Reactivity Ratio Studies

A detailed study of the reactivity ratios of the isobutylene and fipimm Q tem has been undertaken. We fmind that the reactivity rath product is dose to unity over the whole temperature range studied from —50° to —130°, indicating a random copolymer system. Further, we discovered that while the reactivity ratios are quite insendtive to the particular Lewis acid used, they can be controlldl by temperature and that the individual reactivity ratios b x me equal to unity below about —110°. In other words, at very low temperatures the copolymerization becomes azeotropic (the composition of tte feed and that of the copolymer become equal). 

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Thus by the use of Wood s approximation we were able to construct a complete 7 /composition curve for the polyfisobutylene-co- -pinene) system. In view of the experimental difficulties in obtaining the Tg for poiy( -pinene), the accuracy of the dot is questionable beyond V2 = 63 volume% ie., above 63 volume% pinene in the copolymer. Howewr, in the range from 0 to 63 volume% /3-pinene, the curve is considered to be accurate and, importantly, reveals that useful rubbery poly(isobutylene-co-j3 pinene) could be made with up to about 28 vohime% pinene, re, to a Tg of —40 °C. In agreement with cnir spectroscopic and reactivity ratio studies, the (admittedly somewhat limited) applicability of Eq. (1) developed for random copolymers also suggests a statistically random copdymer structure for our poly(isobutylene-co- pinene) products. 

Reactivity Ratio Studies. DHA-4VP System. A series of DHA-4VP (MrM2) copolymerizations was carried out to low conversions (<10% ) with the monomer pair ratio being varied. Table III summarizes the data. In the corresponding copolymerization diagram, the composition of the copolymer is plotted as a function of the composition of the initial monomer concentration... 

DHA monomer copolymerizes readily with both 4VP and NVP to form two new reactive polymers. The copolymers are easily obtained in high yield, with polymerization rates and molecular weights strongly affected by initial monomer feeds. Reactivity ratio studies, with r1 = 0.41 and r2 = 0.77 for DHA-CO-4VP and rt = 0.35 and u — 0.15 for DHA-co-NVP, clearly demonstrate that alternating copolymers are obtained. 

Effects of Composition. Reactivity ratio studies of the NaAMPS-AMPDAC pair showed high alternation as expected for ion-paired systems (i, 33). The acrylamide functionality of both monomers would also be expected to terpolymerize well with acrylamide. Therefore, the series of ter-polymers shown in Table II was prepared. Molecular weights, second virial coefficients, and intrinsic viscosities at 25 °C in 1.0 M NaCl solutions are given in Table III. The numbers following the AD AS AM acronym represent... 

Reactivity ratio studies were carried out and the reactivity of was compared to that of vinylferrocene, 1,... 

As we have already seen, it is the reactivity ratios of a particular copolymer system that determines both the composition and microstructure of the polymer. Thus it is important to have reliable values for these parameters. At the same time it suggests that experimental studies of composition and microstructure can be used to evaluate the various r s. 

Microstructure studies, by contrast, offer both a means to evaluate the reactivity ratios and also to test the model. The capability to investigate this type of structural detail was virtually nonexistent until the advent of modern instrumentation and even now is limited to sequences of modest length. 

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

Copolymers of diallyl itaconate [2767-99-9] with AJ-vinylpyrrolidinone and styrene have been proposed as oxygen-permeable contact lenses (qv) (77). Reactivity ratios have been studied ia the copolymerization of diallyl tartrate (78). A lens of a high refractive iadex n- = 1.63) and a heat distortion above 280°C has been reported for diallyl 2,6-naphthalene dicarboxylate [51223-57-5] (79). Diallyl chlorendate [3232-62-0] polymerized ia the presence of di-/-butyl peroxide gives a lens with a refractive iadex of n = 1.57 (80). Hardness as high as Rockwell 150 is obtained by polymerization of triaHyl trimeUitate [2694-54-4] initiated by benzoyl peroxide (81). 

In studies of the polymerization kinetics of triaUyl citrate [6299-73-6] the cyclization constant was found to be intermediate between that of diaUyl succinate and DAP (86). Copolymerization reactivity ratios with vinyl monomers have been reported (87). At 60°C with benzoyl peroxide as initiator, triaUyl citrate retards polymerization of styrene, acrylonitrile, vinyl choloride, and vinyl acetate. Properties of polyfunctional aUyl esters are given in Table 7 some of these esters have sharp odors and cause skin irritation. 

Vinyhdene chloride copolymerizes randomly with methyl acrylate and nearly so with other acrylates. Very severe composition drift occurs, however, in copolymerizations with vinyl chloride or methacrylates. Several methods have been developed to produce homogeneous copolymers regardless of the reactivity ratio (43). These methods are appHcable mainly to emulsion and suspension processes where adequate stirring can be maintained. Copolymerization rates of VDC with small amounts of a second monomer are normally lower than its rate of homopolymerization. The kinetics of the copolymerization of VDC and VC have been studied (45—48). 

Absolute rate data for Friedel-Crafts reactions are difficult to obtain. The reaction is complicated by sensitivity to moisture and heterogeneity. For this reason, most of the structure-reactivity trends have been developed using competitive methods, rather than by direct measurements. Relative rates are established by allowing the electrophile to compete for an excess of the two reagents. The product ratio establishes the relative reactivity. These studies reveal low substrate and position selectivity. 

Studies of chlorination and bromination of 2//-cyclopenta[reactivity differences dependent on substituents and halogenation conditions. In monochlorination the unsubstituted compound was more reactive than its 2-methyl and 2-phenyl derivatives, the reactivity ratio being 7.1 1.7 1 [78H(11)155]. Chlorination occurred most readily in the 5- and 7-positions of the cyclopentadienyl moiety, but once all three positions had been substituted, NCS attacked the methyl group... 

Similar anomalies have been encountered by several workers in the bulk and solution polymerization of this monomer induced by classical free-radical initiators84-86) also, particularly low rates of conversion were observed. The most thorough kinetic study was carried out by Aso and Tanaka86) who again found normal results and a value of k jkt much lower than that for styrene. Copolymerization studies of 2-vinylfuran (Mj) have given the following values of the reactivity ratios ... 

Kunitake, Yamaguchi and Aso149 studied the copolymerization of 2-furaldehyde with olefins and vinyl ethers using BF3 Et20 in methylene chloride or toluene at —78 °C. No copolymers were obtained with olefins, but p-tolyl vinyl ether or 2,3-dihydropyran gave polyethers. With the former co-monomer the values of the reactivity ratios were rx = 0.15 0.15 and r2 = 0.25 0.05 (Mj = 2-furaldehyde). 

For copolymerizations between non protie monomers solvent effects are less marked. Indeed, early work concluded that the reactivity ratios in copolymerizations involving only non-protic monomers (eg. S, MMA, AN, VAe, etc.) should show no solvent dependence.100101 More recent studies on these and other systems (e.g. AN-S,102-105 E-VAc,106 MAN-S,107 MMA-S,10s "° MMA-VAc1" ) indicate small yet significant solvent effects (some recent data for AN-S copolymerization are shown in Table 8.5). However, the origin of the solvent effect in these cases is not clear. There have been various attempts to rationalize solvent effects on copolymerization by establishing correlations between radical reactivity and various solvent and monomer properties.71,72 97 99 None has been entirely successful. 

Studies on the reactions of small model radicals with monomers provide indirect support but do not prove the bootstrap effect.111 Krstina et ahL i showed that the reactivities of MMA and MAN model radicals towards MMA, S and VAc were independent of solvent. However, small but significant solvent effects on reactivity ratios are reported for MMA/VAc111 and MMA S 7 copolymerizations. For the model systems, where there is no polymer coil to solvate, there should be no bootstrap effect and reactivities are determined by the global monomer ratio [Ma0]/[Mb0].1j1... 

One might also anticipate that the influence of bootstrap effects (Section 8.3.1.2) would be quite different in living and non-living processes. 68 A comprehensive study of reactivity ratios in living and conventional radical polymerization may provide a test of the various hypotheses for the origin of this effect. 

The kinetics of the chlorination of some alkylbenzenes in a range of solvents has been studied by Stock and Himoe239, who again found second-order rate coefficients as given in Table 57. Although the range of rates varies by a factor of 104, there was no marked change in the toluene f-butylbenzene reactivity ratio, and it was, therefore, concluded that the Baker-Nathan order is produced by a polar rather. than a solvent effect. 

Our theoretical studies [38] showed that the hyperbranched polymers generated from an SCVP possess a very wide MWD which depends on the reactivity ratio of propagating and initiating groups, r=kjk. For r=l, the polydispersity index where P is the number-average degree of polymerization. 

Copolymerization equations for systems of more than two monomers have been derived, and several experimental studies of copolymerizations involving three monomers have been reported. Six reactivity ratios are required for treatment of the composition in a three-compo-... 

Samples taken during three different AN/S polymerizations were analyzed chromatographically. Target composition was 70/30 AN/S for all three polymerizations. It is difficult to prepare high nitrile copolymers of styrene because reactivity ratios of the two monomers are very different. This study used continuous addition of monomers to achieve the desired polymer composition. Addition rates were those needed to maintain an excess of acrylonitrile. 

Preparation of Copolymers Containing Both Electrophilic and Nucleophilic Groups. Our first implementation of this reaction scheme involved the preparation of a series of copolymers incorporating both a latent electrophile and an electron-rich aromatic moiety which, being phenolic, also provides access to swelling-free development in aqueous medium. The copolymers are prepared as shown in Figure 1 by copolymerization of 4-t-butyloxycarbonyloxy-styrene with 4-acetyloxymethyl-styrene. Although the reactivity ratios of these two monomers are different [11], our study of this system has confirmed that they copolymerize essentially in random fashion. 

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