Abstract
A novel alkyl lithium-based initiator with relatively large steric hindrance, tert-butyldimethylsiloxydimethylpropyl lithium (TBDMSODPrLi), was designed and synthesized. By using TBDMSODPrLi, hydroxyl-terminated polybutadiene (HTPB) was prepared via anionic polymerization. The macromolecular structure of HTPB was characterized and verified by FTIR and 1H-NMR. It was found that 1,4 unit content in HTPB initiated by TBDMSODPrLi was significantly higher (over 90%) compared to a HTPB (1,4 unit content of 70%) initiated with another initiator possessing smaller steric hindrance. The possible mechanism, which was based on initiator steric hindrance affecting monomer chain addition behaviour, was deduced. It was that the initiator's larger steric hindrance blocked lithium's intermolecular association during anionic polymerization; as a result, it could effectively increase the 1,4 unit content in HTPB. To further study how to obtain higher and stable 1,4 unit content, the optimal anionic polymerization technique for HTPB was explored including polymerization temperature, time and the amount of initiator used. The study concluded that utilization of an initiator with larger steric hindrance and reducing the polymerization temperature were two important factors to raise the 1,4 unit content in HTPB.
Highlights
Hydroxyl-terminated polybutadiene (HTPB) is an important liquid rubber with high transparency, low viscosity, ageing2018 The Authors
The synthesis routes of both TBDMSODPrCl and TBDMSODPrLi are shown in figure 3
To increase the branching coefficient of initiator, methyl was introduced in the structure of the initiator, which could raise the steric hindrance of initiator and reduce the end chain association
Summary
Hydroxyl-terminated polybutadiene (HTPB) is an important liquid rubber with high transparency, low viscosity, ageing. There are two methods to block the initiator’s association: one is addition of Lewis alkali such as tetrahydrofuran (THF) This method can weaken the binding force between carbon anion and lithium, the ability to control the end chain is reduced, and the content of 1,4 unit could not be effectively increased. The other method is to increase the steric hindrance of alkyl lithium initiator, and utilize the 3 geometrical size of the initiator to stop end chain association. It needs at least 22 h to synthesize the initiator to guarantee the conversion of TIPSODMPrCl, which is inefficient for industrialization Based on this analysis, in this article, a novel initiator, tert-butyldimethylsiloxydimethylpropyl lithium (TBDMSODPrLi), with high steric hindrance was designed and synthesized by low-cost materials in an efficient way. To select a stable and systematic technique for carrying out HTPB preparation, the preparation process, including the effect of polymerization temperature, time and amount initiator used, was explored, and an optimal synthesis technique was determined
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