Methyl resonance polymerization

Inomata 211) studied the H-NMR spectra of poly(penta-1,3-diene) and concluded that with hexane as polymerization medium the polymers were about 49% cis-1,4 and 40% trans-1,4 enchained. The polymer derived from the cis monomer had 12% of 1,2-units which were exclusively trans that from the trans monomer had some 10 % of 1,2-units, two thirds of which were trans. Aubert et al.2I6) made a more extensive study of pentadiene polymers using both1H and 13C-NMR spectroscopy and modified the cis and trans-1,4 methyl resonance assignments made by Inomata 2U). 

It should be noted that the resonances observed at 0.95 and 1.25 6 In the spectrum of the catalytlcally-prepared polymer (14) are virtually absent In the present case. These peaks coincide closely with the strong methyl resonances In the B-plnene oxide monomer (Figure 3(a)), and so It Is conceivable that they arise from contamination by monomer. In a study of the polymerization of B-plnene... 

Rhodium-catalyzed polymerization of ethylene was reported very early in the history of organometallic chemistry, but the more recent hard-ligand triaza-cyclononane complexes of rhodium (Cn = 1,4,7-trimethyl-1,4,7-triazacyclononane) are the best characterized. At room temperature, ethylene is slowly polymerized by [(Cn)RhMe(H20)(OH)]+ in water and more rapidly in acetone. At 50 °C, the disappearance of the methyl resonance in the presence of ethylene is first order in ethylene and in rhodium. The polymerization rate profile versus pH in water indicates that [CnRhMe(OH2)(OH)]+ is the most effective catalyst, [CnRhMe(OH2)2] is much slower, and... 

Summaiy In this short review, selected experimental approaches for probing the mechanism and kinetics of RAFT polymerization are highlighted. Methods for studying RAFT polymerization via varying reaction conditions, such as pressure, temperature, and solution properties, are reviewed. A technique for the measurement of the RAFT specific addition and fragmentation reaction rates via combination of pulsed-laser-initiated RAFT polymerization and j,s-time-resolved electron spin resonance (ESR) spectroscopy is detailed. Mechanistic investigations using mass spectrometry are exemplified on dithiobenzoic-acid-mediated methyl methacrylate polymerization. 

The nmr spectmm of PVAc iu carbon tetrachloride solution at 110°C shows absorptions at 4.86 5 (pentad) of the methine proton 1.78 5 (triad) of the methylene group and 1.98 5, 1.96 5, and 1.94 5, which are the resonances of the acetate methyls iu isotactic, heterotactic, and syndiotactic triads, respectively. Poly(vinyl acetate) produced by normal free-radical polymerization is completely atactic and noncrystalline. The nmr spectra of ethylene vinyl acetate copolymers have also been obtained (33). The ir spectra of the copolymers of vinyl acetate differ from that of the homopolymer depending on the identity of the comonomers and their proportion. 

In addition, Bamford, Jenkins and coworkers (19) previously reported on the behavior of occluded radicals in the heterogeneous polymerizations of acrylonitrile, methyl acrylate, methyl methacrylate and vinylidene chloride. From their electron spin resonance studies, they concluded that the degree of occlusion was ... 

Methylene cyclopropene (5), the simplest triafulvene, is predicted to be of very low stability. From different MO calculations5 it has been estimated to possess only minor resonance stabilization ranging to 1 j3. Its high index of free valency4 at the exocyclic carbon atom causes an extreme tendency to polymerize, a process favored additionally by release of strain. Thus it is not surprising that only one attempt to prepare this elusive C4H4-hydrocarbon can be found in the literature. Photolysis and flash vacuum pyrolysis of cis-l-methylene-cyclopropene-2,3-dicarboxylic anhydride (58), however, did not yield methylene cyclopropene, but only vinyl acetylene as its (formal) product of isomerization in addition to small amounts of acetylene and methyl acetylene65 ... 

In many cases, homopolymerization can be initiated by the anion-radicals of the monomers themselves. Of course, such monomers must have pronounced electron affinity (EA) and be stabilized by delocalization of an unpaired electron. Typical examples are represented by the anion-radicals of 1,1-dicyanoethylene (EA = 1.36 eV) and methyl or ethyl 2-cyanoacrylates (EA = 1.08 eV). In all of these anion-radicals, an unpaired electron is primarily localized on C atom of the CH2 segment and characterized by appreciable resonance stabilization (Brinkmann et al. 2002). These anion-radicals are nucleophilic and attack the neutral monomers to initiate polymerization. 

Alkene polymers such as poly(methyl methacrylate) and polyacrylonitrile are easily formed via anionic polymerization because the intermediate anions are resonance stabilized by the additional functional group, the ester or the nitrile. The process is initiated by a suitable anionic species, a nucleophile that can add to the monomer through conjugate addition in Michael fashion. The intermediate resonance-stabilized addition anion can then act as a nucleophile in further conjugate addition processes, eventually giving a polymer. The process will terminate by proton abstraction, probably from solvent. 

We observe the a-methylene carbons of the methyl tetrahydro-furanium ion, the a-carbons of the two types of propagating chain heads, the macroion and the macroester (17). The observation of the a-methylene carbon resonances of the acyclic tertiary oxonium ion provides a direct proof of chain transfer reaction in THF polymerization. 

Figure 11 shows the a carbon resonance region of such a THF/ OXP copolymerization in CH3NO2. At about 55 ppm we observe the peak due to the methoxy methyl carbons of the chain ends, and further downfield a solvent peak and then the methylene carbons of the unreacted monomers, THF and OXP. There are two peaks attributable to the polymeric methylene carbons. The higher field one is due to THF and the other one to OXP. Similarly, two peaks are observed for the methylene carbons attached to the methoxy chain ends. The fact that the intensities of these two peaks are similar indicates that both THF and OXP participate in the initiation step. 

Many substituents stabilize the monomer but have no appreciable effect on polymer stability, since resonance is only possible with the former. The net effect is to decrease the exothermicity of the polymerization. Thus hyperconjugation of alkyl groups with the C=C lowers AH for propylene and 1-butene polymerizations. Conjugation of the C=C with substituents such as the benzene ring (styrene and a-methylstyrene), and alkene double bond (butadiene and isoprene), the carbonyl linkage (acrylic acid, methyl acrylate, methyl methacrylate), and the nitrile group (acrylonitrile) similarly leads to stabilization of the monomer and decreases enthalpies of polymerization. When the substituent is poorly conjugating as in vinyl acetate, the AH is close to the value for ethylene. 

Polar monomers, such as methyl (meth)acrylate, methyl vinyl ketone, and acrylonitrile, are more reactive than styrene and 1,3-dienes because the polar substituent stabilizes the carba-nion propagating center by resonance interaction to form the enolate anion. However, the polymerizations are more complicated than those of the nonpolar monomers because the polar... 

Nishioka, Watanabe, Abe, and Sono (48) carried out an extensive study of the Grignard reagent catalyzed polymerization of methyl methacrylate in toluene with respect to tactidty of the resulting polymers. The tactidty of the polymer was determined quantitatively by nuclear magnetic resonance analysis. It was found that the stereo-regularity depended on the nature of the R group of the Grignard... 

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