Graft polymerization radical catalysts

A radical initiator based on the oxidation adduct of an alkyl-9-BBN (47) has been utilized to produce poly(methylmethacrylate) (48) (Fig. 31) from methylmethacrylate monomer by a living anionic polymerization route that does not require the mediation of a metal catalyst. The relatively broad molecular weight distribution (PDI = (MJM ) 2.5) compared with those in living anionic polymerization cases was attributed to the slow initiation of the polymerization.69 A similar radical polymerization route aided by 47 was utilized in the synthesis of functionalized syndiotactic polystyrene (PS) polymers by the copolymerization of styrene.70 The borane groups in the functionalized syndiotactic polystyrenes were transformed into free-radical initiators for the in situ free-radical graft polymerization to prepare s-PS-g-PMMA graft copolymers. 

Grafting of Vinyl Monomer on Radiation-Peroxidized Polymer. The polymer is irradiated in the presence of air or oxygen and then immersed in monomer. The peroxides in the irradiated polymer are decomposed by heat or catalysts to form free radicals, capable of initiating graft polymerization. 

Many other chemical redox systems have been reported as initiators of macrocellulosic radicals and as catalysts for graft polymerization. One variation has been to modify cellulose chemically to increase its reactivity with selected oxidizing and reducing agents which on reaction yield macrocellulosic radicals (14, 15). 

Another method is based on the metal-catalyzed polymerization from carbon—halogen bonds in the main-chain units, which was applied for the synthesis of C-3 and C-4.430 For C-3, the main chain polymers with controlled molecular weights were prepared via the copper-catalyzed radical polymerization of tri-methylsilyl-protected HEMA followed by the transformation of the silyloxyl group into 2-bromoisobu-tyrate. The pendant C—Br bonds were subsequently activated by the copper catalysts to polymerize styrene and nBA. A more direct way is employed for C-4 i.e., via conventional radical polymerization of 2-[(2-bromopropinonyl)oxy]ethyl acrylate followed by the copper-catalyzed graft polymerization of styrene and nBA from the C—Br substituent. 

The grafting of an alternating copolymer on a substrate polymer occurs when the alternating copolymer is prepared under conditions normally used without the substrate polymer. When comonomers that are subject to spontaneous or thermal initiation are polymerized in the presence of a suitable polymer, graft copolymers are formed despite the absence of a radical catalyst (10). 

Typical results in the graft polymerization of poly (styrene-alt-maleic anhydride) onto polystyrene in solution without a radical catalyst are shown in Table I. Analogous results are obtained in the presence of a radical catalyst. 

Although a radical catalyst is not necessary for graft polymerization when the alternating copolymer forms spontaneously, a radical catalyst is required when alternating copolymer formation requires radical initiation (10). 

The use of a radical catalyst increases polymerization rate and the extent of grafting. When the reaction is done in an aqueous medium, as when the substrate polymer is charged as a latex, a water-soluble catalyst is necessary, as previously observed for the preparation of alternating copolymers in an aqueous medium (15). Although the latex... 

Radiation-induced grafting is a process where, in a first step, an active site is created in the preexisting polymer. This site is usually a free radical, where the polymer chain behaves like a macroradical. This may subsequently initiate the polymerization of a monomer, leading to the formation of a graft copolymer structure where the backbone is represented by the polymer being modified, and the side chains are formed from the monomer (Fig. 1). This method offers the promise of polymerization of monomers that are difficult to polymerize by conventional methods without residues of initiators and catalysts. Moreover, polymerization can be carried out even at low temperatures, unlike polymerization with catalysts and initiators. Another interesting as-... 

Radical catalysts have been used to promote the graft polymerization of MAH with saturated polymers including polyethylene. Catalysts have been used at high concentrations and/or at temperatures where the half-life is extremely short, conditions similar to those used in the homopolymerization of MAH. 

An alternative route to the compatibilization of a filler such as clay with LDPE and HDPE, through the radical catalyzed polymerization of maleic anhydride (MAH) in the presence of the polymer and clay, has been shown to yield PE-g-MAH-clay composites having better mechanical properties than unfilled PE or PE-clay mixtures prepared in the absence of MAH and a radical catalyst, In the present paper, further improvements in the preparation and properties of HDPE-clay composites are described. These result from the use of high melt index HDPE as "coating PE" in the preparation of the PE/clay masterbatch and low melt index HDPE as "matrix PE" in the final HDPE-clay composite. The crosslinking which accompanies the graft polymerization of MAH onto PE also plays a significant role in the enhancement of the mechanical properties of the composite, ... 

Other milestones are grafting by radical, cationic, anionic and coordinative polymerization use of semisynthetic and synthetic polymers as ion exchange resins " and catalysts which, in some cases, mimic and even surpass the efficiency of enzymes and solid phase peptide synthesis by Merrifield and Letsinger. ... 

Goto and coworkers attached a surface-immobilizing initiator IHE onto a siliccm wafer (Scheme 5) and prepared polymer brushes by RTCP [66]. The IHE-immobUized silicon wafer was inunersed in a mixture of MMA, 2-cyanopropyl iodide (a free iodide initiator), azobis(isobutyronitrile) (a radical source), and NIS (a catalyst). The system was purged with an inert gas and heated at 70°C for 4 h to induce polymerization. The and M IM values of the free polymer were 15,000 and 1.31, respectively. From the thickness of the graft polymer and the M of the free polymer, the a value was calculated to be 0.28. This result indicates the successful controlled synthesis of a concentrated polymer brush by RTCP. Another example of the graft polymerization is depicted in Fig. 6,... 

Copolymers of VDC can also be prepared by methods other than conventional free-radical polymerization. Copolymers have been formed by irradiation and with various organometaHic and coordination complex catalysts (28,44,50—53). Graft copolymers have also been described (54—58). 

If a vinyl monomer is polymerized in the presence of cellulose by a free radical process, a hydrogen atom may be abstracted from the cellulose by a growing chain radical (chain transfer) or by a radical formed by the polymerization catalyst (initiator). This leaves an unshared electron on the cellulose chain that is capable of initiating grafting. As cellulose is a very poor transfer agent [10], very little copolymer results from the abstraction of hydrogen atoms by a growing chain radical. The... 

Here we discuss dispersion polymerizations that are not related to vinyl monomers and radical polymerization. The first one is the ring-opening polymerization of e-caprolactone in dioxane-heptane (30). A graft copolymer, poly(dodecyl acrylate)-g-poly(e-caprolactone), is used as a stabilizer. The polymerization proceeds via anionic or pseudoanionic mechanism initiated by diethylaluminum ethoxide or other catalysts. The size of poly(caprolactone) particles depends on the composition of stabilizer, ranging from 0.5 to 5 i,m. Lactide was also polymerized in a similar way. Poly(caprolactone) and poly(lactide) particles with a narrow size distribution are expected to be applied as degradable carriers of drugs and bioactive compounds. 

The introduction of functional groups is suitable to control the chemical and physical properties of the polymer. However, the introduction of functional groups may cause a reaction of the unshared electron pairs of the functional groups with the active catalytic sites. Thus, the active sites of the catalyst are destroyed. In order to overcome this problem, a procedure has been developed, where the functionalized monomers, such as maleic acid, nadic acid or their anhydrides are grafted after the polymerization reaction (4,37). Grafting takes place as a radical reaction, using e.g., dicumyl peroxide. Other attempts use excessive amounts of catalysts. 

PE-g-PS graft copolymer was produced by a coupling reaction, too. a-Carboxyl PS prepared by an ATRP technique was reacted with PE-g-glycidyl methacrylate (GMA) to produce PE-g-PS [121]. A PP-fc-PMMA block copolymer was synthesized using a magnesium bromide terminated PP as an initiator for the radical polymerization of MMA, which was prepared from the vinylidene terminated PP obtained with the Et(Ind)2ZrCl2/MAO catalyst system [122],... 

The emulsion polymerization process involves the polymerization of liquid monomers that are dispersed in an aqueous surfactant micelle-containing solution. The monomers are solubilized in the surfactant micelles. A water-soluble initiator catalyst, such as sodium persulfate, is added to the aqueous phase. The free radicals generated cause the dispersed monomers to react to produce polymer molecules within the micellar environment. The surfactant plays an additional role in stabilizing dispersion of the produced polymer particles. Thus, the surfactants used both provide micelles to house the monomers and macroradicals, and also stabilize the produced polymer particles [193,790], Anionic surfactants, such as dodecylbenzene sulfonates, are commonly used to provide electrostatic stabilization [193], These tend to cause production of polymer particles having diameters of about 0.1-0.3 pm, whereas when steric stabilization is provided by, for example, graft copolymers, then diameters of about 0.1-10 pm tend to be produced [790,791]. 

---