基于聚乙烯的无规和嵌段接枝共聚物的制备

采用乙烯配位聚合和巯基-烯点击化学相结合的方法制备了羟基封端的线性聚乙烯,末端羟基含量接近100%;利用酰氯与羟基的高效反应,将羟基封端的聚乙烯转化为降冰片烯封端的聚乙烯大单体(PE-NB).使用Grubbs II代催化剂,将大分子单体与降冰片烯(NB)单体进行开环易位共聚,通过调整单体的投料比和加料方式制备了分子量和组成可控的聚降冰片烯-g-聚乙烯(PNB-g-PE)接枝共聚物.其中,无规共聚时,大单体的转化率接近100%,所得无规接枝共聚物的重均分子量为1.79×10~4~3.14×10~4,分子量分布指数为2.09~2.60,聚乙烯链段的质量分数为4.6%~16.8%;而嵌段共聚时,由于空间位阻原因,大单体的转化率约为80%.热分析研究发现,由于空间位阻,接枝共聚物的结晶度较聚乙烯前驱体略有下降,且接枝度越大,结晶能力下降得越多.

Preparation of Polyethylene-Based Random and Block Graft Copolymers

Catalyzed by a phenoxycyclopentylimine ligated zirconium complex/dried methylaluminoxane (dMAO) system at 40 ℃ under 1 atm of ethylene pressure,linear vinyl-terminated polyethylene (PE-ene) with nearly 100％ of end-functionality was produced via ethylene polymerization.With high efficiency,hydroxyl-terminated polyethylene (PE-OH) was subsequently prepared via the radical initiated thiol-ene click chemistry between the PE-ene and 2-mercaptoethanol.The PE-OH was then used to react with norbornene-5-carbonyl chloride to synthesize the norbornene-terminated polyethylene macromonomer (PE-NB) through the esterification reaction.The ring-opening metathesis copolymerization (ROMP) of PE-NB and norbornene (NB) with the different feed ratio was conducted in the presence of Grubbs Ⅱ generation catalyst to produce the polyethylene-based graft copolymers.Both random and block polynorbornene-g-polyethylene graft copolymers (PNB-g-PE) were then obtained with different feeding mode,i.e.,the one-batch feeding mode and the sequential feeding mode,respectively.The structures of the graft copolymers were characterized by Fourier transform-infrared and proton nuclear magnetic resonance spectroscopy.The molecular weight and molecular weight distribution of PNB-g-PE were determined by high-temperature gel permeation chromatography;the thermal properties of PNB-g-PE were also tested by differential scanning calorimetry.The conversion of macromonomer was found to be nearly 100％ in the random copolymerization of macromonomer and norbornene.The random graft copolymers were obtained with the molecular weight of 1.79 × 104-3.14 × 104 with relatively narrow molecular weight distribution (2.09-2.60).The molecular weight of the copolymers decreased gradually with the increase in the feed ratio of PE-NB.The molar fraction of PE in the random copolymers varied from 4.6％ to 16.8％,depending on the feed ratio of PE-NB to NB,whereas the molar fraction of PE showed slight influence on the melting point of the PE segments.However,the conversion of macromonomer reached about 80％ in the block copolymerization of PE-NB and NB.Due to the steric hindrance,the crystallinity of the graft copolymers slightly decreased in comparison with the corresponding PE precursor.