Utilization of steric hindrance of alkyl lithium-based initiator to synthesize high 1,4 unit-containing hydroxyl- terminated polybutadiene

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.

Understanding and controlling the glass transition of HTPB oligomers

“…More concentrated polymerization solutions, with higher amounts of initiator, achieved increased insertion of v units and reduced insertion of c units. 63 The t, c and v contents of MHTPB samples polymerized in toluene at temperatures between 10°C and 85°C using TBDMSPLi 10% w/w and their HTPB analogues, show a marked trend (ESI, Table S2 †). Thus, the proportion of v units decreases from 37% to 18% across this temperature range, whilst c and t units increase fairly evenly from an aggregate of 63% to 82% (Fig.…”

“…13,[19][20][21][22][23] Two types of initiator can be used: (i) dilithium initiators creating a propagating species with two active centres; 9,24 or (ii) functionalized initiators with a protective group such as t-alkoxy or t-butyldimethylsiloxy which is easily removed, giving a free hydroxyl group. 13,18,[25][26][27] The kinetics and stereochemistry of this polymerization process are strongly contingent on the aggregate states of the initiator and of the propagating species in the polymerization media. 28 These aggregate states are themselves dependent on the structure of the chosen alkyllithium compound 28 and the polarity of the solvent.…”

“…20,21 Historically, the investigation and development of binder materials has been achieved through the costly and time-consuming combination of chemical synthesis and analysis. 2,18,26,[29][30][31][32][33][34][35][36] Whilst effective for honing specific material systems, this approach has not directly addressed more general questions such as: what, at a molecular scale, characterizes a "good" binder? Further, it has systemically limited both the transferability of understanding between systems, and the consideration of design principles with which to guide the development of future binder materials.…”

Kinetic study of HTPB (Hydroxyl Terminated Polybutadiene) Synthesis Using Infrared Spectroscopy

“…(9) then integrated with β = (ktc/kt) and α = (-kdt) to express initial monomer concentration [A]o, expressed in Eq. 13where MA is the molecular formula of monomer butadiene [11].…”

“…This approach resembles the olefin polymerization kinetics for rubber applications [7][8]. The kinetic of radical polymerization of butadiene and other diene compounds and olefins for rubber applications has been explored since 1940 [11][12]. The polybutadiene in rubber applications has hydrogen-ended functional groups, and polymerization usually uses a peroxide initiator, a redox system, and other strong oxidizers.…”