通过消除分子链中的缺陷结构提高PMMA树脂的热稳定性和透光性

Poly(methyl methacrylate) (PMMA) is a transparent thermoplastic with highest visible light transmittance, widely used in many sophisticated applications， and has become an important product for the manufactures of thermoplastics. Unfortunately, For a long time, foreign company dominant the PMMA resin market. Our country does not have the technology to produce PMMA. The polymerization of methyl methacrylate is probably the best described polymerization reaction in polymer science. However, the PMMA polymer is not as simple as we expected, the polymer chain of PMMA contain some weak linkages, such as: head-to-head bonds that form by combination termination of two active polymer chains; unsaturated end groups that form by disproportionation termination of two active polymer chains; and the trace amount of leaking oxygen copolymerize into the polymer main chain. What is the exact structure of these weak linkages? How was it formed? How to eliminate it? All these problems have not competely solved yet. It was, therefore, necessary to study the relation of polymerization, weak linkage in the polymer chain, and the properties of end products. In this study, novel polymerization recipe will be designed to prevent the formation of weak linkage, and as a results, it will improve the transmittance and the thermal-stability of the PMMA final products. Furthermore, an in situ thioester antioxidants was designed by using GMA as a co-monomer to copolymerize with MMA and then to react with the chain transfer agent tert-dodecyl mercaptan, this in situ thioester antioxidants can completely eliminate the yellowing of PMMA resin. This study will provide a scientific support to the industrial development of a high-temperature radical polymerization process of PMMA resin.

PMMA是透光性最好的聚合物，但热稳定性差，其原因是PMMA分子链中含有结构缺陷。例如：分子链的不饱和端基、单体单元之间的头头链接、微量氧进入聚合物主链等等。如果不在聚合过程中对这些缺陷加以控制，得到的PMMA树脂在成型加工过程中会发生热分解，损害透光性和力学性能，甚至导致产品变黄。虽然这些结构缺陷的含量极微，但对产品性能却起着决定性作用。这些结构缺陷的确切化学结构是什么？如何产生的？怎样去消除？一直是工业界和学术界关注的问题，至今仍然没有完全搞清楚。本项目将要研究聚合工艺、聚合物链的结构缺陷、终端产品性能之间的关系。本项目将通过对MMA高温连续本体聚合的工艺创新，实现对PMMA分子链结构的控制，防止分子链内结构缺陷的形成，达到提高PMMA树脂产品热稳定性和透光性的目的。本项目还将探索用共聚单体GMA与聚合体系中的链转移剂硫醇反应，原位生成硫醚类大分子抗氧剂，彻底根治PMMA树脂变黄。

聚甲基丙烯酸甲酯（PMMA）因其出色的光学性能，广泛应用各种灯具、照明器材、光学玻璃、光导纤维、导光板、飞机座舱玻璃、飞机和汽车的防弹玻璃等领域。作为一种通用的塑料，PMMA的合成很简单，可以通过各种自由基聚合工艺实现，但合成出高透光性PMMA却并非易事。目前，合成高透光性PMMA最为先进的聚合技术是连续本体聚合技术，其优点是过程连续，产能大，光透光率高，热稳定性好。但技术难度较大，只有日本，德国等的少数企业掌握该技术。国内市场上的PMMA均由这些公司生产，已形成对国内PMMA市场的完全垄断。PMMA作为一个大的塑料品种，在国家的经济建设中具有不可或缺的作用，不可能长期依赖于进口。因此，有必要进行MMA的高温连续本体聚合研究，打破技术垄断，解决国内PMMA产业的技术需求。.甲基丙烯酸甲酯的高温连续本体聚合研究，不仅要开发稳定的聚合工艺，能够生产出PMMA，还要保证生产的PMMA性能优异。由于在项目申报前已完成大量基础性工作，项目执行期间重点研究了聚合工艺 、链结构与产品热稳定性及透光性之间的关系；连续本体聚合宏观动力学及有解聚的高温自由基聚合行为；分子链内热不稳定弱键的结构及成因。通过对这些问题的研究，明确过氧键和末端不饱和双键是导致PMMA热稳定性下降的主要因素；聚合及脱挥过程中残留或渗入氧是导致PMMA发黄的主要原因；聚合过程中氧气氧化产生的低聚物是导致光学性能下降的主要原因。通过反应物预脱氧，控制聚合反应温度，调控配方中的引发剂和链转移剂配比及优化脱挥工艺可以抑制氧对聚合物的氧化、移除PMMA分子链内热不稳定结构制备高热稳定性高透光性的PMMA树脂。获得的PMMA树脂起始热分解温度高达331℃，透光率为92%。.项目的研究成果加深了相关工作者对甲基丙烯酸甲酯高温连续本体聚合过程及产物结构的认识，为甲基丙烯酸甲酯高温连续本体聚合工业奠定了基础，为其他类型烯类单体的连续本体聚合研究提供了理论依据，对推动国内连本体聚合的理论研究具有重要的学术意义。