双峰聚乙烯分子量分布及结构调控的若干途径研究
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摘要
双峰聚乙烯是指分子量呈双峰分布的线性聚乙烯(LPE)和支化聚乙烯(BPE)的共混物。由于双峰聚乙烯能够在许多极端条件下平衡材料的加工性能(低分子量部分)和使用性能(高分子量部分)，目前已经成为聚烯烃合成树脂高性能化的重要方向。
目前生产双峰聚乙烯的工艺主要有串联反应器法和单反应器法。单反应器法相比于串联反应器法具有设备投资低，工艺操作简单，开停车方便，而且高、低分子量产物混合比较均匀(较好的产品性能)等优点，是生产双峰聚乙烯的重要研究方向。
论文根据“Reactor Granule Technology——RGT”(颗粒反应器技术)的设计思想，围绕单反应器生产双峰聚乙烯，开展了以下四个方面的工作：(1)反应器浓度变量振荡操作生产双峰聚乙烯；(2)受限空间下LPE／BPE原位挤出共混(3)新型铁催化剂体系——乙酰丙酮铁／双亚胺基吡啶体系制备双峰聚乙烯；(4)负载型铁催化剂的制备及其聚合特性的考察。具体的工作及创新成果如下：
1．单反应器内氢气浓度振荡操作制备双峰聚乙烯。
(a)氢气振荡操作制备双峰聚乙烯的实验研究。在气相间歇聚合反应釜中采用茂金属催化剂，以氢气振荡为操作手段制备双峰聚乙烯。考察了氢气浓度，振荡时间分配和振荡周期对聚乙烯分子量分布的影响。实验结果表明，氢气的加入使得催化剂活性降低，分子量也降低，通过调节氢气振荡操作的实验条件，尤其是振荡时间分配，可以很好地实现聚乙烯产物分子量及分子量分布的调控。
(b)连续聚合过程中氢气振荡操作制备双峰聚乙烯的模拟研究。以Flory分布函数描述单活性中心的分子量分布，模拟单反应器内氢气振荡操作生产双峰聚乙烯。模拟结果表明，在氢气的振荡操作下可获得分子量呈双峰分布的聚乙烯树脂，并且振荡的可操作性与催化剂的相对氢转移速率常数(氢转移速率常数和链增长速率常数之比K_(trH)/K_p)和氢转移速率常数k_(trH)密切相关。k_(trH)/K_P)和K_(trH)越大，通过改变氢气浓度调节树脂分子量分布越容易实现，而且氢气浓度切换所需时间越短，振荡操作生产双峰聚乙烯可操作性就越强。
2．受限空间下LPE／BPE原位挤出共混。
(a)茂金属／后过渡复合催化剂乙烯均相聚合。以二氯二茂钛(Cp_2TiCl_2)和α-二亚胺溴化镍催化剂(ArN=C(CH_3)-C(CH_3)=NAr，Ar=-2，6-(i-Pr)_2 C_6H_3)(DMN)组成复合催化剂体系，MAO为助催化剂催化乙烯聚合制备LPE／BPE共混物，考察了温度和催化剂摩尔比对复合催化剂的乙烯聚合性能的影响。DSC和SEM分析结果发现，LPE／BPE共混物，BPE支化度较低时能均匀混合，而在BPE支化度较高时共混物存在明显的相分离。
(b)受限空间下LPE／BPE原位挤出共混。以介孔分子筛MCM-41为载体，负载茂金属催化剂(Cp_2TiCl_2)和后过渡金属催化剂α-二亚胺溴化镍(ArN=C(CH_3)-C(CH_3)=NAr，Ar=2，6-(i-Pr)_2C_6H_3)(DMN)，以MAO为助催化剂，催化乙烯聚合制备LPE／BPE共混物。DSC，XRD和SEM等分析结果表明，以介孔MCM—41的纳米孔道为聚合场所，通过LPE／BPE的原位“挤出”共混，改变了共混物的结晶过程，促使两种支化度不同的聚乙烯更均匀的共混，有效地消除了相分离。
3．新型铁催化剂体系——乙酰丙酮铁／双亚胺基吡啶体系制备双峰聚乙烯。
(a)乙酰丙酮铁／双亚胺基吡啶体系聚合特性的考察。本文发现一种新的铁催化剂体系——乙酰丙酮铁和双亚胺基吡啶体系，在MAO作用下能原位形成活性中心并催化乙烯聚合得到分子量呈宽峰／双峰分布的聚乙烯。本文系统考察了聚合条件对7种乙酰丙酮铁／双亚胺基吡啶(2-R_1N=C(Me)-6-R_2N=C(Me)C_5H_3N)(L_1：R_1=R_2=2，6-Me_2C_6H_3；L_2：R_1=R_2=2-Me-6-(i-Pr)C_6H_3；L_3：R_1=R_2=2，6-(i-Pr)_2C_6H_3；L_4：R_1=R_2=2-MeC_6H_4；L_5：R_1=R_2=2-(i-Pr)C_6H_4；L_6：R_1=2-MeC_6H_4，R_2=2，6-(i-Pr)_2C_6H_3；L_7：R_1=cyclohexyl，R_2=2，6-(i-Pr)_2C_6H_3)催化剂体系的聚合活性和产物性能的影响。实验结果表明双亚胺基吡啶在催化剂体系中起着至关重要的作用，单独的Fe(acac)_3在MAO作用下不能催化乙烯聚合。但是Fe(acac)_3和双亚胺基吡啶组成的催化剂体系，在MAO作用下能够原位形成活性中心催化乙烯聚合，得到分子量呈宽峰／双峰分布的聚乙烯。Fe(acac)_3／L_1～L_3催化剂体系的产物为高分子量聚乙烯；Fe(acac)_3／L_4，L_5催化剂体系的产物中既有低聚物，也有高聚物；而Fe(acac)_3／L_6，L_7催化剂体系的产物除了高聚物还有极少量的低聚物。升高聚合温度有利于生成低分子量的聚乙烯，而且配体位阻越小，聚合温度对分子量分布的影响越明显。当使用商业MAO时(含有三甲基铝)，增加Al／Fe摩尔比可以显著提高向铝的链转移反应，产物分子量变小，而且配体位阻越大，Al／Fe摩尔比对产物分子量分布的影响越明显。
(b)多活性中心和向铝链转移共同作用机理的提出。通过聚合条件对双峰聚乙烯分子量分布变化规律的研究，以及不同助催化剂对分布量分布的影响的佐证，本文认为双峰聚乙烯的产生是多活性中心和向铝链转移共同作用的结果。温度变化时，活性中心的变化对分子量分布的影响占主导地位。而且配体位阻越小，形成高分子量聚乙烯的活性中心越不稳定，聚合温度对活性中心分布的影响就越明显。以商用MAO(含有三甲基铝)为助催化剂，改变Al／Fe比时，向铝链转移的变化对分子量分布的影响占主导地位。配体位阻越大，β-H转移越小，向铝链转移起的作用就越明显，Al／Fe摩尔比对分子量分布的影响就越大。以MAO-2(商用MAO去除三甲基铝)为助催化剂，Al／Fe改变时，活性中心的变化对分子量分布的影响占主导地位。Al／Fe比增大，产生了更多的形成高分子量聚乙烯的活性中心，分子量变大。
4．负载型铁催化剂的制备及其聚合特性的考察。为了能使新的催化剂体系在现有的淤浆和气相流化床反应器中使用，本文以硅胶同时负载催化剂体系和助催化剂。鉴于本文催化剂体系的特殊性，当载体先负载MAO然后负载催化剂体系时，负载型催化剂没有活性。本文以活化的Davison 955硅胶先负载乙酰丙酮铁／双亚胺基吡啶催化剂体系，然后负载助催化剂MAO。得到的负载型催化剂，在聚合过程中不再加入助催化剂情况下，仍然能较好的催化乙烯聚合。聚合过程中加压反应器没有粘壁现象。所得聚乙烯都有很高的熔融温度，达到135℃。在聚合温度较低时，分子量超过100×10~4 g·mol~(-1)，接近超高分子量聚乙烯。实验结果表明负载型催化剂的催化活性与乙烯压力呈一次方关系，而产物分子量不随压力改变；聚合温度对负载型催化剂催化活性和产物分子量的影响规律与均相时基本相同，聚合活性在30℃时达到最大值。随着聚合温度升高产物分子量显著降低，而且产物的分子量分布不断变宽。在本文考察的Al／Fe比范围内，随着Al／Fe摩尔比从32增大到59，催化活性显著增大，但是分子量变化很小。研究表明链增长与乙烯压力呈一次方关系，链转移与乙烯压力也呈一次方关系，而且聚合时β-H转移占绝对优势。
Bimodal polyethylene with bimodal molecular weight distribution and bimodal composition distribution include low molecular species for processability and high molecular weight species for properties have gained considerable popularity due to a better balance of processability and properties. Dual reactor systems and single reactor are main bimodal technology. There are many advantages in using single reactor bimodal technology vis-a-vis dual reactor systems. Lower investment costs, intimate mixing of high and low molecular weight components (improved product property).
New technology and novel catalyst system were developed to produce bimodal polyethylene in single reactor based on the concept of "reactor granule technology". The thesis focuses on the four parts as follows:
1. A new process to produce bimodal polyethylene through oscillating operatio in single reactor was presented.
(a) Bimodal polyethylene was produced with the metallocene catalyst in a gas-phase reactor through oscillating operation of hydrogen periodically. Effects of hydrogen concentration, time distribution, oscillating operation period on molecular weight distributions with oscillating operation were studied. It was viable to control molecular weight distributions of bimodal polyethylene through hydrogen oscillating operation.
(b) Bimodal polyethylene production in a continuous gas-phase fluidized bed through oscillating operation of hydrogen periodically was simulated. Flory distribution function was adopted to simulate the molecular weight distributions of single-site activity. Bimodal polyethylene was obtained through oscillating hydrogen concentration periodically. It shows that the feasibility of oscillating operation depended on the values of k_(trH)/kp (the ratio of termination rate constant by chain transfer to hydrogen to the propagation rate constant) and k_(trH) (termination rate constant by chain transfer to hydrogen). The bimodality of polyethylene will be easier to obtain by metallocene catalyst with the higher of k_(trH)/kp and k_(trH). It also shows that the key factor to achieve oscillating operation is the changeover time from high to low hydrogen concentration in the fluidized bed reactor. The shorter the transition time is, the easier of oscillating operation will be.
2. Blend of linear polyethylene (LPE) and branch polyethylene (BPE) by in-situ space-confined polymerization was studied.
(a) Homogeneous polymerization using Cp_2TiCl_2 (Bis(cyclopentadienyl)titanium
Dichloride)and nickel-diimine (ArN=C(CH_3)-C(CH_3)=NAr, Ar=2,6-(i-Pr)_2C_6H_3) (DMN) binary catalysts with methylaluminoxane (MAO) was studied. The effects of polymerization temperature and Cp_2TiCl_2 molar fraction (X_(Ti)) on binary catalysts performances were investigated. DSC and SEM have been used to study the polyethylene microblends, which both indicate that phase separation occurred in the polyethylene obtained at 50°C, while the linear and branched polyethylene obtained at 0°C are uniformly microblends. The GPC analysis shows that both Cp_2TiCl_2 and nickel-diimine have only one type of active site, while the binary catalysts have two, which are almost the same as that of Cp_2TiCl_2 and nickel-diimine.
(b) Microblend of linear LPE/BPE by in-situ space-confined polymerization was studied. Cp_2TiCl_2 and a-nickel diimine catalysts (DMN) were supported on mesoporous particles having parallel hexagonal nanotube pore structure (MCM-41) for ethylene polymerization with methylaluminoxane (MAO) as cocatalyst. DSC, XRD and SEM have been used to investigate phase structures of LPE/BPE blends. Comparing with polyethylene obtained by homogeneous binary catalysts, it was able to blend LPE and BPE to microscale range through in-situ reaction without the need of a compatibilizer.
3. A novel simple catalyst system, iron (III) acetylacetonate and bis(imino)pyridyl ligandmixture activated with methylaluminoxane (MAO) has been found to exhibit high activity for ethylene polymerization.
(a) Effects of polymerization temperature and Al/Fe molar ratio have been systemically investigated on seven iron (III) acetylacetonate (Fe(acac)_3) and bis(imino)pyridyl ligand ( 2-R_1N=C(Me)-6-R_2N=C (Me)C_5H_3N ) ( L_1: R_1=R_2= 2,6-Me_2C_6H_3; L_2: R_1=R_2=2-Me-6-(i-Pr)C_6H_3; L_3: R,=R_2=2,6- (i-Pr)_2C_6H_3; L_4: R_1=R_2=2-MeC_6H_4; L_5: R_1=R_2=2-(i-Pr)C_6H4; L_6: R_1= 2-MeC_6H_4, R_2=2,6- (i-Pr)_2C_6H_3; L_7: R_1= cyclohexyl, R_2=2,6-(i-Pr)_2C_6H_3) catalyst systems. Fe(acac)_3 could not catalyze ethylene polymerization without bis(imino)pyridyl ligand. However, bimodal polyethylene was produced by active species were formed in situ and when Fe(acac)_3/ bis(imino)pyridyl ligand system was activated by MAO. Bis(imino)pyridyl ligand play a major role in the catalyst system. High molecular weight polyethylene was obtained using Fe(acac)_3/ L_1～L_3 catalyst systems, both liquid and solid products were obtained simultaneously using Fe(acac)_3/L_4, L_5 catalyst systems, and polyethylene with a few oligomers were produced with Fe(acac)_3/L_6, L_7 catalyst systems. As polymerization temperature increased, polyethylene with lower molecular weight was obtained. The effect of polymerization temperature on molecular weight distributions increased when decreasing the bulkiness of
ligand. The chain transfer to MAO occurs more easily at higher equiv of Al/Fe, and the effect of Al/Fe molar ratio on molecular weight distributions increase when increasing the bulkiness of ligand when commercial MAO (including Al(Me)3) was used as cocatalyst.
(b) Mechanisms of active species formation and bimodal polyethylene production were investigated. The results indicated that iron metal sites were coordinated with bis(imino)pyridyl ligand according one equiv molar ratio when MAO was added to Fe(acac)_3/ bis(imino)pyridyl ligand catalyst system by detailed study. Several kinds of active species were formed in situ and its action together with transfer to aluminum lead to the polyethylene product with broad/bimodal molecular weight distributions.
4. Supporting of soluble single-site catalysts on preferably inorganic substrates is essential to provide "drop in" catalysts for use in existing technologies for slurry or gas-phase polymerization processes. Support method different with conventional support sequence was used because there was no activity when MAO was supported first. Activated Davision 955 silica gel was used to support compound by sequence of catalyst system and MAO. The supported catalyst has receivable activity when no more cocatalyst was added to the reactor. In all polymerization runs no reactor fouling was found with the supported catalyst. The polyethylene obtained showed high melting temperature and high molecular weight. The increase in activity is linear and demonstrates that the rate of propagation has a first-order rate dependence on ethylene, in accordance with the proposed Cossee-type mechanism. The fact that molecular weight remained essentially invariant with ethylene pressure indicated that the overall rate of chain transfer must also be first order in ethylene and β-H transfer was predominant chain transfer process. Experiments in which the temperature of the polymerization reaction was varied revealed that an increase in temperature results in large decreased in activity and molecular weight. As Al/Fe molar ratio increased from 32 to 59, the activity of supported catalysts increased obviously, while molecular weight remained constant indicated β-H transfer was predominant chain transfer process.
目前生产双峰聚乙烯的工艺主要有串联反应器法和单反应器法。单反应器法相比于串联反应器法具有设备投资低，工艺操作简单，开停车方便，而且高、低分子量产物混合比较均匀(较好的产品性能)等优点，是生产双峰聚乙烯的重要研究方向。
论文根据“Reactor Granule Technology——RGT”(颗粒反应器技术)的设计思想，围绕单反应器生产双峰聚乙烯，开展了以下四个方面的工作：(1)反应器浓度变量振荡操作生产双峰聚乙烯；(2)受限空间下LPE／BPE原位挤出共混(3)新型铁催化剂体系——乙酰丙酮铁／双亚胺基吡啶体系制备双峰聚乙烯；(4)负载型铁催化剂的制备及其聚合特性的考察。具体的工作及创新成果如下：
1．单反应器内氢气浓度振荡操作制备双峰聚乙烯。
(a)氢气振荡操作制备双峰聚乙烯的实验研究。在气相间歇聚合反应釜中采用茂金属催化剂，以氢气振荡为操作手段制备双峰聚乙烯。考察了氢气浓度，振荡时间分配和振荡周期对聚乙烯分子量分布的影响。实验结果表明，氢气的加入使得催化剂活性降低，分子量也降低，通过调节氢气振荡操作的实验条件，尤其是振荡时间分配，可以很好地实现聚乙烯产物分子量及分子量分布的调控。
(b)连续聚合过程中氢气振荡操作制备双峰聚乙烯的模拟研究。以Flory分布函数描述单活性中心的分子量分布，模拟单反应器内氢气振荡操作生产双峰聚乙烯。模拟结果表明，在氢气的振荡操作下可获得分子量呈双峰分布的聚乙烯树脂，并且振荡的可操作性与催化剂的相对氢转移速率常数(氢转移速率常数和链增长速率常数之比K_(trH)/K_p)和氢转移速率常数k_(trH)密切相关。k_(trH)/K_P)和K_(trH)越大，通过改变氢气浓度调节树脂分子量分布越容易实现，而且氢气浓度切换所需时间越短，振荡操作生产双峰聚乙烯可操作性就越强。
2．受限空间下LPE／BPE原位挤出共混。
(a)茂金属／后过渡复合催化剂乙烯均相聚合。以二氯二茂钛(Cp_2TiCl_2)和α-二亚胺溴化镍催化剂(ArN=C(CH_3)-C(CH_3)=NAr，Ar=-2，6-(i-Pr)_2 C_6H_3)(DMN)组成复合催化剂体系，MAO为助催化剂催化乙烯聚合制备LPE／BPE共混物，考察了温度和催化剂摩尔比对复合催化剂的乙烯聚合性能的影响。DSC和SEM分析结果发现，LPE／BPE共混物，BPE支化度较低时能均匀混合，而在BPE支化度较高时共混物存在明显的相分离。
(b)受限空间下LPE／BPE原位挤出共混。以介孔分子筛MCM-41为载体，负载茂金属催化剂(Cp_2TiCl_2)和后过渡金属催化剂α-二亚胺溴化镍(ArN=C(CH_3)-C(CH_3)=NAr，Ar=2，6-(i-Pr)_2C_6H_3)(DMN)，以MAO为助催化剂，催化乙烯聚合制备LPE／BPE共混物。DSC，XRD和SEM等分析结果表明，以介孔MCM—41的纳米孔道为聚合场所，通过LPE／BPE的原位“挤出”共混，改变了共混物的结晶过程，促使两种支化度不同的聚乙烯更均匀的共混，有效地消除了相分离。
3．新型铁催化剂体系——乙酰丙酮铁／双亚胺基吡啶体系制备双峰聚乙烯。
(a)乙酰丙酮铁／双亚胺基吡啶体系聚合特性的考察。本文发现一种新的铁催化剂体系——乙酰丙酮铁和双亚胺基吡啶体系，在MAO作用下能原位形成活性中心并催化乙烯聚合得到分子量呈宽峰／双峰分布的聚乙烯。本文系统考察了聚合条件对7种乙酰丙酮铁／双亚胺基吡啶(2-R_1N=C(Me)-6-R_2N=C(Me)C_5H_3N)(L_1：R_1=R_2=2，6-Me_2C_6H_3；L_2：R_1=R_2=2-Me-6-(i-Pr)C_6H_3；L_3：R_1=R_2=2，6-(i-Pr)_2C_6H_3；L_4：R_1=R_2=2-MeC_6H_4；L_5：R_1=R_2=2-(i-Pr)C_6H_4；L_6：R_1=2-MeC_6H_4，R_2=2，6-(i-Pr)_2C_6H_3；L_7：R_1=cyclohexyl，R_2=2，6-(i-Pr)_2C_6H_3)催化剂体系的聚合活性和产物性能的影响。实验结果表明双亚胺基吡啶在催化剂体系中起着至关重要的作用，单独的Fe(acac)_3在MAO作用下不能催化乙烯聚合。但是Fe(acac)_3和双亚胺基吡啶组成的催化剂体系，在MAO作用下能够原位形成活性中心催化乙烯聚合，得到分子量呈宽峰／双峰分布的聚乙烯。Fe(acac)_3／L_1～L_3催化剂体系的产物为高分子量聚乙烯；Fe(acac)_3／L_4，L_5催化剂体系的产物中既有低聚物，也有高聚物；而Fe(acac)_3／L_6，L_7催化剂体系的产物除了高聚物还有极少量的低聚物。升高聚合温度有利于生成低分子量的聚乙烯，而且配体位阻越小，聚合温度对分子量分布的影响越明显。当使用商业MAO时(含有三甲基铝)，增加Al／Fe摩尔比可以显著提高向铝的链转移反应，产物分子量变小，而且配体位阻越大，Al／Fe摩尔比对产物分子量分布的影响越明显。
(b)多活性中心和向铝链转移共同作用机理的提出。通过聚合条件对双峰聚乙烯分子量分布变化规律的研究，以及不同助催化剂对分布量分布的影响的佐证，本文认为双峰聚乙烯的产生是多活性中心和向铝链转移共同作用的结果。温度变化时，活性中心的变化对分子量分布的影响占主导地位。而且配体位阻越小，形成高分子量聚乙烯的活性中心越不稳定，聚合温度对活性中心分布的影响就越明显。以商用MAO(含有三甲基铝)为助催化剂，改变Al／Fe比时，向铝链转移的变化对分子量分布的影响占主导地位。配体位阻越大，β-H转移越小，向铝链转移起的作用就越明显，Al／Fe摩尔比对分子量分布的影响就越大。以MAO-2(商用MAO去除三甲基铝)为助催化剂，Al／Fe改变时，活性中心的变化对分子量分布的影响占主导地位。Al／Fe比增大，产生了更多的形成高分子量聚乙烯的活性中心，分子量变大。
4．负载型铁催化剂的制备及其聚合特性的考察。为了能使新的催化剂体系在现有的淤浆和气相流化床反应器中使用，本文以硅胶同时负载催化剂体系和助催化剂。鉴于本文催化剂体系的特殊性，当载体先负载MAO然后负载催化剂体系时，负载型催化剂没有活性。本文以活化的Davison 955硅胶先负载乙酰丙酮铁／双亚胺基吡啶催化剂体系，然后负载助催化剂MAO。得到的负载型催化剂，在聚合过程中不再加入助催化剂情况下，仍然能较好的催化乙烯聚合。聚合过程中加压反应器没有粘壁现象。所得聚乙烯都有很高的熔融温度，达到135℃。在聚合温度较低时，分子量超过100×10~4 g·mol~(-1)，接近超高分子量聚乙烯。实验结果表明负载型催化剂的催化活性与乙烯压力呈一次方关系，而产物分子量不随压力改变；聚合温度对负载型催化剂催化活性和产物分子量的影响规律与均相时基本相同，聚合活性在30℃时达到最大值。随着聚合温度升高产物分子量显著降低，而且产物的分子量分布不断变宽。在本文考察的Al／Fe比范围内，随着Al／Fe摩尔比从32增大到59，催化活性显著增大，但是分子量变化很小。研究表明链增长与乙烯压力呈一次方关系，链转移与乙烯压力也呈一次方关系，而且聚合时β-H转移占绝对优势。
Bimodal polyethylene with bimodal molecular weight distribution and bimodal composition distribution include low molecular species for processability and high molecular weight species for properties have gained considerable popularity due to a better balance of processability and properties. Dual reactor systems and single reactor are main bimodal technology. There are many advantages in using single reactor bimodal technology vis-a-vis dual reactor systems. Lower investment costs, intimate mixing of high and low molecular weight components (improved product property).
New technology and novel catalyst system were developed to produce bimodal polyethylene in single reactor based on the concept of "reactor granule technology". The thesis focuses on the four parts as follows:
1. A new process to produce bimodal polyethylene through oscillating operatio in single reactor was presented.
(a) Bimodal polyethylene was produced with the metallocene catalyst in a gas-phase reactor through oscillating operation of hydrogen periodically. Effects of hydrogen concentration, time distribution, oscillating operation period on molecular weight distributions with oscillating operation were studied. It was viable to control molecular weight distributions of bimodal polyethylene through hydrogen oscillating operation.
(b) Bimodal polyethylene production in a continuous gas-phase fluidized bed through oscillating operation of hydrogen periodically was simulated. Flory distribution function was adopted to simulate the molecular weight distributions of single-site activity. Bimodal polyethylene was obtained through oscillating hydrogen concentration periodically. It shows that the feasibility of oscillating operation depended on the values of k_(trH)/kp (the ratio of termination rate constant by chain transfer to hydrogen to the propagation rate constant) and k_(trH) (termination rate constant by chain transfer to hydrogen). The bimodality of polyethylene will be easier to obtain by metallocene catalyst with the higher of k_(trH)/kp and k_(trH). It also shows that the key factor to achieve oscillating operation is the changeover time from high to low hydrogen concentration in the fluidized bed reactor. The shorter the transition time is, the easier of oscillating operation will be.
2. Blend of linear polyethylene (LPE) and branch polyethylene (BPE) by in-situ space-confined polymerization was studied.
(a) Homogeneous polymerization using Cp_2TiCl_2 (Bis(cyclopentadienyl)titanium
Dichloride)and nickel-diimine (ArN=C(CH_3)-C(CH_3)=NAr, Ar=2,6-(i-Pr)_2C_6H_3) (DMN) binary catalysts with methylaluminoxane (MAO) was studied. The effects of polymerization temperature and Cp_2TiCl_2 molar fraction (X_(Ti)) on binary catalysts performances were investigated. DSC and SEM have been used to study the polyethylene microblends, which both indicate that phase separation occurred in the polyethylene obtained at 50°C, while the linear and branched polyethylene obtained at 0°C are uniformly microblends. The GPC analysis shows that both Cp_2TiCl_2 and nickel-diimine have only one type of active site, while the binary catalysts have two, which are almost the same as that of Cp_2TiCl_2 and nickel-diimine.
(b) Microblend of linear LPE/BPE by in-situ space-confined polymerization was studied. Cp_2TiCl_2 and a-nickel diimine catalysts (DMN) were supported on mesoporous particles having parallel hexagonal nanotube pore structure (MCM-41) for ethylene polymerization with methylaluminoxane (MAO) as cocatalyst. DSC, XRD and SEM have been used to investigate phase structures of LPE/BPE blends. Comparing with polyethylene obtained by homogeneous binary catalysts, it was able to blend LPE and BPE to microscale range through in-situ reaction without the need of a compatibilizer.
3. A novel simple catalyst system, iron (III) acetylacetonate and bis(imino)pyridyl ligandmixture activated with methylaluminoxane (MAO) has been found to exhibit high activity for ethylene polymerization.
(a) Effects of polymerization temperature and Al/Fe molar ratio have been systemically investigated on seven iron (III) acetylacetonate (Fe(acac)_3) and bis(imino)pyridyl ligand ( 2-R_1N=C(Me)-6-R_2N=C (Me)C_5H_3N ) ( L_1: R_1=R_2= 2,6-Me_2C_6H_3; L_2: R_1=R_2=2-Me-6-(i-Pr)C_6H_3; L_3: R,=R_2=2,6- (i-Pr)_2C_6H_3; L_4: R_1=R_2=2-MeC_6H_4; L_5: R_1=R_2=2-(i-Pr)C_6H4; L_6: R_1= 2-MeC_6H_4, R_2=2,6- (i-Pr)_2C_6H_3; L_7: R_1= cyclohexyl, R_2=2,6-(i-Pr)_2C_6H_3) catalyst systems. Fe(acac)_3 could not catalyze ethylene polymerization without bis(imino)pyridyl ligand. However, bimodal polyethylene was produced by active species were formed in situ and when Fe(acac)_3/ bis(imino)pyridyl ligand system was activated by MAO. Bis(imino)pyridyl ligand play a major role in the catalyst system. High molecular weight polyethylene was obtained using Fe(acac)_3/ L_1～L_3 catalyst systems, both liquid and solid products were obtained simultaneously using Fe(acac)_3/L_4, L_5 catalyst systems, and polyethylene with a few oligomers were produced with Fe(acac)_3/L_6, L_7 catalyst systems. As polymerization temperature increased, polyethylene with lower molecular weight was obtained. The effect of polymerization temperature on molecular weight distributions increased when decreasing the bulkiness of
ligand. The chain transfer to MAO occurs more easily at higher equiv of Al/Fe, and the effect of Al/Fe molar ratio on molecular weight distributions increase when increasing the bulkiness of ligand when commercial MAO (including Al(Me)3) was used as cocatalyst.
(b) Mechanisms of active species formation and bimodal polyethylene production were investigated. The results indicated that iron metal sites were coordinated with bis(imino)pyridyl ligand according one equiv molar ratio when MAO was added to Fe(acac)_3/ bis(imino)pyridyl ligand catalyst system by detailed study. Several kinds of active species were formed in situ and its action together with transfer to aluminum lead to the polyethylene product with broad/bimodal molecular weight distributions.
4. Supporting of soluble single-site catalysts on preferably inorganic substrates is essential to provide "drop in" catalysts for use in existing technologies for slurry or gas-phase polymerization processes. Support method different with conventional support sequence was used because there was no activity when MAO was supported first. Activated Davision 955 silica gel was used to support compound by sequence of catalyst system and MAO. The supported catalyst has receivable activity when no more cocatalyst was added to the reactor. In all polymerization runs no reactor fouling was found with the supported catalyst. The polyethylene obtained showed high melting temperature and high molecular weight. The increase in activity is linear and demonstrates that the rate of propagation has a first-order rate dependence on ethylene, in accordance with the proposed Cossee-type mechanism. The fact that molecular weight remained essentially invariant with ethylene pressure indicated that the overall rate of chain transfer must also be first order in ethylene and β-H transfer was predominant chain transfer process. Experiments in which the temperature of the polymerization reaction was varied revealed that an increase in temperature results in large decreased in activity and molecular weight. As Al/Fe molar ratio increased from 32 to 59, the activity of supported catalysts increased obviously, while molecular weight remained constant indicated β-H transfer was predominant chain transfer process.
引文
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