Chain transfer to polymer

Chain transfer to polymer is another important reaction that can greatly affect the structure of macromolecules and, hence, the properties of the polymers. Moreover, contrary to chain transfer to solvent that can be eliminated or at least suppressed by the use of another solvent, transfer to polymer is an inherent property of the polymerizing system. The effect of this reaction on molecular weight characteristics of polymers has been studied only for free radical polymerization [23-25]. In particular, it was shown that transfer to polymer may cause gelation if termination proceeds via combination. Principal results for living polymerization were obtained in Ref [26]. 

The occurrence of chain transfer to polymer gives rise to the formation of macromolecules that simultaneously contain more than one active center even if the initiator is monofunctional. If one denotes R(i,l) the concentration of macromolecules containing i active centers and I monomer units, the reaction of chain transfer to polymer can be represented as follows  

This reaction does not affect the rate of monomer consumption hence, the dependence of monomer conversion on time is given by Eq. (3.3). There is no effect on the number average DP, either, because the number of chains does not change due to this reaction type. Nevertheless, chain transfer to polymer leads to the formation of 

The average degree of branching per monomer unit, Qm = Ntr/Mo c, is given by the well-known Flory expression [27] that is valid for all addition polymerization processes independent of their mechanisms  

Cp is the relative constant of chain transfer to polymer, Cp = ktp/kp. When chain transfer to solvent proceeds along with chain transfer to polymer, this expression for nonterminating polymerization changes to [26] 

The previous discussion has ignored the possibility of chain transfer to polymer molecules. Transfer to polymer results in the formation of a radical site on a polymer chain. The polymerization of monomer at this site leads to the production of a branched polymer, for example 

Ignoring chain transfer to polymer does not present a difficulty in obtaining precise values of C, Cm, and Cs, since these are determined from data at low conversions. Under these conditions the polymer concentration is low and the extent of transfer to polymer is negligible. 

Transfer to polymer cannot, however, be neglected for the practical situation where polymerization is carried to complete or high conversion. The effect of chain transfer to polymer plays a very significant role in determining the physical properties and the ultimate applications of a polymer [Small, 1975], As indicated in Chap. 1, branching drastically decreases the crystallinity of a polymer. 

The transfer constant Cp for chain transfer to polymer is not easily obtained [Yamamoto and Sugimoto, 1979]. Cp cannot be simply determined by introducing the term Cp[P]/[M] into Eq. 3-108. Transfer to polymer does not necessarily lead to a decrease in the overall degree of polymerization. Each act of transfer produces a branched polymer molecule of 

The evaluation of Cp involves the difficult determination of the number of branches produced in a polymerization relative to the number of monomer molecules polymerized. This can be done by polymerizing a monomer in the presence of a known concentration of polymer of known molecular weight. The product of such an experiment consists of three different types of molecules  

In discussions above, we have ignored the possibility that chain transfer may take place to polymer molecules present in the system. At low conversions, the polymer concentration is low and so the extent of transfer to polymer is negligible. However, transfer to polymer cannot be neglected at high conversions. Chain transfer to polymer is also significant with very reactive propagating radicals like those in the polymerizations of vinyl chloride, vinyl acetate, ethylene, and other monomers in which there is no significant resonance stabilization. 

Chain transfer to polymer produces a radical on the polymer chain and polymerization of monomer from this site results in the formation of a branch, for 

While long branches are formed by the normal chain transfer to polymer, as shown above [Eq. (6.124)], reactive radicals like those of polyethylene can also undergo self-branching by a backbiting intramolecular transfer reaction (see Fig. 6.9) in which the chain-end radical abstracts a hydrogen atom from a methylene unit of the same chain resulting in the formation of short branches (as many as 30-50 branches per 1000 carbon atoms in the main chain) that outnumber the long branches by a factor of 20-50. 

Chain transfer to polymer does not change either the number of monomer molecules which have been polymerized or the number of polymer molecules over which they are distributed. Chain transfer to polymer thus has no effect on DP,i and it is not included in Eq. (6.121). It, however, broadens the molecular weight distribution because the polymers which are already large are more likely to suffer transfer reactions and become yet bigger due to branching. 

Chain transfer to polymer produces a radical on the polymer chain and polymerization of 

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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. 

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. 

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. 

Reactivity Is typical of an acrylamide. For example, compound 1 shows essentially 1 1 copolymerizablllty with butyl acrylate. Copolymerizablllty has also been demonstrated with styrene, other acrylates and methacrylates, vinyl acetate (VAc), VAc/ethylene and vinyl chlorlde/ethylene. High molecular weight polymers and copolymers remain soluble. Indicating any chain transfer to polymer, e.g. through abstraction of the acetal hydrogen. Is minor. 

In many real polymerisation reactions, the kinetic scheme given above will be inadequate. Other reaction steps may have to be included amd the results of chain transfer to polymer are not always easy to describe. There is clear evidence which suggests that the chain termination rate coefficient is reduced in value when the concentration of polymer is high [43, 44]. The quantitative assessment for such changes is still a subject of much research [45, 46]. At very high concentrations, the value of kp may also be reduced [47]. Other physical events may also be important, particularly when the reaction becomes heterogeneous. 

Several chain transfer to polymer reactions are possible in cationic polymerization. Transfer of the cationic propagating center can occur either by electrophilic aromatic substituation or hydride transfer. Intramolecular electrophilic aromatic substituation (or backbiting) occurs in the polymerization of styrene as well as other aromatic monomers with the formation of... 

Transfer and termination occur by the modes described previously for cyclic ether polymerizations. Chain transfer to polymer (both inter- and intramolecular) is facilitated in cyclic acetal polymerizations compared to cyclic ethers because acetal oxygens in the polymer chain are more basic than the corresponding ether oxygens [Penczek and Kubisa, 1989a,b]. Working at high monomer concentrations, especially bulk polymerizations, is used to depress cyclic oligomer formation. 

The chain transfer to polymer process that produces long-chain branching is also a graft polymerization process (Sec. 3-6d). 

In radical template polymerization, when only weak interaction exists between monomer and template and pick-up mechanism is commonly accepted, the reaction partially proceeds outside the template. If macroradical terminates by recombination with another macroradical or primary radical, some macromolecules are produced without any contact with the template. In fact, such process can be treated as a secondary reaction. Another very common process - chain transfer - proceeds simultaneously with many template polymerizations. As a result of chain transfer to polymer (both daughter and template) branched polymers appear in the product. The existence of such secondary reactions is indicated by the difficulty in separating the daughter polymer from the template as described in many papers. For instance, template polymerization of N-4-vi-nyl pyridine is followed, according to Kabanov et aZ., by the reaction of poly(4-vinylpyridine) with proper ions. The reaction leads to the branched structure of the product ... 

Chain transfer reactions in THF polymerizations have not been considered until rather recently. Compounds known to be effective chain transfer agents include dialkyl ethers, orthoesters, and water. In addition, chain transfer to polymer and with gegenion is possible. 

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