Copolymers ethylene with polar monomers

Post-polymerization fimctionalization has been used to this end, but most research has been directed toward the copolymerization of ethylene with polar monomers. In this manner, inexpensive monomers can be used to create novel polymeric materials with a wide range of applications. The major drawback to this methodology is the inherent difference in reactivity between ethylene and other vinyl monomers during chain polymerization. This phenomenon is known to yield copolymers with low polar monomer incorporation and increased branch content arising from chain transfer events caused by side reactions with polar and/or protic functionaUties [45]. 

The copolymerization of ethylene with polar monomers is one potential method of improving various properties of PE [146, 147]. ADMET offers a method of synthesizing linear polymers identical to sequence-specific or random copolymers of ethylene and polar monomers with a variety of comonomer compositions. 

RUBBERY COPOLYMERS OF ETHYLENE WITH POLAR MONOMERS... 

Polyethylene has limited adhesion to paints and inks. This is because it is a non-polar hydrocarbon incapable of forming hydrogen bonds. Adhesion can be improved by copolymerizing ethylene with polar monomers such as ethyl acrylate or vinyl acetate to give ethylene ethyl acrylate (EEA) copolymers or ethylene vinyl acetate (EVA) copolymers. EVA is often used for shoe soles. 

Very low-density (VLDPE) is an extreme version of LLDPE Ethylene copolymers, typically with polar monomers such as vinyl acetate, are made by LDPE technology... 

FIGURE 13.2 Calculated relation between the solubility parameter and glass transition temperature (Jg) for a variety of ethylene-propylene copolymers (EPMs) grafted with polar monomers the window for rubbers with an oil resistance similar to or better than hydrogenated acrylonitrile-butadiene copolymer (NBR) (20 wt% acrylonitrile) is also shown. 

However, the practical, direct synthesis of functionalized linear polyolefins via coordination copolymerization olefins with polar monomers (CH2 = CHX) remains a challenging and industrially important goal. In the mid-1990s Brookhart et al. [25, 27] reported that cationic (a-diimine)palladium complexes with weakly coordinating anions catalyze the copolymerization of ethylene with alkylacrylates to afford hyperbranched copolymers with the acrylate functions located almost exclusively at the chain ends, via a chain-walking mechanism that has been meticulously studied and elucidated by Brookhart and his collaborators at DuPont [25, 27], Indeed, this seminal work demonstrated for the first time that the insertion of acrylate monomers into certain late transition metal alkyl species is a surprisingly facile process. It spawned almost a decade of intense research by several groups to understand and advance this new science and to attempt to exploit it commercially [30-33, 61]. 

The dual function of the precatalysts 4 opened the way to well-controlled block polymerization of ethylene and MMA (eq. (5)) [89, 90]. Homopolymerization of ethylene (Mn = 10000) and subsequent copolymerization with MAA (Mn 20000) yielded the desired linear AB block copolymers. Mono and bis(alkyl/silyl)-substituted flyover metallocene hydride complexes of type 8 gave the first well-controlled block copoymerization of higher a-olefins with polar monomers such as MMA or CL [91]. In contast to the rapid formation of polyethylene [92], the polymerization of 1-pentene and 1-hexene proceeded rather slowly. For example, AB block copolymers featuring poly( 1-pentene) blocks (M 14000, PDI = 1.41) and polar PMMA blocks (M 34000, PDI = 1.77) were obtained. Due to the bis-initiating action of samarocene(II) complexes (Scheme 4), type 13-15 precatalysts are capable of producing ABA block copolymers of type poly(MMA-co-ethylene-co-MMA), poly(CL-co-ethylene-co-CL), and poly(DTC-co-ethylene-co-DTC DTC = 2,2-dimethyltrimethylene carbonate) [90]. 

Recently, much attention has been devoted to modeling polyolefins and copolymers of ethylene and polar monomers. Eor example, polymers with regularly spaced methyl groups on a polyethylene backbone have been synthesized and display very interesting and surprising thermal properties and microstructure [12]. This represents a rational synthesis of branched polyethylene that cannot be achieved by any other means at this time (Scheme 6.12). 

Elends of the polyethylene variants (HDPE, LLDPE, LDPE, VLDPE) and ethylene copolymers with polar monomers (EAA, EMAc, EEA, EVA)) are employed in many variations in commercial apphcations, particularly involving film apphcations. LLDPE/LDPE blends were commercially introduced in film apphcations shortly after the introduction of LLDPE in the late 1970s. The earher version of LLDPE required LDPE addition to achieve good processabihty and reduced haze to take advantage of the mechanical properties offered by LLDPE. Several more recent papers wih be briefly discussed, showing the continuing interest in this... 

Ethylene can also be copolymerized with polar monomers in order to widely modify the characteristics of the corresponding materials. The comonomers are most often (meth)acrylic monomers or vinyl acetate, with the latter being the most used for the production of EVA copolymers (ethylene/vinyl acetate). EVAs generally contain about 20% mass of comonomer and are very interesting due to their adhesive properties. 

In order to increase the solubiUty parameter of CPD-based resins, vinyl aromatic compounds, as well as other polar monomers, have been copolymerized with CPD. Indene and styrene are two common aromatic streams used to modify cyclodiene-based resins. They may be used as pure monomers or contained in aromatic steam cracked petroleum fractions. Addition of indene at the expense of DCPD in a thermal polymerization has been found to lower the yield and softening point of the resin (55). CompatibiUty of a resin with ethylene—vinyl acetate (EVA) copolymers, which are used in hot melt adhesive appHcations, may be improved by the copolymerization of aromatic monomers with CPD. As with other thermally polymerized CPD-based resins, aromatic modified thermal resins may be hydrogenated. 

Since the reactivity ratios of ethylene-polar monomer pairs are quite different, the preparation of copolymers with precisely the same comonomer composition can be a challenging endeavor. Earlier in this chapter, we described the synthesis and characterization of precisely placed methyl groups on a polyethylene... 

As described in Section 9.1.2.2.3, several lanthanocene alkyls are known to be ethylene polymerization catalysts.221,226-229 Both (188) and (190) have been reported to catalyze the block copolymerization of ethylene with MMA (as well as with other polar monomers including MA, EA and lactones).229 The reaction is only successful if the olefin is polymerized first reversing the order of monomer addition, i.e., polymerizing MMA first, then adding ethylene only affords PMMA homopolymer. In order to keep the PE block soluble the Mn of the prepolymer is restricted to <12,000. Several other lanthanide complexes have also been reported to catalyze the preparation of PE-b-PMMA,474 76 as well as the copolymer of MMA with higher olefins such as 1-hexene.477... 

The Lewis acidity and reactivity of these alkyl aluminum cocatalysts and activators with Lewis basic polar monomers such as acrylates make them impractical components in the copolymerization of ethylene with acrylates. To address this shortcoming, Brookhart et al. developed well-defined cationic species such as that shown in Fig. 2, in which the counterion (not illustrated) was the now-ubiquitous fluorinated arylborate family [34] such as tetrakis(pentaflurophenyl)borate. At very low methyl acrylate levels the nickel catalysts gave linear copolymers but with near-zero levels of acrylate incorporation. 

These catalysts represent the current state-of-the-art in ethylene copolymerization with polar olefinic monomers, being able to copolymerize a wide variety of polar monomers containing both O and N heteroatoms to generate completely linear, high molecular weight, random copolymers. There are leads to enhance the modest activity of these catalysts, and it will be interesting to watch further developments over the next few years. 

The cationic Pd(II) catalysts exhibit effective copolymerizations of ethylene and other a-olefins with polar-functionalized comonomers, with the majority of insertions occurring at the ends of branches. Among the best tolerated monomers are those bearing fluorine or oxygen-containing functionalities, such as esters, ketones, and ethers. The copolymerization of ethylene and acrylates, attractive because the monomers are inexpensive and the copolymers exhibit unique physical properties, has been well-studied mechanistically [27,69], Examples of copolymerizations of ethylene and a-olefins with methyl acrylate are shown in Table 4. In general, the amount of comonomer incorporation varies linearly with its reaction concentration and... 

The applicability of organolanthanide metallocenes as polymerisation catalysts can also be seen from the results of the block copolymerisation of ethylene and methyl methacrylate. The persistence of the lanthanide-alkyl bond has been utilised to prepare ethylene copolymers with polar poly(methyl methacrylate) blocks. For this purpose, ethylene is introduced as the first monomer into the polymerisation system with the samarocene catalyst, and then methyl methacrylate is polymerised, which leads to block copolymer formation [532-534] ... 

Historically, high-pressure free radical copolymerization has been used to produce highly branched, ill-defined copolymers of ethylene and various polar monomers. Although these materials are in production and extensively used throughout the world, the controlled incorporation of polar functionality coupled with linear polymer structure is still desired to improve material properties. Recent focus in this area has led to the development of new transition metal catalysts for ethylene copolymerization however, due to the electro-philicity of the metal centers in these catalysts, polar functional groups often coordinate with the metal center, effectively poisoning the catalyst. There has b een some success, but comonomer incorporation is hard to control, leading to end-functionalized, branched polyethylenes [44, 46]. These results are undesirable due to low incorporation of polar monomer into the polymer as well... 

ADMET offers a synthetic route to strictly linear, fimctionalized polyethyl-enes through the polymerization of a,co-dienes followed by exhaustive hydrogenation. Researchers have been able to use metathesis catalysts in conjimction with the functionalized monomers to produce statistical or sequenced copolymers of ethylene with various polar monomers. With the improved tolerance and reactivity of [ Ru], the broadening of ADMET methodology will allow the syntheses of numerous functionalized systems [4]. However, due to the well known olefin isomerization that occurs during the metathesis polymerization with [Ru], monomer sequence control is lost and the methylene run length between functional groups varies widely. 

Ionomers consist of statistical copolymers of a non-polar monomer, such as ethylene, with (usually) a small proportion of ioniz-able units, like methacrylic acid. Ethylene-co-methacrylic acid copolymers (-5% methacrylic acid) are used to make cut-proof golf balls (see Fascinating Polymers opposite). The protons on the carboxylic acid groups are exchanged with metal ions to form salts. These ionic species phase-separate into microdomains or clusters which act as crosslinks, or, more accurately, junction zones (Figure 6-4). (We discuss interactions in a little more detail in Chapter 8.)... 

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