Transfer-to-polymer

Transfer to polymer can be studied particularly well by adding comparatively low-molecular-weight polymers of the same type (a-polymers) to the polymerizing monomer mixture. The transfer const nt Cp iy can then be calculated from the degrees of polymerization X or found for polymerizations with and without the addition of these a-polymers  

The concentration [poly] should be given in terms of monomeric units. 

In poly(styrene) at 60°C, Cp iy = 2 x 10 i.e., somewhat higher than the transfer constant for the monomer. Cp jy is also independent of the degree of polymerization in poly(styrene), but not in poly(methyl methacrylate), where Cp jy increases as the degree of polymerization falls, and therefore must depend on the end groups. 

A transfer to polymer produces a radical site on the polymer, and this site can add on further monomer, thereby producing branched polymer. If transfer to polymer occurs intramolecularly as in poly(ethylene) (see Section 25.2.1), then short-chain branching will result  

In other polymers, such as poly(vinyl acetate), the branching that results from transfer to polymer is predominantly intermolecular  

The intramolecular process does not give rise to a new polymer chain and is considered in Section 4.4.3. It will not be considered further in this section. 

For some polymers, the value of CV depends on the polymer molecular weight (e.g. Section 6.2.7.2). This may help account for the wide range of values for CP in the literature (Table 6.15). 

Numbers art taken from the Polymer Handbook figures. 

Multifunctional transfer agents are of no practical importance. They can, however, contribute significantly to our knowledge of transfer to polymer. 

Each polymer is really a potential multifunctional transfer agent. It depends only on the reactivity of the growing radicals or ions under the given conditions as to how often it will be attacked. Transfer to polymer has many variants. In principle, the reaction is always of the type  

Teblina et al. [26] studied the kinetics of radical polymerization of metha-crylic acid in the presence of a hexafluoropropene-vinylidene fluoride copolymer. The polymerization rate increased with conversion. A grafted copolymer was formed by a reaction analogous to eqn. (40) (with F instead of 

Transfer to polyalkenes is similar when these are present during radical polymerization of vinyl monomers. Cross-linking transfer was also observed in ionic polymerizations [27]. 

The value of the radio kyt jkp, where kyc P is the rate constant of transfer to polymer, inereases with temperature. Unwanted branching can therefore be suppressed by polymerization at the lowest possible temperature and by limited conversion. 

Not all transfers to polymer give rise to branched polymers. Many lead to a redistribution of chain lengths. 

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The magnitude of the individual terms in the summation depends on both th( specific chain transfer constants and the concentrations of the reactants undei consideration. The former are characteristics of the system and hence quantitie over which we have little control the latter can often be adjusted to study particular effect. For example, chain transfer constants are generally obtainec under conditions of low conversion to polymer where the concentration o polymer is low enough to ignore the transfer to polymer. We shall return belov to the case of high conversions where this is not true. 

As noted above, chain transfer to polymer does not interfere with the determination of other transfer constants, since the latter are evaluated at low conversions. In polymer synthesis, however, high conversions are desirable and extensive chain transfer can have a dramatic effect on the properties of the product. This comes about since chain transfer to polymer introduces branching into the product ... 

A moment s reflection reveals that the effect on v of transfer to polymer is different from the effects discussed above inasmuch as the overall degree of polymerization is not decreased by such transfers. Although transfer to polymer is shown in one version of Eq. (6.84), the present discussion suggests that this particular transfer is not pertinent to the effect described. Investigation of chain transfer to polymer is best handled by examining the extent of branching in the product. We shall not pursue the matter of evaluating the transfer constants, but shall consider instead two specific examples of transfer to polymer. 

The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

Chain transfer to monomer and to other small molecules leads to lower molecular weight products, but when polymerisation occurs ia the relative absence of monomer and other transfer agents, such as solvents, chain transfer to polymer becomes more important. As a result, toward the end of batch-suspension or batch-emulsion polymerisation reactions, branched polymer chains tend to form. In suspension and emulsion processes where monomer is fed continuously, the products tend to be more branched than when polymerisations are carried out ia the presence of a plentiful supply of monomer. 

Investigation has shown that chain transfer to polymer occurs predominantly on the acetate methyl group in preference to the chain backbone one estimate of the magnitude of the predominance is 40-fold (92,93). The number of branches per molecule of poly(vinyl acetate) polymerised at 60°C is ca 3, at 80% conversion. It rises rapidly thereafter and is ca 15 at 95% conversion and 1-2 x lO" number-average degrees of polymerisation. 

It has been shown that intramolecular chain transfer to polymer occurs during the polymerisation of vinyl acetate, lea ding to short-chain branching (81,235—238). The number of short-chain branches has been estimated by nmr to be in the range of 0.12—1.7 mol % (81). The number of short-chain branches increases significantly at low monomer concentration. 

Vinyl acetate may be easily polymerised in bulk, solution, emulsion and suspension. At conversions above 30%, ehain transfer to polymer or monomer may occur. In the case of both polymer and monomer transfer two mechanisms are possible, one at the tertiary carbon, the other (illustrated in Figure 14.4) at the acetate group. 

The non-bonded interaction energy, the van-der-Waals and electrostatic part of the interaction Hamiltonian are best determined by parametrizing a molecular liquid that contains the same chemical groups as the polymers against the experimentally measured thermodynamical and dynamical data, e.g., enthalpy of vaporization, diffusion coefficient, or viscosity. The parameters can then be transferred to polymers, as was done in our case, for instance in polystyrene (from benzene) [19] or poly (vinyl alcohol) (from ethanol) [20,21]. 

Branches in PVC can be formed by transfer to polymer during polymerization. Short branches in PVC have usu-... 

Chain transfer to polymer is reported as a major complication and is thought to be unavoidable in the polymerization of alkyl acrylates.200 202 The mechanism is believed to involve abstraction of a tertiary backbone hydrogen (Scheme 6.32). It has been proposed that this process and the consequent formation of branches may contribute to the early onset of the gel or Norrish-Trommsdorff effect in the polymerization of these monomers. At high temperatures the radicals formed may undergo fragmentation. 

PVAc is known to contain a significant number of long chain branches. Branches to the acetate methyl may arise by copolymerization of the VAe macromonomcr produced as a consequence of transfer to monomer (Section 6.2.6.2). Transfer to polymer may involve either the acetate methyl hydrogens (Scheme 6.34) or the methine (Scheme 6.35) or methylene hydrogens of the polymer backbone. 

The extent of branching, of whatever type, is dependent on the polymerization conditions and, in particular, on the solvent and temperature employed and the degree of conversion. Nozakura et at.1 1 found that, during bulk polymerization of VAc, the extent of transfer to polymer increased and the selectivity (for abstraction of a backbone vs an acetoxy hydrogen) decreases with increasing temperature. 

The microstrueture of PVC has been the subject of numerous studies (Sections 4.3.1.2 and 6.2.6.3).214 Starnes el n/.l6S determined the long chain branch points by NMR studies on PE formed by Bu,SnlI reduction of PVC. They concluded that the probable mechanism for the formation of these branches involved transfer to polymer that occurred by hydrogen abstraction of a backbone methine by the propagating radical (Scheme 6.32),... 

Crosslihkinq Density Distribution. Let us consider the statistical copolymerization of vinyl/divinyl monomers without chain transfer to polymer for simplicity. In this case the crosslinking density p is defined as follows. 

LDPE polymerization reaction consists of various elementary reactions such as initiation, propagation, termination, chain transfer to polymer and monomer, p-scission and so forth [1-3], By using the rate expression of each elementary reaction in our previous work [4], we can construct the equations for the rate of formation of each component. 

C. H. Bamford and H. Tompa, J. Polymer Sci.j 10, 345 (1953), first derive the moments of the distribution for the case of chain transfer to polymer. They then obtain the molecular weight distribution from these moments by appropriate mathematical methods. Their procedure should be applicable to a wide variety of polymerization mechanisms. 

The molecular weight distributions in chain polymerizations are broader than in step polymerization. The ratio Xw/Xn can reach as high as 5-10 due to the autoaccelerative effect and as high as 20-50 due to chain transfer to polymer. 

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