Methyl acrylate termination mechanism

If chain transfer of the radical center to a previously formed polymer molecule is followed ultimately by termination through coupling with another similarly transferred center, the net result of these two processes is the combination of a pair of previously independent polymer molecules. T. G. Fox (private communication of results as yet unpublished) has suggested this mechanism as one which may give rise to network structures in the polymerization of monovinyl compounds. His preliminary analysis of kinetic data indicates that proliferous polymerization of methyl acrylate may be triggered by networks thus generated. 

It is very clear that if the initiator has hydroxyl groups, and if the termination takes place exclusively by recombination then a polymeric diol is obtained [2, 3], which is ideal for polyurethane. If the termination takes place by disproportionation, only monofunctional compounds are obtained, which cannot be used in PU. The vinylic and dienic monomers used in practice have various termination mechanisms. Some monomers give only recombination reactions, such as styrene, acrylates (methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethyl hexyl acrylate), acrylonitrile and butadiene. Other monomers give both mechanisms of termination, around 65-75% disproportionation and 25-35% recombination, such as methacrylates (methyl methacrylate, ethyl methacrylate, butyl methacrylate etc.), substituted styrenes and other monomers [2, 3, 4]. 

In addition to the work previously described, the studies of Bielawski and coworkers in which hemi-telechelic pyridine-end-terminated poly(methyl acrylate) was utilized to initiate ionic polymerization upon mechanically activated scission of the boron-nitrogen coordination bond should be mentioned [97]. Although the design of these systems is in principle very interesting, an expression of concern was published and editorial review is currently underway to investigate the reliability of the data reported in the article. Nevertheless, at this stage, no definitive conclusion has been drawn. 

Block Copolymers. Several methods such as ultrasonics (100), radiation (101), and chemical techniques (102,103), including the use of polymer ions, polymer radicals, and organometallic initiators, are available to prepare Block Copolymers of acrylonitrile. Acrylonitrile can be used as either the first-or the second-phase monomer. Depending on the mechanism of termination, a diblock of the AB type and a triblock of the ABA type can be formed by disproportionation or transfer for the former, and recombination for the latter. Some of the comonomers are styrene, methyl acrylate, vinyl chloride, methyl methacrylate, vinyl acetate, acrylic acid, and re-butyl isocyanate. An overview and survey of alternating and block copolymers can be found in Reference 104. 

Capek et al. polymerized various alkyl acrylates, methyl (MA), ethyl (EA), butyl (BA), hexyl (HA) and 2-ethylhexyl (EHA) acrylate, and alkyl methacrylates in microemulsion [100]. Microemulsion polymerizations of BA and EHA reached in a short time a conversion close to 100%. In case of PMMA the polydispersity index varied from 2 to 4. This can be taken as evidence that the chain transfer events contribute to the termination mechanism [57]. 

Vinylidene terminal groups can be formed by disproportioning they cause degradation by a zipper mechanism that reaches its maximum degradation rate at 280 °C [620]. Small amounts of statistically incorporated methyl acrylate disrupt this mechanism and increase thermal stability. 

These structures, together with the sextet stractures (22) and (23) are usually assumed to be the important valence-bond structures for the construction of the electronic mechanism for cycloaddition . For the latter two structures, the terminal atoms are both nucleophilic (-) and electrophilic (+), and it is these properties of the 1,3 dipole that are often assumed to be implicated for the electronic mechanism of the cycloaddition. As CH Nj approaches the methyl acrylate, one set of Ji-electron atomic orbitals of CHjNj overlaps with the Ji-orbitals of methyl acrylate (see Fig. 22-3). 

Aromatic nitro compounds act as inhibitors and show greater tendency toward more reactive and electron-rich radicals. Nitro compounds have very little effect on methyl acrylate and methyl methacrylate [5,10,11] but inhibit vinyl acetate and retard styrene polymerization. The effectiveness increases with the number of nitro groups in the ring [1 13]. The mechanism of radical termination involves attack on both the aromatic ring and the nitro group. The reactions are represented as follows ... 

Organo-cobalt Porphyrins in LRP of Methyl Acrylate hy the Reversible Termination Mechanism... 

Pentadienyl-terminated poly(methyl methacrylate) (PMMA) as well as PSt, 12, have been prepared by radical polymerization via addition-fragmentation chain transfer mechanism, and radically copolymerized with St and MMA, respectively, to give PSt-g-PMMA and PMMA-g-PSt [17, 18]. Metal-free anionic polymerization of tert-butyl acrylate (TBA) initiated with a carbanion from diethyl 2-vinyloxyethylmalonate produced vinyl ether-functionalized PTBA macromonomer, 13 [19]. 

Abbreviations y x AFM AIBN BuMA Ca DCP DMA DMS DSC EGDMA EMA EPDM FT-IR HDPE HTV IPN LDPE LLDPE MA MAA MDI MMA PA PAC PB PBT PBuMA PDMS PDMS-NH2 interfacial tension viscosity ratio atomic force microscopy 2,2 -azobis(isobutyronitrile) butyl methacrylate capillary number dicumyl peroxide dynamic mechanical analysis dynamic mechanical spectroscopy differential scanning calorimetry ethylene glycol dimethacrylate ethyl methacrylate ethylene-propylene-diene rubber Fourier transform-infra-red high density polyethylene high temperature vulcanization interpenetrating polymer network low density polyethylene linear low density polyethylene maleic anhydride methacrylic acid 4,4 -diphenylmethanediisocyanate methyl methacrylate poly( amide) poly( acrylate) poly(butadiene) poly(butylene terephtalate) poly(butyl methacrylate) poly(dimethylsiloxane) amino-terminated poly(dimethylsiloxane)... 

A similar mechanism was proposed when 1,5-dithiocin 838g underwent polymerizations with methyl methacrylate (MMA) and styrene (STY). The activated double bound of 838g was found to have a profound affect on reactivity. In fact, co-polymerization of 838g with MMA at 70 °C the 5-terminated sulfanyl radicals preferred to undergo homopropagation, while cross-propagation is favored for MMA-terminated radicals. Both monomers possessed an electron-deficient acrylate double bond with similar possibilities for conjugative stabilization of the adduct radical by the ester functionality, which would explain the apparent equal reactivity of the MMA radical to either monomer. 

It cannot be excluded that the back biting depicted in Scheme 10 could be responsible for the observed absence of methyl proton.389 Scheme 10 would leave most of the dead polymer molecules without methyl end groups. The chemical shifts of the vinylic protons in the resulting oligo-acrylamides are at 5.75 and 6.25 ppm, which is similar to the methacrylate end group of polyBA which has been terminated by back biting. In oligo-acrylates produced by CCT, the vinylic protons have resonances at 6.8 and 5.85 ppm.383 The mechanism of Scheme 11 in acrylate polymerization requires additional study. 

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