Synthetic Routes of Hindered Amine Light Stabilizer 944: A Discussion

        Hindered amine light stabilizers (HALS) are a new type of light stabilizer developed in the late 1970s and have become the dominant light stabilizer product worldwide [1,2]. Among them, GW-944, introduced by foreign companies in the early 1990s, is one of the varieties with the best HALS performance in the world [3]. In addition to the excellent light-stabilizing effect of general HALS [4], its molecular structure contains UV-absorbing and antioxidant functional groups and has no easily hydrolyzed ester groups; therefore it offers the advantages of high efficiency, low dosage, good thermal stability, hydrolysis resistance, good extraction resistance, and non-toxicity, and is widely used in agricultural and other plastic films, household appliances, automotive, and food packaging materials. It is one of the few additives approved by the U.S. FDA for use in food packaging materials [5,6]. As a result, its synthesis process has attracted widespread attention. At present, Ciba-Geigy Fine Chemicals Ltd. of Switzerland is the company that produces GW-944 best. Although extensive domestic research has been conducted on the synthesis of GW-944, the fatal problems of broad molecular weight distribution and low light transmittance have never been solved, failing to meet customer requirements. Based on previous work [7], the author conducted further research on the synthesis process of GW-944 and proposed three process routes for its synthesis, fundamentally solving the problems of broad molecular weight distribution and poor light transmittance, and providing a basis for the industrial production of GW-944 in China.

    1. Experiment
    1.1 Main Instruments
    300 MHz NMR spectrometer (Varian, USA); ZAB-HS mass spectrometer (VG, UK); 1515 GPC (Waters, USA); VIS 7220 spectrophotometer (Beijing Analytical Instrument Factory); digital microscope melting point apparatus (Tianjin University Precision Instrument Factory).
    1.2 Synthesis
    1.2.1 Improved two-step synthesis method
    1.2.3 Four-step synthesis method
    The synthesis of intermediates I and II is as described above.

    2. Discussion
    2.1 Design of the three synthesis routes
    The main factors affecting the quality and application performance of GW-944 are the molecular weight, its distribution, and the light-transmittance performance. If the molecular weight is too low, GW-944 is volatile and has poor solvent-extraction and heat resistance. If it is too high, it affects the migration and diffusion of GW-944 in polymer materials, hindering the effective replenishment of GW-944 molecules from the interior to the surface of the product, and limiting the full play of its light-stabilizing activity. The distribution range of the molecular weight affects its light-transmittance performance: the narrower the molecular weight distribution, the better the product's light transmittance. Therefore, the key to improving product quality is to control the molecular weight and its distribution range. Domestic manufacturers have carried out extensive research on this, but have never solved the shortcomings of broad molecular weight distribution and low light transmittance of GW-944. By stepwise substitution of the three chlorine atoms on the cyanuric chloride ring under different conditions, the author effectively controlled the polymerization degree during GW-944 synthesis, achieving control over the molecular weight distribution and thereby improving the product's light-transmittance performance. Through research, three reaction routes for synthesizing GW-944 were established: the three-step route, the improved two-step route, and the four-step route.
    2.1.1 Three-step synthesis
    The design of this synthesis route has been described in detail in reference [7].
    2.1.2 Improved two-step synthesis route
    The traditional two-step route first synthesizes intermediate I, which then polymerizes with adipic acid piperidine; the substitution of the second and third chlorines on the cyanuric chloride ring is completed in one pot. Because such a process cannot well control the polymerization, the resulting GW-944 has a broad molecular weight distribution, many impurities, and low light transmittance, failing to meet product standards. The author improved this by referring to the production of K-type reactive dyes [8]: through staged heating of the polymerization process, the stepwise substitution of the second and third chlorines on the cyanuric chloride ring was controlled, effectively controlling the molecular weight of GW-944 and improving the product's light transmittance.
    2.1.3 Four-step synthesis route
    Referring to related patented synthesis methods [9-12] and improving on the two-step and three-step processes, the author designed a four-step route for synthesizing GW-944. That is, the polymeric monomers II and III are synthesized separately, and then the target product is obtained by condensation. This breaks the complex polymerization reaction into independent synthesis steps, making it easy to control the degree of polymerization during the reaction and thus control the molecular weight distribution of the product.
    2.2 Comparison of the three routes
    At present, the best GW-944 in the world is produced by Ciba Fine Chemicals Ltd. Its GW-944 has a controlled molecular weight of 2400-3000, with 425 nm light transmittance of 95% and 450 nm light transmittance of 95%. After establishing the three synthesis routes for GW-944 and optimizing the process conditions, the degree of hydrolysis of cyanuric chloride in the reaction can be reduced and the influence of impurities eliminated; not only is the molecular weight range controlled, but the light-transmittance performance is also improved. The author compared the raw material consumption and final product quality indicators of the three routes, with the results shown in Table 1.

        As can be seen from Table 1, the average molecular weight of GW-944 synthesized by the three routes is controlled at 2400-3000, and the light transmittance at 425 nm and 450 nm wavelengths is greatly improved, all greater than 97%, reaching the quality indicators of Ciba's GW-944. This shows that the stepwise substitution of the three chlorine atoms on the cyanuric chloride ring under different conditions can effectively control the polymerization degree during GW-944 synthesis, achieving control over the molecular weight distribution and thereby improving the product's light-transmittance performance. On comprehensive comparison, because the improved two-step synthesis route is simple to operate, has a short process flow, good product-quality control, little "three-waste" (waste gas, water, residue), and is easy to treat, it is more conducive to industrialized production.

    3. Conclusion
    Through experimental research, it is feasible to synthesize the hindered amine light stabilizer GW-944 by the improved two-step, three-step, and four-step routes, using cyanuric chloride, tert-octylamine, and adipic acid piperidine as raw materials. Experiments prove that these three routes can effectively control the molecular weight distribution of the product, and the light-transmittance performance is also greatly improved, reaching the level of similar foreign products. Because the improved two-step synthesis route has a simple operation process, it is more conducive to the industrial production of GW-944.