Styrene-acrylonitrile copolymer reactivity ratios

Example 13.6 The following data were obtained using low-conversion batch experiments on the bulk (solvent-free), free-radical copol)mierization of styrene (X) and acrylonitrile (Y). Determine the copolymer reactivity ratios for this pol5Tnerization. 

P.G. Sanghvi, A.C. Patel, K.S. Gopalkrishnan, and S. Devi, Reactivity ratios and sequence distribution of styrene-acrylonitrile copolymers synthesized in microemulsion medium, Eur. Polym. /., 36(10) 2275-2283, October 2000. 

Errors in variables methods are particularly suited for parameter estimation of copolymerization models not only because they provide a better estimation in general but also, because it is relatively easy to incorporate error structures due to the different techniques used in measuring copolymer properties (i.e. spectroscopy, chromatography, calorimetry etc.). The error structure for a variety of characterization techniques has already been identified and used in conjunction with EVM for the estimation of the reactivity ratios for styrene acrylonitrile copolymers (12). 

The randomness (which is usually desirable) and the final composition depend on the reactivity ratio of the various monomers (r, r ). If they are grossly different then one monomer may be consumed before the other. This, however, can be overcome for example styrene/acrylonitrile copolymer (which has good gas-barrier properties) can be made as a 50/50 copolymer despite the reactivity ratios, 0.4, Tacryionitriie 4, by adding the more reactive styrene... 

Acrylonitrile copolymeri2es readily with many electron-donor monomers other than styrene. Hundreds of acrylonitrile copolymers have been reported, and a comprehensive listing of reactivity ratios for acrylonitrile copolymeri2ations is readily available (34,102). Copolymeri2ation mitigates the undesirable properties of acrylonitrile homopolymer, such as poor thermal stabiUty and poor processabiUty. At the same time, desirable attributes such as rigidity, chemical resistance, and excellent barrier properties are iacorporated iato melt-processable resias. 

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. 

Compositionally uniform copolymers of tributyltin methacrylate (TBTM) and methyl methacrylate (MMA) are produced in a free running batch process by virtue of the monomer reactivity ratios for this combination of monomers (r (TBTM) = 0.96, r (MMA) = 1.0 at 80°C). Compositional ly homogeneous terpolymers were synthesised by keeping constant the instantaneous ratio of the three monomers in the reactor through the addition of the more reactive monomer (or monomers) at an appropriate rate. This procedure has been used by Guyot et al 6 in the preparation of butadiene-acrylonitrile emulsion copolymers and by Johnson et al (7) in the solution copolymerisation of styrene with methyl acrylate. 

Acrylonitrile copolymerizes readily with many electron-donor monomers oilier than styrene. Hundreds of acrylonitnle copolymers have been reported, and a comprehensive listing of reactivity ratios for acrylonitrile copolymerizations is readily available. 

Table 6.8 Parameters (2.4) of the penultimate model (2.3) describing copolymerization of styrene M, with acrylonitrile M2 in toluene solution at T = 60 °C. The values of reactivity ratios were obtained [283] from the data on copolymer composition (I) and triad distribution (II)...

The acrylate- and methacrylate-derivatized r 5-(benzene)tricarbonylchromium monomers 20 65,66,68,72 21,69>72 and 2273 (Scheme 1.2) were synthesized from benzyl alcohol or 2-phenylethanol when reacted with Cr(CO)6. The alcohols were esterified with either acrylyl or methacrylyl chloride in ether/pyridine and purified by multiple recrystallizations from CS2. Homopolymerizations proceeded in classic fashion with no special electronic effects from the rr-complexed Cr(CO)3 moiety.65,73 Acrylate 20 was copolymerized with styrene and methyl methacrylate and the reactivity ratios were obtained.65 Acrylate 21 and methacrylate, 22, copolymerized readily with styrene, methyl acrylate, acrylonitrile, and 2-phenylethyl acrylate to give bimodal molecular-weight distributions using AIBN initiation.69 Copolymerization of 20 with ferrocenylmethyl acrylate, 2, generates copolymers with varying mole ratios of two transition metals, Cr and Fe (see structure 34).65... 

Novel iron carbonyl monomer, r)4-(2,4-hexadien-l-yl acrylate)tricarbonyl-iron, 23, was prepared and both homopolymerized and copolymerized with acrylonitrile, vinyl acetate, styrene, and methyl methacrylate using AIBN initiation in benzene.70,71 72 The reactivity ratios obtained demonstrated that 23 was a more active acrylate than ferrocenylmethyl acrylate, 2. The thermal decomposition of the soluble homopolymer in air at 200°C led to the formation of Fe203 particles within a cross-linked matrix. This monomer raised the glass transition temperatures of the copolymers.70 The T)4-(diene)tricarbonyliron functions of 23 in styrene copolymers were converted in high yields to TT-allyltetracarbonyliron cations in the presence of HBF4 and CO.71 Exposure to nucleophiles gave 1,4-addition products of the diene group.71... 

Monomer 13 homopolymerization was very sluggish, but it copolymerized in good yields with acrylonitrile, methyl methacrylate, and /V-vinyl-2-pyrrolidonc (Scheme 1.6).59,61 However, styrene copolymerizations required several subsequent reinitiations to get good yields of copolymers. The reactivity ratios obtained in 13/styrene copolymerizations were fj = 0.16 and r2 = 1.55 (when Mj = 13),61 giving values of Q = 1.66 and e = —1.98 for monomer 13 in direct accord with the Qe values found for monomers 1, 8, 10, 11, and 12, as discussed earlier.61... 

The polyester resin used in this study, MR 13006 (Aristech Corporation), was supplied as a 60-wt% solution in styrene monomer. The epoxy resin, a digly-cidyl ether of bisphenol A (Epon 828), was obtained from Shell Chemical Company. The reactive liquid rubber, an amino-terminated butadiene-acrylonitrile copolymer (ATBN 1300 x 16), was provided by the BFGoodrich Company. The resin was mixed with additional styrene monomer to maintain the ratio of reactive unsaturation in the polyester-to-styrene monomer at 1 to 3. We added 1.5 wt% of tert-butylperbenzoate initiator to the solution, which we then degassed under vacuum. The mixture was poured between vertical, Teflon-coated, aluminum plates and cured under atmospheric pressure at 100 °C. In the modified compositions, the rubber was first dissolved in the styrene monomer, and then all the other components were added and the solution cured as described. In all the compositions, the ratio of the amine functions with respect to the epoxy functions was kept at 1 to ensure complete cure of the epoxy. 

The reactivity ratios for the free-radical copolymerization of styrene (rj = 0.4) and acrylonitrile (r2 = 0.04) result in uneven incorporation of each monomer into the copolymer as seen in Figure 3. Thus, most SAN and ABS polymers are made at the crossover point (A in Figure 3) to avoid composition drift. 

Random copolymers of styrene/isoprene and styrene/acrylonitrile were prepared by the stable free radical polymerization process. The molecular weight of the polymers increased as a function of conversion, as expected for a living radical polymerization. The microstructure of the copolymers and reactivity ratios of the monomers were found to be very similar to what would be obtained for a conventional free radical polymerization. The propagating living radical chain reacts similarly to a conventionally propagating chain. 

If the nitroxide does leave the vicinity of the propagating chain end then the reactivity ratios for the radicals should also be the same as in conventional radical polymerizations. However, if the capping and uncapping rates for the two monomers are different this would lead to different concentrations of the two types of propagating chain ends relative to what would be present in a conventional radical polymerization. To address this issue, stable free radical copolymerizations of styrene-isoprene and styrene-acrylonitrile were studied in detail to compare the low conversion copolymer compositions to those prepared by conventional radical polymerization. The microstructure of the polymers was also examined. 

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. 

A new organotin monomer tributyltin -chloroacrylate (TCA) was synthesized in our laboratory. Detailed studies on homopolymerization and copolymerization were undertaken. Copolymerization was carried out with styrene (ST), methyl methacrylate (MMA) and acrylonitrile (AN). Both homopolymer and copolymers were characterized by IR, IF and C-13 NMR and tin analysis. Reactivity ratios were determined using Kelen-Tudos method. Reactivity ratios were r =0.500 and r = 0.170 for TCA-ST, r = 1.089 and r = 0.261 for TCA-MMA and r =1.880 and r = 0.243 for TCA-AN respectively. Micro-structures of homopolymer and copolymers were studied using C-13 NMR spectroscopy. Data obtained were compared with those of tributyltin methacrylate (TBTMA) and its corresponding copolymers. The results indicate that TCA is more reactive than TBTMA. 

There are several cases where NMR spectroscopy has been used to investigate copolymers which deviate from the terminal model for copolymerisation (see also chapter 3). For example, Hill and co-workers [23, 24] have examined sequence distributions in a number of low conversion styrene/acrylonitrile (S/A) copolymers using carbon-13 NMR spectroscopy. Previous studies on this copolymer system, based on examination of the variation of copolymer composition with monomer feed ratio, indicated significant deviation from the terminal model. In order to explain this deviation, propagation conforming to the penultimate (second-order Markov) and antepenultimate (third-order Markov) models had been proposed [25-27]. Others had invoked the complex participation model as the cause of deviation [28]. From their own copolymer/comonomer composition data. Hill et al [23] obtained best-fit reactivity ratios for the terminal, penultimate, and the complex participation models using non-linear methods. After application of the statistical F-test, they rejected the terminal model as an inadequate description of the data in comparison to the other two models. However, they were unable to discriminate between the penultimate and complex participation models. Attention was therefore turned to the sequence distribution of the polymer. 

---