyl radical addition to substituted styrenes, deducing from the value of p + = — 0.50 in the Hammett plot that the sulfonyl radical has an electrophilic character (equation 21). Further indications that sulfonyl radicals are strongly electrophilic have been obtained by Takahara and coworkers62, who measured relative reactivities for the addition reactions of benzenesulfonyl radicals to various vinyl monomers and plotted rate constants versus Hammett s Alfrey-Price s e values these relative rates are spread over a wide range, for example, acrylonitrile (0.006), methyl methacrylate (0.08), styrene (1.00) and a-methylstyrene (3.21). The relative rates for the addition reaction of p-methylstyrene to styrene towards methane- and p-substituted benzenesulfonyl radicals are almost the same in accord with their type structure discussed earlier in this chapter. 

Alkyl-substituted phenols have different reactivities than phenol toward reaction with formaldehyde. Relative reactivities determined by monitoring the disappearance of formaldehyde in phenol-paraformaldehyde reactions (Table 7.3) show that, under basic conditions, meta-cresol reacts with formaldehyde approximately three times faster titan phenol while ortho- and para-cresols react at approximately one-third the rate of phenol.18 Similar trends were observed for the reactivities of acid-catalyzed phenolic monomers with formaldehyde. 

The relative rate of cationic homopolymerization is governed by three factors, ie. the concentration of the propagating species, the ring-opening reactivity of the growing species and the nucleophilic reactivity of the monomer. From kinetic studies196 197 of the polymerization of oxazolines and oxazines it was found that the second factor was the most important. On the other hand, the relative reactivity in the cationic copolymerization is mainly determined by the nucleophilicity of the monomer and for 2-substituted 2-oxazolines this is in the order of benzyl > methyl > > isopropyl > H > phenyl195. ... 

Thus ri represents the ratio of the rate constants for the reaction of a radical of type 1 with monomer Mi and with monomer M2, respectively. The monomer reactivity ratio similarly expresses the relative reactivity of an M2 radical toward an M2 compared with an Ml monomer. The quantity d[M /d M given by Eq. (5) represents the ratio of the two monomers in the increment of polymer formed when the ratio of unreacted monomers is The former ratio... 

Table XX.—Relative Reactivities (1/r) of Monomers with Various Polymer Radicals (ca. 60°C)... 

Determination of Compositional Sequences. In the copolymerization of MMA and MAA or TBTM, compositional dyads and triads are generated. These sequences are determined by the relative concentration of monomers as well as by their relative reactivity. These compositional sequences characterize the material and allow predictions of activity based on structure by comparison with field tested polymers. 

The relative reactivities (in free-radical copolymerizations) of TBTM and MMA are 0.79 and 1.00 respectively (15). With equal concentrations of monomer, an excess of MMA in the polymer would be expected. In the following discussion A will represent the MAA or TBTM unit and B will represent the MMA unit. For A-centered triads four different arrangements are possible AAA, AAB, BAA, and BAB. Analogous sequences apply to the B-centered triads. For a random compositon, Bernoullian statistics should apply (14). With P (the proportion of TBTM) equal to 0.5, the probabilities of each of the A-centered triads is P 2 or 0.25. The AAB and BAA triads are indistinguishable and appear as a single resonance. 

A high concentration of B will help to compensate for its lesser reactivity, since the growing chain has a greater chance of meeting B molecules more often than A molecules. The relative reactivity of monomers depends on polar and steric effects of the substituents on the monomers. 

The relative reactivity ratios rx and r2 are the ratios of rate constant for a given radical adding to its own monomer to the rate constant for its adding to the other monomer. For example when r, >1, radical prefers to add to Mi while if rx < 1, i.e. M prefers to add to M2. Now, we can write... 

This very efficient, time-honoured pathway suffers however a very severe limitation, i.e. the relative reactivity of the living centre C must be adapted to the structure and reactivity of monomer M2, a requirement which is not very often met. 

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