负载型Ziegler-Natta催化剂的乙烯、1-已烯聚合体系机理研究
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摘要
过去几十年来,氯化镁负载型钛系催化剂在聚烯烃工业生产中起到了重要的作用。国内外学者对活性中心和聚合机理也开展了深入研究,并提出了许多机理模型。但是有关乙烯-α-烯烃共聚机理,以及内给电子体和氢调等因素影响活性中心结构和性质的微观机理还有一些关键问题没有搞清楚。本论文主要围绕这些问题开展基础研究。
为了考察内给电子体对催化剂性质的影响,首先利用改进的共沉淀法,原位合成了含有酯类和醚类内给电子体的三种MgCl_2载体醇合物和无内给电子体的MgCl_2载体醇合物,然后利用二次载钛的工艺制备了相应的TiCl_4负载型催化剂。利用这些催化剂研究了:(1)不同内给电子体对乙烯及1-已烯聚合及其共聚反应的影响;(2)建立了一种能够直接、准确的测量活性中心数目的方法—肉桂酰氯淬灭测活性中心数目的方法;(3)不同内给电子体对活性中心分布和性质的影响;(4)不同共单体对聚合特性和活性中心的影响;(5)氢气对聚合特性和活性中心的影响;(6)烷基铝对聚合特性和活性中心的影响等。目的是探索调控Ziegler-Natta催化剂的活性中心分布和聚合物分子量分布、组成分布的新途径,掌握调控原理,为开发线型低密度聚乙烯(LLDPE)和聚丙烯(PP)新产品提供依据。
在制备醇合物的过程中,加入的苯酐(PA)与乙醇原位反应生成了邻苯二甲酸二乙酯(DEP),DEP起到了内给电子体的作用。DEP(PA)的存在降低了催化剂的钛含量;而苯甲醚(Anisole)的存在却未降低钛含量。制备的四种催化剂分别是TiCl_4/MgCl_2(Cat-1),TiCl_4/DEP/MgCl_2(Cat-2),TiCl_4/Anisole/MgCl_2(Cat-3),TiCl_4/(DEP,Anisloe)/MgCl_2(Cat-4)。在本论文中,所用催化剂为以上四种催化剂和DQ催化剂(含二酯类),助催化剂为三乙基铝(TEA)和三异丁基铝(TIBA);采用淤浆聚合,聚合温度范围0～70℃之间,乙烯压力0.11～0.3 Mpa之间。在乙烯均聚及乙烯与1-已烯共聚反应中,内给电子体的加入提高了催化剂活性;使聚乙烯和乙烯与1-已烯共聚物的熔点提高,结晶度降低(特别是DEP加入催化剂后这种效果更加明显),能提高共聚物的不溶级分(HI)中共单体的含量,使共聚物组成分布变窄,共聚物链结构的嵌段性增强。
建立了一种能够直接、准确地测量活性中心数(C_p)的方法-肉桂酰氯淬灭直接测活性中心浓度法。通过~1H-NMR检测淬灭产物链端基团及分析淬灭条件对产物中酰基含量的影响,验证了此方法的反应机理和可靠性。利用此方法测量了内给电子体对1-已烯聚合体系活性中心浓度的影响,发现内给电子体的加入导致了活性中心数目有不同程度的增多,特别是Anisole的加入使活性中心数出现最大幅度的增加。同时发现,共单体、氢气的加入都会对聚合中的活性中心数有明显的增强作用;助催化剂浓度、温度的变化也能使聚合中的活性中心数发生显著变化,进而对到聚合的活性和产物的性质产生影响。
内给电子体对聚合产物的分子量及分子量分布有明显的影响。加入内给电子体后,聚乙烯和聚1-已烯的重均分子量都有不同程度的增大。对MWD曲线进行Flory分峰拟合,发现加入内给电子体并没有改变活性中心种类,但改变了各类活性中心的分子量大小及分布,导致Ⅰ、Ⅱ型的高分子量的活性中心的含量和分子量增大。对酰氯淬灭的聚合产物进行沉淀分级并测定各级分的活性中心数,通过各级分的活性中心数与其分子量的关系发现,DEP的存在导致高分子量部分的活性中心数明显增多,并且减少了低分子量的活性中心数;当Anisole作为内给电子体时,低分子量部分的活性中心数有明显增多。
分别研究了1-已烯和乙烯作为共单体时,加入到乙烯和1-已烯聚合体系中产生的影响。发现在乙烯或1-已烯聚合中,少量的共单体的加入均能使催化剂活性增加,共聚物的分子量也有不同程度的下降。其他条件不变时,增加共单体的浓度使催化剂的活性逐渐提高。在1-已烯聚合中,加入少量乙烯能增加聚合的活性中心数(C_p),且C_p随着乙烯分压的增加而逐渐增多,而增加的活性中心大多是产生低分子量产物的活性中心。不论是1-已烯均聚物还是1-已烯与少量乙烯的共聚物,级分的等规度都是随着分子量的增加而逐渐增加。加入乙烯后,在高分子量部分的等规度会有所降低。乙烯单元在不同级分中的含量是不同的,在低分子量级分以及高分子级分中含量较高,在中间分子量级分中含量相对较低,这表明乙烯倾向于在生产高分子量或低分子量的活性中心上配位。
内给电子体在不同共聚体系中的作用有明显差别。在乙烯为主的共聚体系中,内给电子体增加了低分子量活性中心(Ⅲ～Ⅴ)的位阻,使其不易进入休眠态,减弱了共聚中共单体活性增强的作用,并能缓解共聚产物分子量下降的程度。而在1-已烯为主的共聚体系中,内给电子体提高了催化剂的立体定向能力,使2,1-反插减少,导致可激活的休眠种相对减少,以上作用缓解了共聚中活性中心数增加的幅度,减弱了催化剂活性增强的程度,并加剧了共聚产物分子量下降的程度。
氢气是烯烃聚合中应用最广泛的链转移剂,对不同的烯烃聚合反应产生不同的作用,并影响产物的结构。在乙烯聚合体系中,加氢后聚合活性和产物的分子量都明显下降,分子量分布变窄;而内给电子体的存在加剧了催化剂活性下降和产物分子量下降的程度。而在1-已烯聚合体系中,加入少量的氢气使聚合活性增加,产物的分子量下降,分子量分布变宽;并随氢气浓度的增加,聚合活性逐渐增大并趋于稳定,分子量逐渐下降,达到最低值后有小幅度提高。发现加入少量的氢气能明显增加聚合中活性中心数,并随着加入量的增多,逐渐达到最大值;在高氢气量下,活性中心数会有所降低。加氢对聚乙烯和聚1-已烯两种产物分子量及活性中心分布影响是不同的。加氢降低了聚乙烯的高分子量的活性中心的含量和平均分子量;提高了低分子量的活性中心(Ⅲ～Ⅴ型)的含量和平均分子量,导致产物的分子量分布变窄。而在1-已烯聚合中,氢气的存在可能阻碍了1-已烯单体对Ⅰ型活性中心Ti-H键的插入,使这部分活性中心失活;同时产生了一种分子量很低、含量较高的活性中心(Ⅴ型),导致产物的分子量下降,产物的PDI变宽。
助催化剂烷基铝浓度的改变对烯烃聚合及活性中心分布有显著影响。研究表明,随着Al/Ti比的增加,1-已烯的聚合活性先增加后降低,产物的分子量逐渐下降,并趋于稳定;1-已烯聚合中活性中心数目先是逐渐增加,达到最大值后有小幅度下降,增加的活性中心主要是低分子量的活性中心(Ⅳ和Ⅴ型)。对MWD曲线进行Flory分峰拟合,发现当烷基铝在低浓度变化(Al/Ti=30-200之间)时,高分子量的活性中心(Ⅰ、Ⅱ型)含量随烷基铝浓度的增加而急剧下降,而低分子量的活性中心(Ⅳ、Ⅴ型)含量会大幅度提高。当高烷基铝用量时(Al/Ti摩尔比超过200),高分子量的活性中心有所增加,导致聚合物的分子量在Al/Ti=300时的分子量反而增大。
根据实验结果提出了一个活性中心分类及催化机理模型,按照活性中心的性质、共聚能力、立体定向性以及动力学等性质,把活性中心分成两组来讨论。详细探讨了在烯烃均聚和共聚体系中两组活性中心的性质和变化,描述了内给电子体、共单体、氢气等同这些活性中心之间的作用情况。这一模型能很好地解释大部分实验规律,希望能为负载型Ziegler-Natta催化体系的改进和创新提供理论支撑。
本论文的主要创新点:
1.建立了一种能够直接、准确地测量Ziegler-Natta催化体系活性中心数的方法-肉桂酰氯淬灭直接测活性中心浓度法。通过~1H-NMR检测淬灭产物链端基团及分析淬灭条件对产物中酰基含量的影响,验证了此方法的反应机理和可靠性,并利用此方法研究了内给电子体、共单体、氢气及烷基铝等因素对催化体系活性中心数、活性中心分布及性质的影响。
2.系统地考察了TiCl_4/MgCl_2类催化剂中内给电子体的存在与否及其结构变化对其催化乙烯与1-已烯共聚反应的影响。内给电子体的存在增加了乙烯聚合物的分子量和共聚物不溶级分的含量,使乙烯与1-已烯共聚物的沸腾正庚烷不溶级分中1-已烯含量提高,并使该级分中[HH]结构含量增大,共聚物链结构嵌段性增强。不同类型的内给电子体表现出对共聚体系不同的影响。发现二酯类内给电子体导致可溶级分含量显著降低,α-烯烃结合量降低;而含单醚类内给电子体的催化剂所得乙烯-1-已烯共聚,产物可溶级分含量变化不大,但α-烯烃结合量提高。
3.通过对聚合物MWD的Flory分峰拟合分析,酰氯淬灭法测定活性中心数、聚(1-已烯)沉淀分级级分的活性中心数测定等研究,系统、深入地研究了内给电子体影响活性中心分布和性质的规律,提出了其作用机理。研究发现,在乙烯为主的共聚体系中,内给电子体增加了Group B中心的位阻,使其不易进入休眠态,减弱了共聚中共单体活性增强的作用,并能缓解共聚产物分子量下降的程度。而在1-已烯为主的聚合体系中,内给电子体提高了催化剂的立体定向能力,使2,1-反插减少,导致可被乙烯激活的休眠种相对减少。在1-已烯聚合时,向TiCl_4/MgCl_2类催化剂加入二酯类内给电子体并未使其五种活性中心的本征活性发生显著改变,但明显改变了活性中心分布,使合成高分子量产物的活性中心数目量增大。而单醚类内给电子体使合成低分子量产物的活性中心相对增多,同时使合成中高分子量产物的活性中心的本征活性有所降低。这些发现为正确认识给电子体的作用机理提供了重要线索。
4.通过对加氢产物的MWD的Flory分峰拟合分析,酰氯淬灭法测定活性中心数、聚(1-已烯)沉淀分级级分的活性中心数测定等研究,系统、深入地研究了氢气影响活性中心分布和性质的规律,提出了其作用机理。在乙烯聚合体系中,少量氢气对高分子的活性中心(Ⅰ、Ⅱ型)起到失活作用;对低分子量的活性中心(Ⅲ～Ⅴ型)有活化作用,将休眠态的中心释放出来。这造成了加氢后催化剂活性降低,产物分子量下降,分子量分布变窄。而对于1-已烯体系,少量的氢气解放了中等位阻活性中心的2,1-反插休眠态,活化了Ⅱ型活性中心;并使部分低价态的活性中心变成了对1-已烯有活性的中心(Ⅴ型),导致了加氢之后活性中心数大大增多,产物分子量下降,并使产物的PDI变宽。这些发现为正确认识氢气的作用机理提供了重要依据。
5.通过对聚合物MWD的Flory分峰拟合分析,酰氯淬灭法测定活性中心数等研究,研究了助催化剂浓度影响1-已烯聚合体系活性中心分布和性质的规律。在1-已烯聚合中,烷基铝浓度的改变对聚合行为和产物有明显的影响。低的铝钛比(Al/Ti=30)生成了少量的本征活性很高的Ti~(3+)中心;随着Al/Ti比(30～150)的增加,活性中心数增加明显,但新增的活性中心k_p值相对较低。当在高的Al/Ti比(＞150)时,部分产生低分子量的Ti~(3+)被还原成无活性的Ti~(2+),导致其k_p值反而增加。由于过量的烷基铝会与活性中心发生络合作用,造成在高的烷基铝浓度下产物的分子量反而增加。这些发现为正确认识助催化剂浓度对聚合的作用机理提供了重要依据。
6.研究了共单体(主要是乙烯加入1-已烯体系)对活性中心分布和性质的影响,发现加入少量的乙烯对1-已烯聚合行为产生影响。在1-已烯聚合中,加入少量的乙烯也会使聚合活性增大,且活性并随乙烯压力的增加而逐渐增大,产物的分子量减小。发现乙烯引起的活性增加是由于活性中心数增加所致,但增加的活性中心大多是活性较低、产物分子量较低的活性中心,这也使聚合体系中活性中心的平均活性降低。乙烯在不同沉淀分级级分中的含量不同,分布不均匀,在低分子量级分及高分子量级分中含量相对较高。这些发现为正确认识共单体对1-已烯聚合的作用提供了重要线索。
In the past several decades,supported catalysts of the TiCl_4/ID/MgCl_2 type were widely used in polyolefin industry.However,questions concerning the mechanism of ethylene copolymerization withα-olefins,the effect of internal electron donor(ID) and hydrogen on the structure and nature of active centers have not yet found satisfactory answers.In this paper,some work was carried out in these directions.
Using four kinds of TiCl_4/MgCl_2 catalysts[Cat-1(TiCl_4/MgCl_2),Cat-2(TiCl_4/diethyl phthalate(DEP)/MgCl_2),Cat-3(TiCl_4/Anisole/MgCl_2),Cat-4(TiCl_4/(DEP,Anisole)/MgCl_2)] and DQ Catalyst(TiCl_4/dibutyl(o-)phthalate/MgCl_2),we carried out following studies in this paper:(1) the role of internal electron donor(ID) on ethylene homo-polymerization and copolymerization;(2) A new method for measuring the number of active centers was developed,based on use of cinnamoyl chlorides;(3) the effect of IDs on distribution of active centers;(4) the role of H_2 on polymerization and distribution of active centers;(5) the effect of AlR_3,polymerization temperature on polymerization and distribution of active centers.It was attempted to find the new approach to accommodate the ACD(active center distribution) of Ziegler-Natta catalyst and the MWD(molecular weight distribution),CD (composition distribution) of polyolefins products,and provide the useful information of developing new linear low density polyethylene(LLDPE) and polypropylene(PP) products.
In the preparation of the supported catalysts,MgCl_2·nEtOH/ID adducts as the precursor of the support were prepared in situ by novel co-precipitation method.Four kinds of MgCl_2·nEtOH/ID adducts were prepared in situ by novel co-precipitation method,and the catalysts of TiCl_4 anchored on the supports were prepared.FT-IR spectra of the catalysts and PA as well as DEP were also recorded.DEP was found to be the actual ID existing in the final catalyst prepared based on MgCl_2·nEtOH/PA.Titanium content of the catalysts containing different internal donors is different:DEP decreases the Ti content and Anisole as internal donor didn't decreases the Ti content.
Four kinds of catalysts(Cat-1,Cat-2,Cat-3 and Cat-4) were applied to ethylene polymerization and ethylene-co-1-hexene copolymerization.The addition of IDs increased activity of catalyst and the T_m of products,especially when the catalyst contains DEP as ID. 1-hexene content of the HI(boiling n-heptane insoluble) fraction increased,the difference in 1-hexene content of the HI fraction and the HS fraction decreased,and the blockiness of HI fractions increased.
A new method for determining the number of active centers(C_p) in supported Ziegler-Natta catalysts based on quenching by cinnamoyl chlorides was developed.The optimal quenching conditions were found to be:n(Cinnamoyl chloride/Al)=3～5;t_(quenching)=10 min;T_(quenching)=20℃～40℃.The change of C_p by adding IDs in the catalyst were studied by this method.The introduction of IDs,especially anisole,increased the number of active center in 1-hexene polymerization.
It was found that the introduction of IDs in the catalyst during its preparation can enhance the molecular weight(MW).The Flory non-linear-fitting of products MWD curves showed that the addition of ID didn't change the type of active centers,but change the ACD (active center distribution),increase both the MW and the number of active centerⅠandⅡ. From distribution of the number of active centers determined by quenching and fractionation, the addition of PA increases the amount and C_p of fraction with high molecular weight.
In this paper,the change from homo-polyethylene to ethylene copolymerization with small amount of 1-hexene,from homo-poly(1-hexene) to 1-hexene copolymerization with small amount of ethylene were studied,respectively.Adding small amount of 1-hexene in ethylene polymerization caused marked activation of the low molecular weight components of the polymer.In 1-hexene polymerization system,the activity can also be greatly enhanced by introducing small amount of ethylene.The total number of active centers is markedly increased by adding small amount of ethylene in 1-hexene polymerization,but the average activity of the active centers decreased.The broad composition distribution of the ethylene-1-hexene copolymer can be well understood from the ACD of catalyst and its dependence on the monomer.Adding small amount of ethylene markedly increase the number of active centers that produce polymer with low molecular weight.There is a nearly linear increase of isotacticity with molecular weight of the polymer fraction obtained by precipitationed fraction.However,the addition of small amount of ethylene in 1-hexene polymerization decreased the isotacticity of the fractions with high molecular weight.The distribution of ethylene content in fractions was broad.The content of ethylene in both the low MW and high MW fractions were higher than that of others.So,ethylene tends to be incorporated in both the low MW and high MW active centers in 1-hexene polymerization.
Hydrogen(H_2) has been used as a standard chain transfer agent in industrial polyolefin production.Hydrogen played different roles in ethylene and propylene/α-olefin polymerization.In ethylene polymerization system,the addition of small amount of hydrogen markedly decreased catalyst efficiency and the molecular weight of polymers,and increased the polydispersity index(PDI) of polymers.In 1-hexene polymerization system, the addition of small amount of hydrogen caused a marked increased in polymerization rate and a decrease in the molecular weight of polymers.The total number of active centers is markedly increased by adding small amount of H_2 in 1-hexene polymerization.The Flory non-linear-fitting of polyethylenes MWD curves showed that the addition of H_2 decreased both the M_w and the amount of active centerⅠandⅡ,meanwhile increased both the MW and the amount of active centerⅢ～Ⅴ,which caused decrease in PDI.In 1-hexene polymerization, however,a new type of active center(Ⅴ) with low molecular weight product emerged after adding H_2,which decreased the MW and broaden the PDI of the products.
In 1-hexene polymerization,the change of cocatalyst concentration markedly influences the polymerization behaviors and distribution of active centers.With the concentration of TEA rising,the catalyst activity increased and the molecular weight of products decreased gradually.When the ratio of Al/Ti rising at low TEA concentration,the number of active centers is increased,which mainly increase the active centers producing polymer with low molecular weight.
A new model of two group active centers in polymerization was proposed in this paper, in which the interactions between active centers and IDs,comonomer,hydrogen were elaborated.This model can exactly explain the results in this paper.This model afford deep understand of mechanism of olefin polymerization and provide a new approach to improving and modifying the performance of the supported Ziegler-Natta catalyst.
为了考察内给电子体对催化剂性质的影响,首先利用改进的共沉淀法,原位合成了含有酯类和醚类内给电子体的三种MgCl_2载体醇合物和无内给电子体的MgCl_2载体醇合物,然后利用二次载钛的工艺制备了相应的TiCl_4负载型催化剂。利用这些催化剂研究了:(1)不同内给电子体对乙烯及1-已烯聚合及其共聚反应的影响;(2)建立了一种能够直接、准确的测量活性中心数目的方法—肉桂酰氯淬灭测活性中心数目的方法;(3)不同内给电子体对活性中心分布和性质的影响;(4)不同共单体对聚合特性和活性中心的影响;(5)氢气对聚合特性和活性中心的影响;(6)烷基铝对聚合特性和活性中心的影响等。目的是探索调控Ziegler-Natta催化剂的活性中心分布和聚合物分子量分布、组成分布的新途径,掌握调控原理,为开发线型低密度聚乙烯(LLDPE)和聚丙烯(PP)新产品提供依据。
在制备醇合物的过程中,加入的苯酐(PA)与乙醇原位反应生成了邻苯二甲酸二乙酯(DEP),DEP起到了内给电子体的作用。DEP(PA)的存在降低了催化剂的钛含量;而苯甲醚(Anisole)的存在却未降低钛含量。制备的四种催化剂分别是TiCl_4/MgCl_2(Cat-1),TiCl_4/DEP/MgCl_2(Cat-2),TiCl_4/Anisole/MgCl_2(Cat-3),TiCl_4/(DEP,Anisloe)/MgCl_2(Cat-4)。在本论文中,所用催化剂为以上四种催化剂和DQ催化剂(含二酯类),助催化剂为三乙基铝(TEA)和三异丁基铝(TIBA);采用淤浆聚合,聚合温度范围0～70℃之间,乙烯压力0.11～0.3 Mpa之间。在乙烯均聚及乙烯与1-已烯共聚反应中,内给电子体的加入提高了催化剂活性;使聚乙烯和乙烯与1-已烯共聚物的熔点提高,结晶度降低(特别是DEP加入催化剂后这种效果更加明显),能提高共聚物的不溶级分(HI)中共单体的含量,使共聚物组成分布变窄,共聚物链结构的嵌段性增强。
建立了一种能够直接、准确地测量活性中心数(C_p)的方法-肉桂酰氯淬灭直接测活性中心浓度法。通过~1H-NMR检测淬灭产物链端基团及分析淬灭条件对产物中酰基含量的影响,验证了此方法的反应机理和可靠性。利用此方法测量了内给电子体对1-已烯聚合体系活性中心浓度的影响,发现内给电子体的加入导致了活性中心数目有不同程度的增多,特别是Anisole的加入使活性中心数出现最大幅度的增加。同时发现,共单体、氢气的加入都会对聚合中的活性中心数有明显的增强作用;助催化剂浓度、温度的变化也能使聚合中的活性中心数发生显著变化,进而对到聚合的活性和产物的性质产生影响。
内给电子体对聚合产物的分子量及分子量分布有明显的影响。加入内给电子体后,聚乙烯和聚1-已烯的重均分子量都有不同程度的增大。对MWD曲线进行Flory分峰拟合,发现加入内给电子体并没有改变活性中心种类,但改变了各类活性中心的分子量大小及分布,导致Ⅰ、Ⅱ型的高分子量的活性中心的含量和分子量增大。对酰氯淬灭的聚合产物进行沉淀分级并测定各级分的活性中心数,通过各级分的活性中心数与其分子量的关系发现,DEP的存在导致高分子量部分的活性中心数明显增多,并且减少了低分子量的活性中心数;当Anisole作为内给电子体时,低分子量部分的活性中心数有明显增多。
分别研究了1-已烯和乙烯作为共单体时,加入到乙烯和1-已烯聚合体系中产生的影响。发现在乙烯或1-已烯聚合中,少量的共单体的加入均能使催化剂活性增加,共聚物的分子量也有不同程度的下降。其他条件不变时,增加共单体的浓度使催化剂的活性逐渐提高。在1-已烯聚合中,加入少量乙烯能增加聚合的活性中心数(C_p),且C_p随着乙烯分压的增加而逐渐增多,而增加的活性中心大多是产生低分子量产物的活性中心。不论是1-已烯均聚物还是1-已烯与少量乙烯的共聚物,级分的等规度都是随着分子量的增加而逐渐增加。加入乙烯后,在高分子量部分的等规度会有所降低。乙烯单元在不同级分中的含量是不同的,在低分子量级分以及高分子级分中含量较高,在中间分子量级分中含量相对较低,这表明乙烯倾向于在生产高分子量或低分子量的活性中心上配位。
内给电子体在不同共聚体系中的作用有明显差别。在乙烯为主的共聚体系中,内给电子体增加了低分子量活性中心(Ⅲ～Ⅴ)的位阻,使其不易进入休眠态,减弱了共聚中共单体活性增强的作用,并能缓解共聚产物分子量下降的程度。而在1-已烯为主的共聚体系中,内给电子体提高了催化剂的立体定向能力,使2,1-反插减少,导致可激活的休眠种相对减少,以上作用缓解了共聚中活性中心数增加的幅度,减弱了催化剂活性增强的程度,并加剧了共聚产物分子量下降的程度。
氢气是烯烃聚合中应用最广泛的链转移剂,对不同的烯烃聚合反应产生不同的作用,并影响产物的结构。在乙烯聚合体系中,加氢后聚合活性和产物的分子量都明显下降,分子量分布变窄;而内给电子体的存在加剧了催化剂活性下降和产物分子量下降的程度。而在1-已烯聚合体系中,加入少量的氢气使聚合活性增加,产物的分子量下降,分子量分布变宽;并随氢气浓度的增加,聚合活性逐渐增大并趋于稳定,分子量逐渐下降,达到最低值后有小幅度提高。发现加入少量的氢气能明显增加聚合中活性中心数,并随着加入量的增多,逐渐达到最大值;在高氢气量下,活性中心数会有所降低。加氢对聚乙烯和聚1-已烯两种产物分子量及活性中心分布影响是不同的。加氢降低了聚乙烯的高分子量的活性中心的含量和平均分子量;提高了低分子量的活性中心(Ⅲ～Ⅴ型)的含量和平均分子量,导致产物的分子量分布变窄。而在1-已烯聚合中,氢气的存在可能阻碍了1-已烯单体对Ⅰ型活性中心Ti-H键的插入,使这部分活性中心失活;同时产生了一种分子量很低、含量较高的活性中心(Ⅴ型),导致产物的分子量下降,产物的PDI变宽。
助催化剂烷基铝浓度的改变对烯烃聚合及活性中心分布有显著影响。研究表明,随着Al/Ti比的增加,1-已烯的聚合活性先增加后降低,产物的分子量逐渐下降,并趋于稳定;1-已烯聚合中活性中心数目先是逐渐增加,达到最大值后有小幅度下降,增加的活性中心主要是低分子量的活性中心(Ⅳ和Ⅴ型)。对MWD曲线进行Flory分峰拟合,发现当烷基铝在低浓度变化(Al/Ti=30-200之间)时,高分子量的活性中心(Ⅰ、Ⅱ型)含量随烷基铝浓度的增加而急剧下降,而低分子量的活性中心(Ⅳ、Ⅴ型)含量会大幅度提高。当高烷基铝用量时(Al/Ti摩尔比超过200),高分子量的活性中心有所增加,导致聚合物的分子量在Al/Ti=300时的分子量反而增大。
根据实验结果提出了一个活性中心分类及催化机理模型,按照活性中心的性质、共聚能力、立体定向性以及动力学等性质,把活性中心分成两组来讨论。详细探讨了在烯烃均聚和共聚体系中两组活性中心的性质和变化,描述了内给电子体、共单体、氢气等同这些活性中心之间的作用情况。这一模型能很好地解释大部分实验规律,希望能为负载型Ziegler-Natta催化体系的改进和创新提供理论支撑。
本论文的主要创新点:
1.建立了一种能够直接、准确地测量Ziegler-Natta催化体系活性中心数的方法-肉桂酰氯淬灭直接测活性中心浓度法。通过~1H-NMR检测淬灭产物链端基团及分析淬灭条件对产物中酰基含量的影响,验证了此方法的反应机理和可靠性,并利用此方法研究了内给电子体、共单体、氢气及烷基铝等因素对催化体系活性中心数、活性中心分布及性质的影响。
2.系统地考察了TiCl_4/MgCl_2类催化剂中内给电子体的存在与否及其结构变化对其催化乙烯与1-已烯共聚反应的影响。内给电子体的存在增加了乙烯聚合物的分子量和共聚物不溶级分的含量,使乙烯与1-已烯共聚物的沸腾正庚烷不溶级分中1-已烯含量提高,并使该级分中[HH]结构含量增大,共聚物链结构嵌段性增强。不同类型的内给电子体表现出对共聚体系不同的影响。发现二酯类内给电子体导致可溶级分含量显著降低,α-烯烃结合量降低;而含单醚类内给电子体的催化剂所得乙烯-1-已烯共聚,产物可溶级分含量变化不大,但α-烯烃结合量提高。
3.通过对聚合物MWD的Flory分峰拟合分析,酰氯淬灭法测定活性中心数、聚(1-已烯)沉淀分级级分的活性中心数测定等研究,系统、深入地研究了内给电子体影响活性中心分布和性质的规律,提出了其作用机理。研究发现,在乙烯为主的共聚体系中,内给电子体增加了Group B中心的位阻,使其不易进入休眠态,减弱了共聚中共单体活性增强的作用,并能缓解共聚产物分子量下降的程度。而在1-已烯为主的聚合体系中,内给电子体提高了催化剂的立体定向能力,使2,1-反插减少,导致可被乙烯激活的休眠种相对减少。在1-已烯聚合时,向TiCl_4/MgCl_2类催化剂加入二酯类内给电子体并未使其五种活性中心的本征活性发生显著改变,但明显改变了活性中心分布,使合成高分子量产物的活性中心数目量增大。而单醚类内给电子体使合成低分子量产物的活性中心相对增多,同时使合成中高分子量产物的活性中心的本征活性有所降低。这些发现为正确认识给电子体的作用机理提供了重要线索。
4.通过对加氢产物的MWD的Flory分峰拟合分析,酰氯淬灭法测定活性中心数、聚(1-已烯)沉淀分级级分的活性中心数测定等研究,系统、深入地研究了氢气影响活性中心分布和性质的规律,提出了其作用机理。在乙烯聚合体系中,少量氢气对高分子的活性中心(Ⅰ、Ⅱ型)起到失活作用;对低分子量的活性中心(Ⅲ～Ⅴ型)有活化作用,将休眠态的中心释放出来。这造成了加氢后催化剂活性降低,产物分子量下降,分子量分布变窄。而对于1-已烯体系,少量的氢气解放了中等位阻活性中心的2,1-反插休眠态,活化了Ⅱ型活性中心;并使部分低价态的活性中心变成了对1-已烯有活性的中心(Ⅴ型),导致了加氢之后活性中心数大大增多,产物分子量下降,并使产物的PDI变宽。这些发现为正确认识氢气的作用机理提供了重要依据。
5.通过对聚合物MWD的Flory分峰拟合分析,酰氯淬灭法测定活性中心数等研究,研究了助催化剂浓度影响1-已烯聚合体系活性中心分布和性质的规律。在1-已烯聚合中,烷基铝浓度的改变对聚合行为和产物有明显的影响。低的铝钛比(Al/Ti=30)生成了少量的本征活性很高的Ti~(3+)中心;随着Al/Ti比(30～150)的增加,活性中心数增加明显,但新增的活性中心k_p值相对较低。当在高的Al/Ti比(＞150)时,部分产生低分子量的Ti~(3+)被还原成无活性的Ti~(2+),导致其k_p值反而增加。由于过量的烷基铝会与活性中心发生络合作用,造成在高的烷基铝浓度下产物的分子量反而增加。这些发现为正确认识助催化剂浓度对聚合的作用机理提供了重要依据。
6.研究了共单体(主要是乙烯加入1-已烯体系)对活性中心分布和性质的影响,发现加入少量的乙烯对1-已烯聚合行为产生影响。在1-已烯聚合中,加入少量的乙烯也会使聚合活性增大,且活性并随乙烯压力的增加而逐渐增大,产物的分子量减小。发现乙烯引起的活性增加是由于活性中心数增加所致,但增加的活性中心大多是活性较低、产物分子量较低的活性中心,这也使聚合体系中活性中心的平均活性降低。乙烯在不同沉淀分级级分中的含量不同,分布不均匀,在低分子量级分及高分子量级分中含量相对较高。这些发现为正确认识共单体对1-已烯聚合的作用提供了重要线索。
In the past several decades,supported catalysts of the TiCl_4/ID/MgCl_2 type were widely used in polyolefin industry.However,questions concerning the mechanism of ethylene copolymerization withα-olefins,the effect of internal electron donor(ID) and hydrogen on the structure and nature of active centers have not yet found satisfactory answers.In this paper,some work was carried out in these directions.
Using four kinds of TiCl_4/MgCl_2 catalysts[Cat-1(TiCl_4/MgCl_2),Cat-2(TiCl_4/diethyl phthalate(DEP)/MgCl_2),Cat-3(TiCl_4/Anisole/MgCl_2),Cat-4(TiCl_4/(DEP,Anisole)/MgCl_2)] and DQ Catalyst(TiCl_4/dibutyl(o-)phthalate/MgCl_2),we carried out following studies in this paper:(1) the role of internal electron donor(ID) on ethylene homo-polymerization and copolymerization;(2) A new method for measuring the number of active centers was developed,based on use of cinnamoyl chlorides;(3) the effect of IDs on distribution of active centers;(4) the role of H_2 on polymerization and distribution of active centers;(5) the effect of AlR_3,polymerization temperature on polymerization and distribution of active centers.It was attempted to find the new approach to accommodate the ACD(active center distribution) of Ziegler-Natta catalyst and the MWD(molecular weight distribution),CD (composition distribution) of polyolefins products,and provide the useful information of developing new linear low density polyethylene(LLDPE) and polypropylene(PP) products.
In the preparation of the supported catalysts,MgCl_2·nEtOH/ID adducts as the precursor of the support were prepared in situ by novel co-precipitation method.Four kinds of MgCl_2·nEtOH/ID adducts were prepared in situ by novel co-precipitation method,and the catalysts of TiCl_4 anchored on the supports were prepared.FT-IR spectra of the catalysts and PA as well as DEP were also recorded.DEP was found to be the actual ID existing in the final catalyst prepared based on MgCl_2·nEtOH/PA.Titanium content of the catalysts containing different internal donors is different:DEP decreases the Ti content and Anisole as internal donor didn't decreases the Ti content.
Four kinds of catalysts(Cat-1,Cat-2,Cat-3 and Cat-4) were applied to ethylene polymerization and ethylene-co-1-hexene copolymerization.The addition of IDs increased activity of catalyst and the T_m of products,especially when the catalyst contains DEP as ID. 1-hexene content of the HI(boiling n-heptane insoluble) fraction increased,the difference in 1-hexene content of the HI fraction and the HS fraction decreased,and the blockiness of HI fractions increased.
A new method for determining the number of active centers(C_p) in supported Ziegler-Natta catalysts based on quenching by cinnamoyl chlorides was developed.The optimal quenching conditions were found to be:n(Cinnamoyl chloride/Al)=3～5;t_(quenching)=10 min;T_(quenching)=20℃～40℃.The change of C_p by adding IDs in the catalyst were studied by this method.The introduction of IDs,especially anisole,increased the number of active center in 1-hexene polymerization.
It was found that the introduction of IDs in the catalyst during its preparation can enhance the molecular weight(MW).The Flory non-linear-fitting of products MWD curves showed that the addition of ID didn't change the type of active centers,but change the ACD (active center distribution),increase both the MW and the number of active centerⅠandⅡ. From distribution of the number of active centers determined by quenching and fractionation, the addition of PA increases the amount and C_p of fraction with high molecular weight.
In this paper,the change from homo-polyethylene to ethylene copolymerization with small amount of 1-hexene,from homo-poly(1-hexene) to 1-hexene copolymerization with small amount of ethylene were studied,respectively.Adding small amount of 1-hexene in ethylene polymerization caused marked activation of the low molecular weight components of the polymer.In 1-hexene polymerization system,the activity can also be greatly enhanced by introducing small amount of ethylene.The total number of active centers is markedly increased by adding small amount of ethylene in 1-hexene polymerization,but the average activity of the active centers decreased.The broad composition distribution of the ethylene-1-hexene copolymer can be well understood from the ACD of catalyst and its dependence on the monomer.Adding small amount of ethylene markedly increase the number of active centers that produce polymer with low molecular weight.There is a nearly linear increase of isotacticity with molecular weight of the polymer fraction obtained by precipitationed fraction.However,the addition of small amount of ethylene in 1-hexene polymerization decreased the isotacticity of the fractions with high molecular weight.The distribution of ethylene content in fractions was broad.The content of ethylene in both the low MW and high MW fractions were higher than that of others.So,ethylene tends to be incorporated in both the low MW and high MW active centers in 1-hexene polymerization.
Hydrogen(H_2) has been used as a standard chain transfer agent in industrial polyolefin production.Hydrogen played different roles in ethylene and propylene/α-olefin polymerization.In ethylene polymerization system,the addition of small amount of hydrogen markedly decreased catalyst efficiency and the molecular weight of polymers,and increased the polydispersity index(PDI) of polymers.In 1-hexene polymerization system, the addition of small amount of hydrogen caused a marked increased in polymerization rate and a decrease in the molecular weight of polymers.The total number of active centers is markedly increased by adding small amount of H_2 in 1-hexene polymerization.The Flory non-linear-fitting of polyethylenes MWD curves showed that the addition of H_2 decreased both the M_w and the amount of active centerⅠandⅡ,meanwhile increased both the MW and the amount of active centerⅢ～Ⅴ,which caused decrease in PDI.In 1-hexene polymerization, however,a new type of active center(Ⅴ) with low molecular weight product emerged after adding H_2,which decreased the MW and broaden the PDI of the products.
In 1-hexene polymerization,the change of cocatalyst concentration markedly influences the polymerization behaviors and distribution of active centers.With the concentration of TEA rising,the catalyst activity increased and the molecular weight of products decreased gradually.When the ratio of Al/Ti rising at low TEA concentration,the number of active centers is increased,which mainly increase the active centers producing polymer with low molecular weight.
A new model of two group active centers in polymerization was proposed in this paper, in which the interactions between active centers and IDs,comonomer,hydrogen were elaborated.This model can exactly explain the results in this paper.This model afford deep understand of mechanism of olefin polymerization and provide a new approach to improving and modifying the performance of the supported Ziegler-Natta catalyst.
引文
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