Homopolymerization of MAH

The rapid decomposition of a peroxide in the presence of MAH results in the excitation and homopolymerization of MAH. When the latter is conducted in the presence of polypropylene, the latter undergoes degradation ( 10) while polyethylene is crosslinked under the same conditions (11). This has been attributed to the presence of excited MAH which increases the radical generation on the polymer beyond that due to the radicals from the peroxide. 

Stearamide is one of many electron donors which donate an electron to the cationic moiety in excited MAH or in propagating -MAH chains. This results in the inhibition of the homopolymerization of MAH and decreases the crosslinking of polyethylene and the degradation of polypropylene which accompany the peroxide-catalyzed reaction of MAH with these polyolefins (8,9). ... 

These conditions are identical to those required for the homopolymerization of MAH, i.e. the catalyst has a half-life of up to about 30 min at the reaction temperature (6). Thus, t-butyl perbenzoate (tBPB) is effective in the preparation of LDEE-clay composites at 130 and 150 c and HDEE-clay composites at 150 C while dicumyl peroxide is less effective in LDFE-clay composites at 130 C than at 150 C. 

The compatibilization of clay with LDPE and HDFE is accomplished by the in situ polymerization of MAH or its precursor maleic acid, in the presence of a radical catalyst. The latter must be capable of initiating the homopolymerization of MAH, i.e. it must be present in high concentration and/or have a half-life of less than 30 min at the reaction temperature, e.g. t-butyl per-benzoate (tBFB) at 150°C. In a one-step process, the clay and PE are mixed with MAH-tBPB in the desired PE/clay ratio. In the preferred two-step process, a 70/30-90/10 clay/PE concentrate is prepared initially in the presence of MAH-tBPB and then blended with additional PE to the desired clay loading. The compatibil-ized or coupled PE-MAH-clay composites have better physical properties, including higher impact strengths, than unfilled PE or PE-clay mixtures prepared in the absence of MAH-tBPB. 

The failure to initiate the homopolymerization of maleic anhydride (MAH) in the presence of a free radical catalyst under normal conditions was long considered evidence of the steric hindrance imposed by 1,2-disubstitution of the double bond. However, in recent years, the radical homopolymerization of MAH has been carried out under the influence of V-radiation, ultraviolet radiation in the presence of a sensitizer, and shock waves, as well as in the presence of free radical catalysts at high concentrations and/or at temperatures where they have a short half-life. These and other aspects of the polymerization of MAH have been reviewed by Gaylord, who has proposed the participation of n excited charge transfer complex and of cationic intermediates in the radical catalyzed homopolymeri zation. 

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. 

The present investigation was undertaken in an attempt to correlate the proposed participation of cationic intermediates in the radical-induced homopolymerization of MAH with the use of MAH polymerization conditions in the graft copolymerization onto PE. 

The distinction suggested by the terms "reactive mononer and "unreactive monomer is not supported by the results obtained in the homopolymerization of styrene and of maleic anhydride (MAH) in the presence of a peroxyester undergoing rapid decomposition. Thus, the addition of 0.5 mmole di-sec-butyl peroxydicarbonate (t.. 1.3 hr) in 4 portions at 2 min intervals (total reaction... 

The polymerization of MAH does not occur under normal conditions but is readily initiated under gamma or ultraviolet radiation and by the use of radical catalysts at high concentrations or having a short half life at the reaction temperature. The radical initiated homopolymerization is promoted by the presence of photosensitizers in the absence of light 2, 2 ). It has been proposed that under these conditions MAH undergoes excitation and the excited monomer, actually an excited dimer or charge transfer complex, polymerizes. The participation of the excimer or excited complex and the cationic character of the propagating chain has been confirmed by the total inhibition of MAH polymerization in the presence of small amounts of dimethyIformamide which has no effect on the polymerization of "reactive acrylic monomers ( ). 

The radical catalyzed homopolymerization of the furan-maleic anhydride (F-MAH) Diels-Alder adduct yields a saturated homopoly-mer at temperatures below 60 C, and an unsaturated equimolar alternating copolymer at elevated temperatures, due to retrograde dissociation of the adduct (10, 11). The copolymerization of monomeric furan and maleic anhydride yields the same unsaturated alternating copolymer, independent of temperature (1C)). 

In contrast, the radical catalyzed homopolymerization of the cyclopentadiene-maleic anhydride (CPD-MAH) Diels-Alder adduct yields a saturated homopolymer at temperatures as high as 220 C, while retrograde dissociation occurs at even higher temperatures. Nevertheless, the copolymerization of monomeric cyclopentadiene and maleic anhydride yields a saturated 1 2 copolymer (12-15). 

The homopolymerization of the endo and exo adducts was carried out in the melt at 150 to 260 C and in chlorobenzene at 120°C (Table I). Polymer was obtained when the catalyst was used at a temperature where the half-life was short, e.g. less than 2 hr, conditions shown to be effective in the homopolymerization of maleic anhydride (16), norbornene (17, 18) and 5-norbornene-2,3-di-carboxylic anhydride (CPD-MAH adduct) (12-15), as well as the... 

It has previously been proposed that the saturated products from the homopolymerization of norbornene (17) and the CPD-MAH Diels-Alder adduct (12-15), and probably from the furan-maleic anhydride Diels-Alder adduct (11), have rearranged structures. An analogous structure would arise from the homopolymerization of the cyclopentadiene-N-phenylmaleimide CPD-NPMI adduct, as follows ... 

MAH is one of the monomers most often used for polyolefins functionalization. It is characterized by an extremely low capacity to homopolymerization, and this fact is explained by the steric features of its structure. The reactivity of MAH to macroradicals, however, is comparatively low. From the chemistry viewpoint, a steric hindrance and a lack of electron density in the double bond explain the low reactivity of MAH, which in MAH is symmetrical owing to the presence of two carbonyl groups. Attempts have repeatedly been made to work out procedures for increasing the chemical activity of MAH. Three methods have been proposed to activate the double bonds in MAH (i) to perform a grafting reaction for MAH in presence of an electron-donating monomer, for example, styrene, which is capable of forming a charge transfer complex (CTC) with MAH (ii) substitution... 

Whereas homopolymerizations of maleic or fumaric add derivatives have not yet found technical interest, copolymers derived from MAH and various vinyl monomers are commercialized by several chemical companies. Such commerdal copolymers may be based on styrene (Scriptset or SMA), ethene (EMA, Malethamer, VINAC), isobutene (ISOBAM), butadiene (Maldene), vinyl alkyl ethers (Gantrez AN or Viscofres), and vinyl acetate (Lyfron, Benex, Amoco DriUing Aid 420,421, or Baroid X tend). 

Copolymerizations of MAH have also found great theoretical interest, because they almost every time yield alternating copolymers and numerous 1,2-di-substituted ethens, that are reluctant to homopolymerization, can be copolymerized. 

The tendency of MAH to form copolymers is so strong, that it can even be copolymerized with thiophene, its 2-methyl or 3-methyl derivatives [1017-1019], furan and 2-methylfuran [1020,1021] all stable aromatic heterocycles, that are reluctant to homopolymerize or copolymerize with other monomers. The repeating units consist of structures with 2,5-linkages (furan, thiophen) and 2,3-linkages across the methyl-substituted derivatives (Figure 12). 

Since the F-MAH and CPD-MAH Diels-Alder adducts yield unsaturated and saturated polymers, respectively, as a result of radical catalyzed homopolymerization at elevated temperatures, it was of interest to investigate the radical catalyzed polymerization of the isomeric exo- and endo-cyclopentadiene-N-phenylmaleimide adducts, models for the norbornene end-capped polyimides whose thermal polymerization products have structures which have been diversely depicted as saturated and unsaturated, without experimental verification. 

S-MAH" " the mechanism of initiation was proposed to be analogous to that of S homopolymerization (Scheme 3.62) but with acrylonitrile acting as the dicnophilc in the fonnation of the Diels-Alder adduct (Scheme 3.66). 

Unlike monosubstituted and 1,1-disubstituted ethylenes, 1,2-disubstituted ethylenes, as a whole, cannot undergo homopolymerization and form high molecular weight polymers (22). Maleic anhydride (MAH) and its mono- and diesters make an example of such compounds. 

The temperature at which a dynamic equilibrium is reached between the formation and the decay of monomer macroradicals is called a ceiling temperature. For certain monomers, there are published ceihng temperatures, heats, and entropy of polymerization (28,29). Their values are, for example, 150°C for MAH, 200°C for methacrylate, 400°C for acrylate and styrene (28). It should be noted that these values are typical of reactions occurring at a constant (atmospheric) pressure and monomer concentration (usually 1 mol). The peak temperature rises with monomer concentration and pressure. That is why MAH was observed to homopolymerize at an extrusion temperature above 160°C (30). 

The choice of maleic anhydride (MAH) as the functionalizing agent is suitable for several reasons. The most important one -- a fact which distinguishes MAH from other unsaturated molecules bearing functional groups - is that MAH, similarly as other 1,2-di-substituted olefins, does not homopolymerize easily. This makes the grafting product, EPM-g-SA, practically unmodified in its rubbery properties and miscible with the parent EPM. 

All of the reactions which follow from the presence and reactions of the MAH" carbenium ion are suppressed by the presence of DMF, i,e, MAH homopolymerization, MAH graft copolymerization onto and from PE and crosslinking of PE,... 

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