Vinyl acetate termination mechanism

Functionality determination is useful for the elucidation of reaction mechanisms. The fact that Fn < 2 [vinyl acetate (Table 2.2)] means that termination by disproportionation is more important than coupling (Fn = 2). In most cases, the obtained results show the hydroxytelechelic character of the polymer, even if transfer reactions introduce functional groups different from those expected according to the initiation mechanism. 

We will consider the MWD in two simple cases. The first is when chain transfer is sufficiently rapid to ensure that all other chain-stopping events can be ignored. In such a situation, whereas the compartmentalized nature of the reaction may affect the rate of initiation of new chains, it will not affect the lifetime distributions of the chains once they are formed. The MWD may then be found from the bulk formulas, provided only that the average number of free radicals per particle, is known. Such an approach has been used by Friis et al. (1974) to calculate the MWD evolved in a vinyl acetate emukion polymerization. These authors included in addition the mechanisms of terminal bond polymerization and of transfer to polymer (both of which cause broadening). The formulas required for the in corporation of these mechanisms could be taken from bulk theory. 

To discriminate between alternative mechanisms for the initiation and termination reactions in the photopoljmerization of vinyl acetate. 

In a study of chain-transfer constants of the monomeric vinyl acetate it was found that the formation of nonhydrolyzable branches is virtually negligible while hydrolyzable branches are formed at position 1 of Structure 1 by a terminal double-bond reaction rather than by a polymer-transfer reaction. The long nonhydrolyzable branches in poly(vinyl alcohol) are, presumably formed almost exclusively by a polymer transfer mechanism [35]. 

The engineers had formulated requirements as to how the desired adhesive layers should deform under a shear loading. These rules were very unusual because normally, engineers make no requirements at all they just test the adhesives offered by the adhesive company and then report if they are satisfied or not. The adhesive company found it difficult to meet the requirements. The most important characteristics required of the adhesive layer were the flexibility as well as the deformability. Obviously, traditional wood adhesive systems like vinyl acetate or formaldehyde-based adhesives could not be used because they only work in very thin adhesive layers. Furthermore, they are brittle and have virtually no mechanical flexibility. Thus only two technologies were considered as potential candidates polyurethanes (including silane-terminated types) and epoxies. 

Unlike transfer to polymer, the mechanism combines two polymer chains (and all of their repeat units) into one chain, affecting both DP and DP . Reaction with terminally unsaturated chains can be important in vinyl acetate [ 19] and higher temperature methacrylate [1]... 

Shaver and coworkers [319] investigated the mechanism of bis(imino)pyridine ligand framework for transition metal systems-mediated polymerization of vinyl acetate. Initiation using azobisisobu-tyronitrile at 120°C results in excellent control over poly(vinyl acetate) molecular weights and polymer dispersities. The reaction yields vanadium-terminated polymer chains which can be readily converted to both proton-terminated poly(vinyl acetate) or poly(vinyl alcohol). Irreversible halogen transfer from the parent complex to a radical derived from azobisisobutyronitrile generates the active species. 

Aoshima, S., and T. Higashimura. 1984. Vinyl ether oligomers with conjugated-polyene and acetal terminals a new chain-transfer mechanism for cationic polymerization of vinyl ethers. Polymer Journal 16(3) 249-258. 

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. 

It has previously been shown for PS, poly (vinyl acetate), and atatic polypropylene that the shift factor of the terminal relaxation or the viscosity aT,n has a weaker temperature dependence than do the softening dispersion ar.s (Fig. 2.11) and the local segmental relaxation ar,a (Figs. 2.19 and 2.20). Therefore, in practice the shift factors ut used to obtain master curves for polymers by time-temperature superposition are actually combinations of the individual shift factors of the several different viscoelastic mechanisms. At low temperatures, aj is principally determined by the shift factor of the local segmental mode aT,a- With increasing temperature, aj is principally determined sequentially by the shift factors of the sub-Rouse modes, r,sR. the Rouse modes, modes in the rubbery plateau, and, finally, the terminal modes, aT,r,- Hence, it is not correct to assume that aj describes the temperature dependence of any or all of the viscoelastic mechanisms in a polymer. 

A different form of retardation occurs when a radical species formed from transfer (S in Scheme 4.3) reinitiates at a slow rate. In addition to the slower reaction rate with monomer to form a polymer radical, the termination of S with other radicals in the system may also need to be considered (Scheme 4.8). Explicit balances must be written for S, and the extra mechanisms must be included when deriving expressions for [Ptot], Rpoi, and DP . As solvent/transfer agent is generally not completely consumed, the retardation effect will last the duration of the polymerization (curve b in Figure 4.2). The degree of retardation depends on the value of which can vary with monomer type many carbon-centered radicals show much lower reactivity toward vinyl esters (for example, vinyl acetate) than (meth)acrylates [3]. 

The problem of incorporation of chains with a terminal double bond (TDB) exists in polymerizations discussed above, such as radical polymerization of vinyl acetate and olefin polymerization with a constrained-geometry metallocene catalyst (CGC). Tobita [15] has developed an MC algorithm for this problem for the PVAc case. It is assumed that TDBs are created by transfer to monomer only, while recombination is absent, which results in a maximum of one TDB per chain. We largely follow Tobita s explanation, but differ in that we will assume that disproportionation is the termination mechanism, while transfer to solvent and to polymer are not yet being accounted for. Later we will address the real PVAc problem, which in fact has two branching mechanisms TDB propagation and transfer to polymer. 

It is interesting to note that the type of polymerization techniques (conventional emulsion polymerization versus microemulsion polymerization) can have a significant influence on the chain transfer reactions and, consequently, polymer properties [59], For example, in the microemulsion polymerization of vinyl acetate, the chain transfer reaction of a polymeric radical to monomer is the predominant mechanism that terminates the free radical reactivity. Thus, the resultant polyvinyl acetate exhibits a lower degree of branching than that produced by the conventional emulsion polymerization process, in which the chain transfer reaction of a polymeric radical to polymer is significant. 

Backbiting is an intramolecular chain transfer reaction. If transfer reactions occur between different chains, long-chain branched polymers are formed. Well-known examples include ethylene and vinyl acetate. Vinyl acetate polymerization could lead to gel formation under certain conditions. It should be pointed out that chain transfer to polymer reaction alone generates only T-type branch structures that do not result in gel formation. Theoretically, some mechanism such as radical termination by combination that brings two chains together to form H-type branch structures is an essential condition for gelation. 

CoMRP In aqueous dispersed systems Similar to TeRP, CoMRP follows the dual mechanism of reversible termination and degenerative chain transfer. It was applied in suspension and in miniemulsion for the polymerization of VAc and allowed wdl-defined polymers to he prepared at low temperamre (0-30 C) with quite a fast rate The miniemulsion process yidded latexes writh small partides (diameter of approximately lOOnm) and good stahUity. CoMRP is one of the best methods (heside RAIT using a xanthate as a chain transfer agent) to produce poly(vinyl acetate) with conttolled molar mass, and its successful implementation to an aqueous dispersed system is an important step. 

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 ... 

---