Radical polymerization chain branching

Radical chain polymerization of ethylene to polyethylene is carried out at high pressures of 120-300 MPa (17,000-43,000 psi) and at temperatures above the Tm of polyethylene (Fig. 3-18) [Doak, 1986]. Batch processes are not useful since the long residence time gives relatively poor control of product properties. Long-chain branching due to intermolecular chain transfer becomes excessive with deleterious effects on the physical properties. Continuous processes allow better control of the polymerization. 

Another general problem, the development of an algorithm for the construction of kinetic models for the quasi-stationary state of the evolution of non-equilibrium chemical system, is solved by the method of linear routes as simple cycles of a graph assigned to sets of elementary reactions and intermediate substances (see Chapter 2). A general algorithm for construction of kinetic models for the linear catalytic and un-branched radical-chain processes, including a free radical polymerization, is proposed. 

Although free radical polymerization is an example of an un-branched radical-chain process, it has characteristics which should be taken into account when interpreting a route and an algorithm of the construction of their kinetic models. 

Using the multiple reactive double bonds, G o can be employed as a co-monomer in an in situ polymerization process with various vinyl based monomers through addition polymerization. For example, Mourey et al. have used radical chain polymerization with Al BN as initiator in 1,2-dichlorobenzene to produce branched structures of Ca) and both PMMA [poly(methyl methacrylate)] and PS (polystyrene)... 

Bulk Polymerization. The bulk polymerization of acryUc monomers is characterized by a rapid acceleration in the rate and the formation of a cross-linked insoluble network polymer at low conversion (90,91). Such network polymers are thought to form by a chain-transfer mechanism involving abstraction of the hydrogen alpha to the ester carbonyl in a polymer chain followed by growth of a branch radical. Ultimately, two of these branch radicals combine (91). Commercially, the bulk polymerization of acryUc monomers is of limited importance. 

Shimizu and Ohtsu [69] have proposed a chemical method to determine head-to-head structures in PVC. Mitani et al. [70] found 2.5-7.0 head-to-head structures per 1,000 monomer units, increasing with the polymerization temperature. It has not been possible to detect internal head-to-head structure by C-NMR spectroscopy with the detection limit of 2 per 1,000 monomer units [71]. Starnes et al. [71] found evidence for the absence of neighboring methylene groups by C-NMR spectroscopy. However, the proposed reaiTangement of head-to-head units at the radical chain ends resulting in chloromethyl branches [Eq. (6)] would partially explain their consumption during polymerization and their absence in the final product. 

Low-density polyethylene (LDPE) is produced under high pressure in the presence of a free radical initiator. As with many free radical chain addition polymerizations, the polymer is highly branched. It has a lower crystallinity compared to HDPE due to its lower capability of packing. 

Raising the temperature of a radical chain reaction causes an increase in the overall rate of polymerization since the main effect is an increase in the rate of decomposition of the initiator and hence the number of primary radicals generated per unit time. At the same time the degree of polymerization falls since, according to Eq. 3.3, the rate of the termination reaction depends on the concentration of radicals (see Example 3-2). Higher temperatures also favor side reactions such as chain transfer and branching, and in the polymerization of dienes the reaction temperature can affect the relative proportions of the different types of CRUs in the chains. 

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 ... 

Probabilities of configurations conducive to the intramolecular back-biting abstraction of a hydrogen atom are evaluated for growing unperturbed PVAc chains. A realistic RIS model is used for the chain statistics, Probabilities are found to be smaller than those seen in an earlier treatment of the polyethylene chain. The smaller probabilities of PVAc contribute to the virtual absence of short branches. The present study therefore provides support for the validity of the Roedel mechanism for the formation of short branches in the free radical initiated polymerization of ethylene. 

Finally, in the last Chap. E the more complex reactions are treated which are observed in free-radical polymerization and in vulcanization of chains. In the course of branching the experimentalist is often confronted with inhomogeneities in branching and chain flexibility and with chemical heterogeneity and steric hindrance due to an overcrowding of segments in space. Some of these problems of great practical importance have been solved in the past and are briefly reported. 

Many researchers have attempted to make branched polystyrene in continuous bulk radical polymerization processes. Approaches involving the addition of additives to the polymerization process which lead to branching inside the polymerization reactor always lead to gel problems. Examples include addition of divinylmonomer [4], vinyl peroxides (e.g. I) [5,6], branched peroxides (e.g. II) [7], vinyl-functional chain transfer agents (III) [8], and the use of addition-fragmentation chain transfer agents that lead to the formation of polystyrene macromonomers (Figure 24.3) [9]. 

Transfer to polymer yields a radical on the polymer chain. Polymerization of monomer from this site produces a polymer with a long branch ... 

The linear polyethylene produced by the Ziegler-Natta process, called hi li-clensity polyethylene, is a highly ctystalline polymer with 4000 to 7000 ethylene units per chain and molecular weights in the range 100,000 to 200,000 amu. High-density polyethylene has greater strength and heat resistance than the branched product of radical-induced polymerization, called low-density polyethylene, and is used to produce plastic squeeze bottles and molded housewares. 

Branching occurs in some. free-radical polymerization of monomers like ethylene, vinyl chloride, and vinyl acetate in which the propagating polymer radical is very reactive and can lead to branching by chain transfer to the backbone chain of another polymer molecule or onto its own chain (see Chapter 6). 

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