2-Phenyl-1-propene (alpha methyl styrene) is soluble in most organic solvents, including esters, hydrocarbons, and ethers, and is chemically stable at room temperature. As a highly effective organotin catalyst, it is commonly used in polyurethane synthesis and as a heat stabilizer for PVC plastics.

Sequence-Controlled 2-Phenyl-1-propene/Styrene Copolymers

In polymer chemistry, control of copolymer sequence distribution has been defined as one of the major challenges in precision polymerization, especially in the case of chain-growth polymerization. For radical, cationic, and anionic polymerization, the challenge stems from the need to control the high activity of the propagation sites by adapting the polymerization parameters. The latter can for instance be achieved by controlling the monomer feed ratio. Based on the same principle as SUA, we focused on a practical in situ method to favor alternating copolymerization of monomers that lack the natural tendency for alternating addition. To the best of our knowledge, the alternating propagation of a simple and well-known system such as 2-Phenyl-1-propene (MSt) and styrene (St) has not been reported as of this writing. The polymerization of MSt is a well-known textbook case, widely studied because of the competition with depolymerization that occurs when reaching the ceiling temperature of ～61–62 °C. Because of steric hindrance of the α-methyl group, depolymerization and polymerization kinetics are similar around this temperature. O’driscoll et al. have reported a theoretical model for the copolymerization of 2-Phenyl-1-propene and St at temperatures equal to or above the ceiling temperature of MSt and claimed that unusual compositions and/or sequence distributions could be achieved by taking advantage of the low ceiling temperature of monomers. However, in many cases, only random or block copolymers have been reported. One of the main reasons stems from the difficulties linked to the characterization of such copolymers. Indeed, the chemical structures of styrene and 2-Phenyl-1-propene are very similar and the quantitative determination of the sequence distribution is therefore rather difficult.[1]

A procedure using the precise addition of monomers at the ceiling temperature of 2-Phenyl-1-propene was designed to synthesize, unit by unit, copolymers with a clear alternating tendency. Well-defined poly(MSt-co-St) copolymers were prepared in terms of targeted molecular weight and dispersity. Diad probabilities P2{MM}, 2P2{MS}, and P2{SS} were obtained thanks to quantitative integrations of 13C NMR spectra in the quaternary aromatic carbon region. The experimental χR values for synthesized poly(MSt-alt-St) copolymers ranged between 1.46 and 1.63 depending on the experimental procedures and molecular weight of the copolymers, emphasizing the ability of the proposed synthetic pathway to favor alternating sequences. The NMR results were corroborated by HRMS characterization. A very narrow CCD was observed in the alternating copolymers, providing direct evidence of sequence-controlled macromolecules. In addition, the copolymers exhibited specific MS2 fragmentation patterns yielding a chain sequence reconstruction (carried out via a proposed assignment model) consistent with a clear alternating tendency. In sum, cutting-edge NMR and MS-based characterization techniques confirm the utility of our synthetic approach as a means of preparing poly(MSt-co-St) with a clear alternating tendency via tailored monomer feeding at the ceiling temperature of 2-Phenyl-1-propene. Furthermore, this has been achieved while maintaining good control over copolymer composition and molecular weight distribution.

Copolymerization of 2-phenyl-1-propene with conjugated dienes

The use of α-methylstyrene as a monomer in anionic solution polymerization has been limited to very low temperatures, which has made it economically unreasonable as a monomer for tire applications. 2-Phenyl-1-propene thermodynamically does not homopolymerize at elevated temperatures due to its thermodynamically low ceiling temperature of the monomer (61 °C, neat monomer). The low ceiling temperature is compounded by the fact that conjugated diene monomers normally polymerize at a much faster rate than α-methylstyrene. Together these effects have precluded the synthesis of such copolymers by solution polymerization on a commercial basis. Higher temperatures generally promote a faster rate of polymerization. Accordingly, it is desirable to utilize moderately high temperatures in commercial polymerizations in order to maximize throughputs. However, it has been virtually impossible to copolymerize 2-phenyl-1-propene with conjugated diene monomers at temperatures above about 60 °C. The copolymerization of α-methylstyrene with conjugated diene in emulsion is well documented in the literature. Such emulsion polymerizations generally result in a high degree of branching in the 2-phenyl-1-propene /butadiene rubber (α-MeSBR) which leads to a high level of hysteresis. A high hysteresis effect is undesirable in tire tread rubber compounds because it causes poor rolling resistance characteristics. Subsequent work has indicated that n-butyl lithium can be substituted for Mg(Bu)2 to produce the desired copolymer at the elevated temperature. We have also found that potassium amylate with TMEDA/Mg(Bu)2 works as well. In this paper, we describe the copolymerization of 2-phenyl-1-propene with conjugated dienes using such formulated catalyst systems at 65 °C in hexane.[2]

Catalyst systems consisting of KOAm/TMEDA/Mg(Bu)2, CsOR/TMEDA/Mg(Bu)2 or CsOR/TMEDA/n-BuLi carried to produce copolymers with randomly incorporated α-methylstyrene at elevated temperatures. The CsOR/TMEDA/Mg(Bu)2 system has similar polymerization rates for 2-phenyl-1-propene and butadiene, resulting in higher α-methylstyrene incorporation levels. Replacing the Mg(Bu)2 with n-BuLi results in a similar material. The similarity between polymers produced from Mg(Bu)2 or n-BuLi combined with the fact that n-BuLi cannot homopolymerize α-methylstyrene above its ceiling temperature suggests that the active catalyst species is the cesium (or potassium) ion, while the Mg(Bu)2 and n-BuLi act as reducing agents. Because of the unique thermodynamics of the system, which dictates that the 2-phenyl-1-propene does not want to react with itself, most of the α-methylstyrene is isolated as singlet units. A fairly uniform distribution of α-methylstyrene is present along the polymer chain, with slightly more frequent 2-phenyl-1-propene incorporated at the end of the reaction when the butadiene is depleted.

Characterization of inhaled alpha-methylstyrene vapor toxicity

2-Phenyl-1-propene (AMS) is a chemical intermediate used in the synthesis of specialty polymers and copolymers. Inhalation studies of AMS were conducted because of the lack of toxicity data and the structural similarity of AMS to styrene, a toxic and potentially carcinogenic chemical. Male and female B6C3F1 mice were exposed to 0, 600, 800, or 1000 ppm 2-Phenyl-1-propene 6 h/day, 5 days/week, for 12 days. After 1 exposure, 21% (5/24) of female mice were found dead in the 1000-ppm group, 56% (10/18) in the 800-ppm group, and 6% (1/18) in the 600-ppm concentration group. After 12 exposures, relative liver weights were significantly increased and relative spleen weights were significantly decreased in both male and female mice at all concentrations. No microscopic treatment-related lesions were observed. A decrease in hepatic glutathione (GSH) was associated with 2-Phenyl-1-propene exposure for 1 and 5 days. Male and female F344 rats were exposed to 0, 600 or 1000 ppm AMS for 12 days. No mortality or sedation occurred in AMS-exposed rats. Relative liver weights were significantly increased in both males and females after 12 exposures to 600 or 1000 ppm. An increased hyaline droplet accumulation was detected in male rats in both concentration groups; no significant microscopic lesions were observed in other tissues examined. Exposure of male and female F344 rats and male NBR rats to 0, 125, 250 or 500 ppm 2-Phenyl-1-propene, 6 h/day for 9 days resulted in increased accumulation of hyaline droplets in the renal tubules of male F344 rats in the 250 and 500 ppm concentration groups. Although AMS and styrene are structurally very similar, AMS was considerably less toxic for mice and more toxic for male rats than styrene.[3]

References

[1]Wolf, A., Desport, J. S., Dieden, R., Frache, G., Weydert, M., Poorters, L., Schmidt, D. F., & Verge, P. (2020). Sequence-Controlled α-Methylstyrene/Styrene Copolymers: Syntheses and Sequence Distribution Resolution. Macromolecules, 53(18), 8032–8040. https://doi.org/10.1021/acs.macromol.0c01649

[2]Halasa, A. F., & Seo, K. S. (2014). Anionic solution copolymerization of α-methylstyrene with conjugated dienes above the ceiling temperature of α-methylstyrene. European Polymer Journal, 51, 80–86. https://doi.org/10.1016/j.eurpolymj.2013.11.017

[3]Morgan, D. L., Mahler, J. F., Kirkpatrick, D. T., Price, H. C., O'Connor, R. W., Wilson, R. E., & Moorman, M. P. (1999). Characterization of inhaled alpha-methylstyrene vapor toxicity for B6C3F1 mice and F344 rats. Toxicological Sciences, 47(2), 187–194. https://doi.org/10.1093/toxsci/47.2.187