Functionalization direct copolymerization

So far, theoretically, there have been two possible approaches for the synthesis of functional polyolefins, namely, (a) direct copolymerization of olefins with functional monomers using polymerization and catalyst technologies, and (b) post-polymerization reaction with polyolefins. 

The first approach has found limited applications. Concerning the direct copolymerization of olefins with functional monomers, studies of copolymerization of olefins by metallocene catalysts have been reported with functional... 

In contrast to Group IV-based polymerization catalysts, late transition metal complexes can carry out a number of useful transformations above and beyond the polyinsertion reaction. These include isomerization reactions and the incorporation of polar monomers, which have allowed the synthesis of branched polymer chains from ethylene alone, and of functional polyolefins via direct copolymerization. The rational design of metallocene catalysts allowed, for the first time, a precise correlation between the structure of the single site catalyst and the mi-crostructure of the olefin homo- or copolymer chain. A similar relationship does not yet exist for late transition metal complexes. This goal, however, and the enormous opportunities that may result from new monomer combinations, provide the direction and the vision for future developments. 

Ionomers of practical interest have been prepared by two synthetic routes (a) copolymerization of a low level of functionalized monomer with an olefinically unsaturated monomer or (b) direct functionalization of a preformed polymer. Typically, carboxyl containing ionomers are obtained by direct copolymerization of acrylic or methacrylic acid with ethylene, styrene and similar comonomers by free radical copoly-merization. Rees (22) has described the preparation of a number of such copolymers. The resulting copolymer is generally available as the free acid which can be neutralized to the degree desired with metal hydroxides, acetates and similar salts. Recently, Weiss et al.(23-26) have described the preparation of sulfonated ionomers by copolymerization of sodium styrene sulfonate with butadiene or styrene. 

Propylene oxide PO was mostly used as comonomer in the preparation of bifunctional THF copolymers by direct copolymerization process. The required products must have a sufficiently low m.p., uniform structure and the functionality equal to two. It is also desirable to have the same OH end groups, either primary (preferentially) or secondary. 

III. Direct Copolymerization of Functional Monomers 1482 with Group IV Catalysts... 

Direct copolymerization [1, 2], In this case, two types of monomers react, one of them having a functional or pendant functional group for instance, the copolymerization of maleic anhydride (MA) and styrene (St) generates the alternating copolymer poly (St-a/t-MA) [1]. Another example is the direct copolymerization of a-olefins (polypropylene, PP, and polyethylene. 

Besides post-modification, a direct copolymerization is suitable not only for introduction of molecular functional groups to the carbon products, but also allows well dispersion of nanoparticles throughout the carbon framework. Researchers from... 

Unlike molecules containing electron-rich heteroatoms, boron compounds do not poison Ziegler-Natta or metallocene polymerization catalysts. Borane-containing olefin comonomers are therefore well suited to produce olefin copolymers while retaining good catalyst activity. The resulting polymers are suitable for subsequent conversion into a variety of functional groups. In principle, two approaches are possible (1) hydroboration of the terminal double bond (formed by typical chain transfer processes) of a preformed polyolefin, and (2) direct copolymerization of propylene or a 1-alkene with an alkenyl borane (Scheme 11.4). 

Interest in homogeneous olefin polymerization catalysts, especially group 4 metallocenes has caused a dramatic increase in the number of publications describing the synthesis of functionalized polyolefins by direct copolymerization. Many soluble metallocenes, such as bridged zirconocenes, have much better ability to incorporate higher a-olefins than do Ti-based Ziegler-Natta catalysts. This also makes them better suited for copolymerizations involving, often very bulky, functional comonomers. 

The second way involves direct copolymerization of functionalized monomers. Modification of monomers is a fundamentally different approach from modification of polymer in that modification of monomer makes feasible control of the molecular structures. Both diphenols and dihalide aryl sulfones can be modified to incorporate functional groups or new counterparts. 

Figure 3.10 (a) Direct copolymerization of H bonds via ROMP and the alternative reaction pathway to yield a similar polymer, (b) End-group-modified telechelic polynorbomenes via quench with functionalized ethyl-vinyl ethers. 

Various strategies for the syntheses of either aliphatic or aromatic functional fluorinated monomers have been proposed in the hterature. Because of their costs, they have been involved in copolymerization with fluoroalkenes, and although a lack of basic research is noted (e.g., no assessment of the reactivity ratios), many apphed investigations have been developed. In fact, most companies producing fluorinated monomers and derivatives have solved the challenge to prepare fluorocopolymers bearing sulfonic acid side groups. Nevertheless, quite a few studies concern phosphonic acid function. Compared with direct copolymerization, the alternative to prepare fluorofunctional copolymers by chemical modification of polymers is often employed. 

Aromatic polymers, such as PESs, poly(ether ether ketone)s, and polyimides, are used as polymer matrices for PEMs due to the high performance described earlier. Sulfonated PEMs are generally prepared by two methods the postsulfonation of aromatic polymers usually leading to a random functionalization along the polymer main chains and direct copolymerization of the sulfonated monomers to afford random copolymers. Both methods are discussed in the following. 

Soapless seeded emulsion copolymerization has been proposed as an alternative method for the preparation of uniform copolymer microspheres in the submicron-size range [115-117]. In this process, a small part of the total monomer-comonomer mixture is added into the water phase to start the copolymerization with a lower monomer phase-water ratio relative to the conventional direct process to prevent the coagulation and monodispersity defects. The functional comonomer concentration in the monomer-comonomer mixture is also kept below 10% (by mole). The water phase including the initiator is kept at the polymerization temperature during and after the addition of initial monomer mixture. The nucleation takes place by the precipitation of copolymer macromolecules, and initially formed copolymer nuclei collide and form larger particles. After particle formation with the initial lower organic phase-water ratio, an oligomer initiated in the continuous phase is... 

It was recently found that j3-PCPY can also be used as a radical initiator to obtain an alternate copolymer of MMA with styrene [35], which was only possible in the presence of Lewis acids [36,37] in the past. The kinetics of the system has been formulated as Rp a[/3-PCPY] a[MMA] (l/a[Styrene] The values of kp /k, and AE were evaluated as 1.43 x 10 L mol -s and 87 kJ/ mol, respectively, for the system. NMR spectroscopy was used to determine the structure composition and stereochemistry of copolymers. Radical copolymerization of AN with styrene [38] by using /3-PCPY as the initiator at 55-65°C also resulted in an alternate copolymer. Rp is a direct function of /3-PCPY and AN, and is inversely related to styrene. 

Only a few quantitative data are available on copolymerization of methacrylates. Direct determination of the cross-propagation constants is readily achieved in living polymer systems whenever the absorption spectra of the two propagating species are different. Unfortunately, this is not the case in the methacrylate series. A new approach to this problem was developed by Muller 43). A mixture of two monomers is copolymerized, the reaction is interrupted at various times, and the concentrations of the residual monomers are determined as functions of time. The pertinent differential equations include 4 constants ku, k12, k21, and k22. Since kn and k22 were independently determined, the remaining cross-propagation constants are obtained by computer fitting the experimental conversion curves to the calculated ones. 

Ionic polymers are a special class of polymeric materials having a hydrocarbon backbone containing pendant acid groups. These are then neutralized partially or fully to form salts. lonomeric TPEs are a class of ionic polymers in which properties of vulcanized rubber are combined with the ease of processing of thermoplastics. These polymers contain up to 10 mol% of ionic group. These ionomeric TPEs are typically prepared by copolymerization of a functionalized monomer with an olefinic unsamrated monomer or direct functionalization of a preformed polymer [68-71]. The methods of preparation of various ionomeric TPEs are discussed below. 

Reactive polymers can be synthesized by either polymerizing or copolymerizing monomers containing the desired functional groups, or performing one or more modifications on a suitable polymer to introduce the essential functionality. Polymers produced directly by polymerization of functionalized monomers have well defined structures, but the physical and mechanical properties of the... 

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]. 

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