Preparation And Functionalization Of Hyperbranched Polyethylene

Hyperbranched polyethylene(HBPE)is a new type of polyethylene materials in addition to low-density polyethylene(LDPE),high-density polyethylene(HDPE),and linear low-density polyethylene(LLDPE).It has a dentrimer-like compact globular structure,and shows Newtonian flow behavior due to the absence of chain entanglements.HBPE exhibits better solubility and lower solution viscosity compared to linear polyethylene,which has attracted great attention in research and development.For example,branching in polyethylene prepared with nickel(â…¡)-or palladium(â…¡)-based late transition metal catalysts can vary from hyperbranched to moderately branched to linear.Their materials properties range from soft wax,to elastomer,to rigid plastic,respectively.This special characteristic helps to improve processing of polyethylene and broadens its range of applications.However,HBPE has not yet achieved industrialization,its relationships between chain structure and materials properties are yet to be established and its applications need to be further explored.Chemical modification and functionalization can introduce novel structures and tailor-made properties to this type of materials and thus expand their application horizon.In this thesis,the progress and prospect in the preparation,structure,and properties,as well as applications of HBPE materials were reviewed.HBPE samples having high molecular weight and high branching density were synthesized with appropriate diimine nickel(â…¡)-based late transition metal catalysts.The structure and properties of these samples were studied using ¹³C NMR,¹H NMR and DSC.HBPE was functionalized through grafting maleic anhydride(MAH)and other chemical modifications.For a comparison purpose,various polyolefins were grafted with MAH. Their grafting degrees(GD),gel contents,loss in chain degradation,molecular weight of the modified polymers were studied in parallel.N,N-diethyl cinnamamide(DECA), having a ring conjugated with a double bond and nitrogen,was synthesized and used as an inhibitor to suppress the side reactions.Two kinds of late transition metal catalysts were synthesized:bi (2,6-diisopropylphenyl)butane diimine nickel dibromide(IBNB)and bi(phenyl)amyl diimine nickel dibromide(ANB).HBPE samples having molecular weight nearly 10⁶ g/mol and branching density(the number of branches per 1000 C)over 100 were synthesized by solution polymerization of ethylene.The methyl content of HBPE1 was much higher than HBPE2,but the contents of propyl,butyl and longer branches were lower.The DSC was employed to characterize thermal properties of HBPE and other commercial polyolefins.The exothermic peaks were analyzed in terms of peak temperature(T_c),starting crystallization temperature(T_o),initial slope(S_i)and width of half-height(â–³w).Increasing the polymerization temperature increased the content of branches in HBPE samples.As a result,their melting temperature,crystallization temperature and crystallinity were reduced remarkably.Both the starting crystallization rate and the total crystallization rate increased.But the size distribution of crystal grains was broadened.The thermal properties of HBPE were similar to the commercial LDPE,ethylene-propylene-diene elastomer(EPDM),and ethylene-hexane copolymer.MAH was grafted onto HBPE through a free radical mechanism.Di-tert amyl peroxide(DTAP)was used as the free radical initiator.However,it was found that the HBPE/MAH/DTAP grafting system experienced severe chain degradation and crosslinking side reactions.The effects of grafting conditions on the performance of grafting products were studied.DECA was successfully and efficiently used as an inhibitor to suppress the side reactions.The gel content in the grafting products was remarkably reduced from 55.38%to 5.15%.The(?)_w was almost doubled in reference to the use of MAH and initiator DTAP only.Compared to the other inhibitors such as styrene(St),N,N-dimethylacetamide(DMAC),3-(2-furyl)acrylic acid(FAA)and ethyl cinnamate(ECA),DECA was found to be the most effective in suppressing the side reactions.The proposed mechanism is that DECA reacts first with HBPE macroradicals to form stable macroradicals when DECA is added to the grafting system.Then these latter react(or copolymerize)with MAH to form MAH-grafted HBPE.Because the formed stable macromolecules contain amide group,both the chain degradation and crosslinking side reactions are effectively suppressed.The grafting reaction kinetics of various polyolefins were also studied and compared with GD,gel content and materials loss in chain degradation analyzed.It was found that DECA could suppress both chain degradation and crosslinking side reactions to a certain extent to various polyolefins.The non-isothermal behaviors of MAH grafting onto various polyolefins with or without DECA were studied by DSC.It was found that the changes in the thermal properties of different modified polyolefins were different.It was related to both polyolefin structure and grafting reaction kinetics.After grafted with MAH,the melting temperature and crystallization temperature of all the HBPE1,HDPE,LDPE, EPDM,the copolymer of ethylene and hexane(LLDPE2),the copolymer of ethylene and octane(LLDPE3)experienced reduction to some extent.However the melting temperature of HBPE2 increased,and the crystallization temperature of isotactic polypropylene(iPP)and the copolymer of ethylene and butane(LLDPE1)increased. The melting enthalpy of every type of polyolefins was also reduced.The crystallization enthalpies of HBPE1,HBPE2,HDPE,iPP,LLDPE1,LLDPE2 and LLDPE3 were reduced.However the crystallization enthalpies of LDPE,EPDM and LLDPE2 that have lower crystallinity increased after grafting.After addition of DECA to the polyolefin/MAH/DTAP system,the melting temperature,crystallization temperature and melting enthalpy of the grafting products of each kind increased.In addition to LDPE,EPDM and LLDPE2,the crystallization enthalpy and crystallinity level of other polyolefins also increased.Oxidized in a mixture of trichloroacetic acid/1,2-dichlorobenzene(TCA/o-DCB) and chromium trioxide/water(Cr₂O₃/H₂O),HBPE was grafted with a relatively high level of carboxylic acid groups,reaching 0.416 mmol/g.With the grafted carboxylic acid,further chemical modification of HBPE could be readily implemented by reduction and esterification reactions and thus expand HBPE's potential applications.