Synthesis of Functional Block Copolymers Based on Polyolefins

Author:Song Xiang Yang

Supervisor:cai zheng guo


Degree Year:2019





Polyolefins are widely used due to their excellent mechanical properties,non-hygroscopicity,high resistivity and low price.In recent years,with the continuous innovation of catalytic technology,many kinds of polyolefins with new structures and applications have been exploited.Among them,polyolefin block copolymers preserve the excellent properties of each segment,which exhibit unique physical and chemical properties.However,non-functionalised polyolefin block copolymers have poor surface properties such as adhesion,coating and miscibility,which restrict their practical application.Therefore,many properties of polyolefin block copolymers can be greatly improved by introducing small amounts of polar functionalities by certain technical means.They also have important research significance for the sustainable development of polyolefin industry.Block copolymers consisting of polyolefins and polar segments are also known as highly desired materials,since they offer a large quantity of functional groups which exhibit phase separation and self-assembly at the microscopic level and act as compatibilizers in polymer blends.However,the synthesis of block copolymers is rather difficult and often proceeds in a multi-step method because of their incompatible techniques with large differences between polar and non-polar vinyl monomers.So,it’s necessary to find suitable catalytic systems and explore new polymerization mechanisms for the synthesis of polyolefin block copolymers containing polar segments with good controllability of chain structures in one pot.In this study,the fluorenylamidodimethyltitanium complexes[t-BuNSiMe2(2,7-tBu2Flu)]TiMe2(1)we selected could be applied for the fast living polymerization of propylene,higherα-olefins,and norbornene as well as their living copolymerization.With the development of the pre-protection of polar monomers and the innovation of polymerization mechanism,some functional polyolefin block copolymers with special properties and applications had been prepared successfully.Firstly,copolymerization of ethylene or NB with 10-undecen-1-ol complexed by iBu3Al(U-OAliBu2)was conducted by using 1.Complex 1 showed high copolymerization activity to produce hydroxy-functionalized polyolefins.The copolymerizaiton activity of ethylene with U-OAliBu2could reached2000 kg·mol-1·h-1.The weight average molecular weight(Mw)of the copolymer obtained was as high as 2.16×106 g/mol.Although the highest hydroxyl content was only 1.6 mol%,the hydrophilicity of the copolymer was significantly improved(WCA=90°).The hydroxyl functionalized UHMWPE was successfully synthesized.The result testified that a small amount of hydroxy group can dramatically alter the surface properties of UHMWPE.Copolymerization of NB with U-OAliBu2 exhibited remarkably high activity of up to 5.96×104 kg-polymer mol-Ti-1 h-1.The content of hydroxy groups(4.758.0%)and thermol properties(235335℃)of the copolymers obtained could be controlled in a wide range.The optical transmittance of all copolymer films was over 90%in the visible light region(350750 nm),and the hydrophilicity and mechanical properties of the copolymer films(WCA60°)were improved with the increase of hydroxyl content in the copolymers.The monomer reactivity ratios of the MMAO system(rNB=3.81,rU-OH=0.13)determined by using Fineman-Ross method indicate a preference for the insertion of NB regardless of the last inserted monomer unit.The product of the reactivity ratios(rNBrU-OH=0.50)indicated a tendency of the formation of random copolymers.In addition,the catalystic system using modified methyl aluminoxane(MMAO)/2,6-di-tert-butyl-4-methylphenol(BHT)as cocatalysts can conduct the copolymerization of NB and U-OAliBu2 in a living manner,which lays a theoretical foundation for the synthesis of functional block copolymers.Secondly,in view of the living nature of system,the synthesis of A-B diblock copolymers consisting of hydroxy-functionalized COC(poly(NB-co-U-OH))and semicrystalline polyolefin sequence such as polyethylene and syndiotactic polypropylene(syn-PP)was achieved using a living catalystic system(1-MMAO/BHT).The block structures of the copolymers were verified by GPC,DSC and NMR,respectively.The sequence lengths of syn-PP or PE and hydroxyl functionalized COC were controlled by changing the amount of olefin monomer in feed and NB/U-OH polymerization time,respectively.The hydroxyl content could be regulated by the comonomer feed ratio of NB to U-OAliBu2.Their hydrophilicity(WCA=80.7102.8)and mechanical properties(maximum stress at break of 43.8 MPa)were also greatly improved.The results above indicated that the functional polyolefin block copolymers had been synthesized in a living manner by using complex 1 for the first time.Next,synthesis of a variety of new class of novel NB based A-B-A polyolefin block copolymer elastomers consisting of hard segment with high Tg and several kinds of soft segments with low Tg values such as atactic polypropylene(aPP),poly(ethylene-co-propylene)and poly(ethylene-co-1-hexene)was achieved.These block copolymers exhibited good tensile strength(up to 16.37 MPa),excellent toughness(above strain at break of 900%)and efficient elastic recovery(maximum elastic recovery of 91±1%).DSC analysis showed the Tg of approximately-30?C and-60?C for the block poly(ethylene-co-propylene)and poly(ethylene-co-1-hexene)as soft segments,respectively,indicating that the materials had more excellent low temperature resistance.Most importantly,the triblock copolymer(PNB-b-aPP-b-P(NB-co-U-OH))synthesized by introduction of hydroxyl group into PNB segment(hydroxyl functionalized COC)gave excellent thermal,improved mechanical properties(The maximum stress at break rises to 20.18 MPa,the strain at break was obve 1100%and the maximum elastic recovery was 92±1%)and surface behavior(The lowest WCA value=90°)for optically transparent material,which was a potential and versatile new material.Then,synthesis of A–B–A polyolefin thermalplastic elastomers with an aPP central segment located between two P(E-co-NB)copolymer hard segments was investigated.This kind of material with the Tg value of below 110?C had improved processability.The properties of all triblock copolymers above could be optimized by changing?Hard values and the hydroxyl content in the copolymers,respectively.It also provided a guideline for exploiting a new family of elastomers with high performance and more applications in the future.Finally,the synthesis of a series of A-B-C block copolymers containing PMMA such as PE-b-PH-b-PMMA and PE-b-PP-b-PMMA was achieved in one pot using 1-MMAO/BHT system by the subsequent addition of monomer.The first olefin polymerizations take place in standard coordination-insertion mechanism to form diblock copolymers and the additional MMA reacts with cationic alkyl titanium species to give cationic titanium enolate,which does not promote MMA polymerization by alone even when neutralized by methyl counter anion or unreacted neutral titanium dimethyl complex because of low nucleophilicity of the titanium enolate.Michael-type addition from titanium enolate to MMA,which is strongly activated by the aluminium Lewis acid(iBu3Al/BHT),would take place.All tri-block copolymers kept relatively narrower molecular-weight distribution(PDI=1.181.56)in each copolymerization.Their block structures were confirmed by GPC,DSC and 13C NMR,respectively.The stereoregularities of both PH and PP blocks were calculated from the integral ratio of the methylene carbon directly connected to the main chain of PH and the methyl carbon of PP were statistically atactic with mr value of 44.7%and45.3%,respectively.The rr triad ratio of the MMA sequence in each block copolymer calculated from integral ratios of the quaternary center carbonyl peak around 178.3 ppm was found to be 74.6%and 71.6%,respectively.The sequence lengths of polyolefin and PMMA were controlled by changing the amount of olefin monomer and MMA polymerization time,respectively.The maximum molecular weight of the triblock copolymers could reach 179,000 g/mol,which broke through the limitation of synthesis of low molecular weight block copolymers with the non-living polymerization system.These block copolymers having a soft amouphous central segment of atactic PH or PP located between two hard segments of semicrystalline PE and glassy PMMA,should function as thermoplastic elastomers with two different processing temperature ranges arising from the presence of two distinct hard blocks.Also,it has potential application in self-assembly,blend compatibilizer and other fields.