GRAFTING OF MALEIC ANHYDRIDE ONTO HYPERBRANCHED POLYETHYLENE AND SUPPRESSION OF CHAIN DEGRADATION AND CROSSLINKING SIDE REACTIONS

Abstract

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 dendrimer-like compact globular structures.Due to lack of chain entanglements,it exhibits better solubility and lower viscosity than the other types of polyethylene.HBPE has attracted great attention in the recent years in both academic and industrial researches.However,as polyolefin materials,HBPE suffers from such shortcomings as nonpolar nature and thus lack of functionalities.These shortcomings limit its applications.Grafting of polar molecules such as maleic anhydride(MAH) has been widely adopted in the modification of polyolefins.In this work,HBPE samples having high molecular weight(M_w=8×10~6 g/mol) and high branching density(the number of branches per 1000C=108) were synthesized by solution polymerization of ethylene with a late transition metal catalyst,namely bi(2,6-diispropylphenyl) butane diimine nickel dibromide(IBNB) at 45℃ and(0.5 MPa.)The Ni catalyst is well-known to experience a chain walking mechanism that is responsible for the hyperbranched chain topology.()~1H-NMR and()~(13)C-NMR were used to examine the branching density and the type of branches,respectively.It was discovered that the types of branches were mainly methyl(50.54%) and butyl(20.30%).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 during the grafting process.N,N-diethyl cinnamamide(DECA) was thus synthesized by the use of cinnamoyl chloride,diethylamine,and triethylamine which was used as an acid-binding reagent.DECA was then used as an inhibitor to suppress the side reactions.The approach was successful and efficient.The gel content in the grafting products was remarkably reduced from 55.38% to 5.15%.The M_w was almost doubled in reference to the use of MAH and peroxide DTAP only.The observation suggests that the chain degradation and crosslinking side reactions were dramatically suppressed with the addition of DECA.Compared to the other investigated 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 both chain degradation and crosslinking side reactions.We also found that FAA and ECA could effectively suppress the chain degradation in the grafting system,but the grafting system had high gel contents and experienced severe crosslinking.St and DMAC had some effects on inhibiting crosslinking,but there was still small amount of crosslinking.The proposed mechanism is that DECA reacts first with HBPE macroradicals to form stable macroradicals when DECA is added to the grafting system.The latter then reacts(or copolymerizes) 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.In general,in the presence of radical initiator,HDPE mainly experiences crosslinking,while polypropylene undergoes chain scission.LDPE and LLDPE have both reactions simultaneously occurring.HBPE is more or less similar to the low-density materials having simultaneous crosslinking and degradation.In the grafting studies,these side reactions need to be controlled.The current work elucidates the grafting mechanisms and provides practical guidance in this regard.