A New View of the Anionic Diene Polymerization Mechanism

We investigated the anionic polymerization of butadiene in d-heptane solvent using tert-butyl lithium as initiator. Two complementary techniques were used to follow the polymerization processes: ¹H NMR and small angle neutron scattering (SANS). The time resolved ¹H NMR measurements allowed us to evaluate quantitatively the kinetics of the processes involved. The initiation event commences slowly and then progressively accelerates. This indicates an autocatalytic mechanism. The microstructure of the first monomer units attached is to a high extent 1,2. The disappearance of initiator --- at about 10% monomer conversion --- signals the onset of the normal ∼6% vinyl content of the chain. Small angle neutron scattering was used to study the aggregation behavior of the carbon lithium head groups. It is well known that the polar head groups aggregate and form micellar structures. For dienes in non-polar solvents the textbook mechanism assumes the formation of only tetramers during the propagation reaction. By combining ¹H NMR and SANS results we were able to determine quantitatively the aggregation number during all stages of the polymerization. Our measurements show the existence of large-scale structures during the initiation period. The initial degree of aggregation of more than 100 living polymer chains diminished as the polymerization progressed. In addition, even larger, giant structures with N_agg >>1000 and R_g ≈ 1000Å were found.     

顺1,4-聚丁二烯在不同溶剂中的环化反应及其机理的研究

以烯丙基氯 一氯二乙基铝 (CH2 CHCH2 Cl AlEt2 Cl)催化体系研究了顺 1 ,4 聚丁二烯在不同溶剂(三甲苯、二甲苯、甲苯、苯和环己烷 )中的环化反应 .结合环化产物红外光谱的测定 ,提出芳烃溶剂分子经受阳碳离子亲电取代作用而参与环化的机理 ,以此解释了若干实验事实 ,并对已有文献报道进行了讨论 .     

羟基封端聚丁二烯的临界点色谱定量方法研究

为建立混合物中羟基封端聚丁二烯（HTPB）的临界点色谱定量方法，以C18为固定相，考察了HTPB在四氢呋喃-乙腈和四氢呋喃-水两种流动相体系下的临界点色谱条件。结果表明，四氢呋喃-乙腈体积比为70.7：29.3以及四氢呋喃-水体积比为92：8时，HTPB的保留值与其相对分子质量无关。将该临界点色谱方法用于胶黏剂混合组分中HTPB的测定，结果表明，HTPB在46.7~216.4 mg/L范围内呈良好线性关系，相关系数（r）>0.99，检出限为4.2 mg/L。自配HTPB和矿物油双组分样品的加标回收率为89.2%~101.1%，相对标准偏差小于0.66%（n=6）。应用该方法对市售聚氨酯胶黏剂进行检测，测定结果为HTPB成分占26.6%。该方法快速、准确，能满足聚合物混合产品的生产质量控制和失效分析的要求。     

Structural characterization of polybutadiene synthesized via cationic mechanism

The microstructure of polybutadiene synthesized via cationic polymerization using TiCl₄‐based initiating systems has been investigated using 1D (¹Н, ²Н, and ¹³С) and 2D (HSQC and HMBC) NMR spectroscopy. It was found that trans‐1,4‐unit is predominant structure of unsaturated part of polymer chain. Besides, the small amount of 1,2‐structures was also detected, while cis‐1,4‐units were totally absent. The signals of carbon atoms of three types of head groups (trans‐1,4‐, 1,2‐, and tert‐butyl) and two types of end groups (trans‐1,4‐Cl and 1,2‐Cl) were identified for the first time in macromolecules of cationic polybutadiene. It was showed that tert‐butyl head groups were formed due to the presence in monomer of admixtures of isobutylene. The new methodology for calculation of the content of different structural units in polybutadiene chain as well as the head and end groups was proposed. It was established that main part of 1,2‐units distributed randomly along the polybutadiene chain as separate units between trans‐1,4‐structures. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 387–398     

Skewered reactions in free‐radical crosslinking (co)polymerizations of liquid polybutadiene as internal olefinic multivinyl crosslinker

As an extension of our continuing studies concerned with the mechanistic discussion of network formation in the free‐radical crosslinking (co)polymerization of multivinyl monomers, this work refers to the skewered reactions in the crosslinking (co)polymerizations of liquid polybutadiene rubber (LBR) as an internal olefinic multivinyl monomer or crosslinker, especially focused on the competitive occurrence of both addition or skewered reaction to internal carbon–carbon (CC) double bonds and abstraction reaction of allylic hydrogens in LBR by growing polymer radical. Thus, LBR is regarded as an internal olefinic multiallyl monomer‐linked allyl groups (? CH?CH? CH₂? ) with methylene units (? CH₂? ). First, gelation in the polymerization of LBR was explored in detail, especially at elevated temperatures. The occurrence of intermolecular crosslinking was easier in the order LBR > LBR containing 20 mol % of 1,2‐structural units > liquid polyisoprene rubber. Then, we pursued the polymerization of LBR using dicumyl peroxide (DCPO) as typical organic peroxide used at elevated temperatures. The primary cumyloxy radical generated by the thermal decomposition of DCPO may add to CC double bond or abstract allylic hydrogen or undergo β‐scission to generate a secondary methyl radical. The initiation by the cumyloxy radical was omitted. The ratio of allylic hydrogen abstraction to β‐scission reaction was estimated; thus, only 39% of cumyloxy radical was used for the allylic hydrogen abstraction reaction. The addition of methyl radical to CC double bond was clearly observed. Finally, we pursued the intermolecular and intramolecular skewered reactions in free‐radical crosslinking LBR/vinyl pivalate copolymerizations. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Influence of Prestrain on the Mechanical Properties and Constitutive Model of Hydroxyl-Terminated Polybutadiene Films as Used for Coatings in Solid Rocket Motors

Low-field ¹H NMR tests and uniaxial tensile tests of hydroxyl-terminated polybutadiene (HTPB) films, subjected to a prestrain before the NMR and tensile tests related to the use of the HTPB as coatings for solid rocket motors, were carried out. The mechanisms of the variations of crosslinking density, tensile strength and maximum elongation of the HTPB films with prestrain were analyzed, and a Gaussian function model of the variation of the crosslinking density with prestrain was obtained. By introducing crosslinking density and prestrain as correction coefficients, two prestrain constitutive models of the stress-strain relationship of HTPB films related to the Mooney-Rivlin and Ogden hyperelastic constitutive models, respectively, were established and the parameters of the models were obtained based on nonlinear least squares fitting. The accuracy and effectiveness of the prestrain constitutive models were determined with that based on the Ogden model proven to be of important engineering value.     

Synthesis of telechelic anthraquinone‐functionalized polybutadiene via ROMP and study of its photo‐oxidation and UV crosslinking behaviors

Telechelic anthraquinone‐functionalized polybutadiene (AQ‐PB‐AQ) was synthesized by ring opening metathesis polymerization to achieve homogeneous dispersion of AQ groups in the polymer matrix. It was observed that the AQ end groups act as a sensitizing group resulting in photo‐induced crosslinking and oxidation reaction at the olefin groups in the PB. The process was confirmed by comparing the polymer's mass loss and Fourier transform infrared (FTIR) spectra with those of telechelic diacetoxy‐functionalized PB (AcO‐PB‐OAc) which did not show any difference before and after ultraviolet irradiation. Homogeneity of the AQ groups in PB matrix results in rapid crosslinking of PB in a short period of time (<4 min) while a simple blend of AQ methyl ester in AcO‐PB‐OAc matrix did not show such behavior. Photo‐oxidative reaction has concurrently occurred during the crosslinking reaction. Generation of  OH,  OOH, and carbonyl groups in the matrix during the photo‐oxidation was verified by FTIR, elemental analysis, and water contact angle measurements. A plausible mechanism for the process was proposed in which the photo‐sensitized AQ groups abstract hydrogen from the PB chain to yield reactive radical species, initiating oxidative crosslinking, and degradation of PB or reduction of AQ to hydroanthraquinone species. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1249–1258     

The nearly constant loss,Johari‐Goldstein β‐relaxation,and α‐relaxation of 1,4‐polybutadiene

From high‐resolution dielectric spectroscopy measurements on 1,4‐polybutadiene (1,4‐PB), we show that in addition to the structural α‐relaxation and higher frequency secondary relaxations in the spectra, a nearly constant loss (NCL) is observed at shorter times/lower temperatures. The properties of this NCL are compared to those of another chemically similar polymer, 1,4‐polyisoprene. The secondary relaxations in 1,4‐PB include the well‐known Johari‐Goldstein (JG) β‐relaxation and two other higher‐frequency peaks. One of these, referred to as the γ‐relaxation, falls between the JG‐relaxation and the NCL. Seen previously by others, this γ‐relaxation in 1,4‐PB is not the JG‐process and bears no relation to the glass transition. At very low temperatures (<15 K), we confirm the existence of a very fast secondary relaxation, having a weak dielectric strength and an almost temperature‐invariant relaxation time. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 342–348, 2007