Ethylene/propylene Copolymerization By Titanium-based Ziegler-Natta Catalyst Systems And Optimization Of Structure And Properties For PP/EPR In-reactor Alloy

Using spherical MgCl₂-supported superactive Ziegler-Natta(Z-N)catalyst, polypropylene/ethylene-propylene copolymer(PP/EPR)in-reactor alloy can be prepared by two-stage polymerization process—a sequential homopolymerization of propylene followed by a downstream ethylene/propylene copolymerization.Its high-impact property makes it one of the most important kinds of improved PP resin and is now commercially produced worldwide.EPR in the alloy has been found to be composed of random ethylene-propylene copolymer part that acts as elastomer,and a considerable amount of PE-b-PP segmented copolymer with crystallizable PE and PP segments.Such segmented copolymer fractions can be partly miscible with both the random copolymer and the PP matrix,and have a positive effect on the impact strength of the alloy.On the other hand, the microstructure of PP also has strong influences on the properties of alloy.However, the relations between the polymerization conditions of PP/EPR alloy synthesis and the alloy's properties have been scarely reported in detail.Mechanism of olefin coordination polymerization with Z-N catalyst system have not been elucidated clearly yet.In this dissertation,MgCl₂/ID/TiCl₄ type Z-N catalysts were used to conduct ethylene-propylene copolymerization,propylene homopolymerization and to produce PP/EPR in-reactor alloy.The influences of polymerization conditions on the polymerization behaviors,microsturcture and properties of the resultant product were symtematically studied.Mechanism of these influences was also explored.The influences of varied condition on ethylene-propylene slurry copolymerization at atomosphric pressure with continuous feed of comonomers were investigated firstly. Ethylene/propylene feed ratio has strong effects on activity,content of crystalline copolymer(or blocky copolymer)and compostion of copolymer.When ethylene conten in feed(f_e)was in the range of 33%⁵0%,the random part of poduct showed medium compostion,while the bloky copolymer part possessed long propylene segment along the chain,which should be favorable to the properties of alloy.Four types of supported catalyst(cat 1,cat 2,cat 3,cat 4)were studied comparatively.It is found that type of catalyst has marked effects on activity,content of C8-insoluble fraction(blocky copolymer)and its microsturcture,but has little influences on microsturcture of C8-soluble fraction(random copolymer).Among the four catalysts,cat 3 tends to form longer PP segment.The kind of cocatalyst also exerted strong influence on copolymerization.Compared with TEA,TiBA activated system produced more blocky copolymer,but the blockiness of its block fraction is lower.At higher and lower value of f_e,the discrepancies between TEA and TiBA in activity and ethylene content of soluble fraction were just reversed.Using TEA/TiBA mixtures as cocatalysts,it is found that the copolymerization behaviors are not simple overlapping of pure TEA and TiBA systems. Reaction temperature and type of external donor(ED)also have strong influence on copolymerization.The effects of ED,composition and concentration of mixed cocatalyst on the copolymerization behaviors and microstructure of copolymer were also investigated at higher monomer pressure in a semicontinuous process.It was demonstrated that DDS and Donor-C as ED have similar influences on copolymerization,while Donor-D showed different influences on activity and content of blocky fraction.Changing ED can hardly affect the composition of random copolymer,but can change the microsturcture of blocky copolymer.In copolymerization runs activated by TEA/TiBA mixtures,it was found that as the content of TEA in cocatalyst increases,yield of the random part of product increases while its ethylene content decreases.Both random part and blocky part of the products were relatively blockier than the products of copolymerizations activated solely by TEA or TiBA.This phenomenon might be ascribed to group exchange reactions between TEA and TiBA,which form new kinds of alkyl aluminium compounds and add more complexity to the catalyst system.It also could be concluded that the active centers forming blocky copolymer are coordinated with electron donor,whereas the active centers producing random copolymer are likely to be complexed with alkyl aluminium compound. Introducing H₂ in the copolymerization system and further insceasing H₂ concentration caused decrease in the yield of blocky copolymer,but its heterogeneity inceased markedly. When the content of TiBA in cocatalyst increases,the extent of this enhancent is more obvious.These effects may be due to frequent reactivation of the active center for blocky copolymer by hydrogen.Yields of both the blocky and random copolymer fractionsincreased and gradually approaching the maximum with increasing molar ratio of Al/Ti, but the ratio of the two fractions kept constant.When Al/Ti increases from 100 to 250,the composition of random copolymer changed little and ethylene content of the blocky part decreased in TEA activated systems,but the ethylene content of both parts decreased in TiBA activated systems.¹³C NMR and DSC analysis prove that increasing Al/Ti leads to increased heterogeneity of the blocky copolymer.Such copolymerization behavior could be closely related to the degree of activation of catalyst,complexation effect of external/internal donor and alkylaluminium compound with active center and chain transfer effet of alkylaluminium compound. An ethylene-propylene copolymer with medium composition synthesized with Z-N catalyst was fractionated by a combination of dissolution/precipitation and temperature-gradient extraction fractionation(TGEF).The fractions were characterized with ¹³C NMR,DSC and FT-IR.It was proved that EP Copolymer prepared with Ti-based catalyst present complicated inter- and intra-chain heterogeneity of composition.DSC and WAXD results revealed that there exist PP crystals originated from long propylene sequences even in copolymer of high ethylene content:Annealing treatment can change a part ofα-PP crystals intoγ-PP crystals.Short propylene sequences Can be included in PE crystals,and short ethylene sequences can also be incorporated into PP crystals.It is shown that in segmented copolymer with high propylene content,the degree of PE crystal expansion by propylene unit is not only affected by propylene content,but depends more on the length of propylene sequence.There are propylene sequences with critical length that can either be included in PE crystals or crystallize by themselves,depending on the crystallization conditions.The influences of reaction conditions on propylene homopolymerization were also investigated.Increasing concentration of ED enhanced isotactic index of PP,but the effects of ED also rely on the cocatalyst type.When TiBA was used as cocatalyst,high isotactic PP is synthesized only at high Si/Ti molar ratio condition.PP products were fractionated by TREF.DSC and ¹³C NMR results showed that the fractionation was carried out mainly according to the isotacticity of PP.When TEA was guadually replaced by TiBA in the mixed cocatalyst,the content of high isotactic fractions increased.GPC results of the fractions demonstrated that with isotacticity of fraction increasing,its molecular weight increases.PP with high molecular weight Cannot crystallize perfectly as a result of chain entangment.According to the above results,plausible model of active centers has been proposed to explain the ED and cocatalyst effects.This model has take into consideration the exchanges reaction between different alkylaluminiums and ED with internal donor(ID)on the surface of catalyst.It was suggested that the propylene polymerization system actually is a complicated 'multiple active center system'. Propylene bulk polymerziations were also conducted and mechnical properties of the products were measured.Changing cocatalyst composition influenced the degree of crystallization,molecular weight and its distribution of PP,which in turn affect the ultimate mechanical performances.Basically both stiffness and toughness of PP are a little lower by using mixed cocatalyst.Properties of PP products synthesized from catalyst DQ and BC-MS respectively were compared. The reaction between cocatalyst and catalyst were investigated.It is found that TEA has a larger ability to react with the components in the catalyst,including Ti species and internal electron donor.More Al atom and ethyl group were anchored to the catalyst after its reaction with TEA-rich cocatalyst.The surface product formed after the catalystalkylaluminum reaction was found to be AlR₂Cl.According to the compostion of gaseous products released in hydrolyzing the products of catalyst-cocatalyst reaction,alkyl group exchanges between Al—Et and Al—iBu have been found,which was also proved by ¹H NMR analysis of TEA/TIBA mixtures.As revealed by ESR analysis,increasing TEA in the cocatalyst caused decrease of Ti species associated with the electron donor,but increase in Ti species associated with the aluminum compounds.Two types of active centers were proposed based on the experimental results to elucidate the relationships between the poIymerzition behaviors and the microsturcture of polymer.Main novelties of the dissertation:1.Using spherical MgCl₂-supported superactive Z-Ncatalyst,ethylene-propylene copolymer,polypropylene and PP/EPR in-reactor alloy were synthesized.The influences of polymerization conditions on polymerization behaviors and microsturcture and property of the resultant product were symtematically studied to explore effective approaches to regulate the polymer properties.It was found that using TEA/TiBA mixtures as cocatalyst is a good option to enhance the performance of PP/EPR alloy.2.By fractionation and detailed characterization of the fractions,it is revealed that composition heterogeneity induces coexsitence of both PE and PP crystals in ethylene-propylene copolymer,even at high ethylene content.Short ethylene sequences can be included in PP crystals and short propylene sequences can be included in PE crystals.The parameters of PE crystal are strongly dependent on the length ofpropylene sequence.3.From the point of exchange reactions between cocatalyst,ED and ID in the catalyst during formation of active center,a complement of the mechanism of Z-N catalytic system was proposed.Two types of active centers were proposed based on the experimental results to elucidate the relationships between the polymerzition behaviors and the microsturcture of polymer.