Branching free radical polymerization

High pressure (60—350 MPa) free-radical polymerization using oxygen, peroxide, or other strong oxidizers as initiators at temperatures of up to 350°C to produce low density polyethylene (LDPE), a highly branched polymer, with densities from 0.91 to 0.94 g/cm. ... 

Branching occurs especially when free radical initiators are used due to chain transfer reactions (see following section, Free Radical Polymerizations ). For a substituted olefin (such as vinyl chloride), the addition primarily produces the most stable intermediate (I). Intermediate (II) does not form to any appreciable extent ... 

Free radical polymerization Relatively insensitive to trace impurities Reactions can occur in aqueous media Can use chain transfer to solvent to modify polymerization process Structural irregularities are introduced during initiation and termination steps Chain transfer reactions lead to reduced molecular weight and branching Limited control of tacticity High pressures often required... 

How does backbiting create branches during free radical polymerization ... 

The architecture of macromolecules is another important synthetic variable. New materials with controlled branching sequences or stereoregularity provide tremendous opportunity for development. New polymerization catalysts and initiators for controlled free-radical polymerization are driving many new materials design, synthesis, and production capabilities. Combined with state-of-the-art characterization by probe microscopy, radiation scattering, and spectroscopy, the field of polymer science is poised for explosive development of novel and important materials. New classes of nonlinear structured polymeric materials have been invented, such as dendrimers. These structures have regularly spaced branch points beginning from a central point—like branches from a tree trunk. New struc-... 

Yoshida et al. developed similar numbered-up capillary pilot reactors for free radical polymerization (Iwasaki et al. 2006). The capillaries were either arranged in parallel fashion similar to conventional multitube reactors or consecutively branched by multiport valves. 

The main topic of interest is the properties of molecules of finite size, having no large rings, and in general having trifunctional branch-points. These are typically produced by chain-transfer with polymer in free-radical polymerizations, though they can of course be made in other ways. Molecules with branch-points of higher functionality are also of interest, especially star-shaped molecules with several arms, as these are both easy to synthesize and relatively easy to discuss theoretically. 

In a real free-radical polymerization, the probability that a given monomer unit in the polymer bears a branch is not constant, but increases with time. Mullikin and Mortimer (tOt) have extended their model to take into account the time-dependence of branching probability and the distribution of residence times in a continuous reactor. They assume that the branching probability b is given by ... 

One method is to measure chain-transfer coefficients with low-MW analogues of the polymer. Thus Gilchrist (140) measured the rate at which 14C labelled decane was incorporated into polyethylene in the free-radical polymerization, and hence obtained an estimate of the transfer coefficient with methylene groups this was in fair agreement with another estimate obtained from the effect of the addition of fractions of linear polyethylene on the Mn of the branched polyethylene, which could be separated from linear polymer plus grafted branched polymer by column extraction. Low MW polymer may be used as a transfer agent Schulz and co-workers (189) obtained chain-transfer coefficients in styrene polymerization from the effect of added low MW polymer on Mn. 

However, it was not until the appearance in 1953 of an important group of papers from the laboratories of Du Pont that any satisfactory evidence became available concerning the nature of the branches in polyethylenes of this type, the low-density polyethylenes (LDPE). Roedel (6) showed that the free-radical polymerization mechanism could be expected to lead not only to short branches containing a few carbon atoms (with which this review is not concerned) but by a mechanism first proposed by Flory (4) and involving the... 

In this section the occurrence of LCB in several other of the more important polymers during free-radical polymerization will be discussed branching due to irradiation or the use of multifunctional monomers (including dienes) will not be... 

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. 

Transfer to polymer, causing reactivation of a polymer molecule al some point along its length, leads to the growth of branches. The process can occur intermolecularly and also intramolecularly the latter process is particularly important in the free radical polymerization of ethylene at high pressure where it leads to the production of numerous short branches which considerably affect the properties of the polymer. 

Polyethylenes obtained by free-radical polymerization have highly branched structures as a consequence of chain-transfer reactions (see eq. 3.42 and the structure below it). Ziegler-Natta polyethylene is mainly linear (CH2CH2)/7- It has a higher degree of crystallinity and a higher density than the polyethylene obtained by the free-radical process. 

The homopolymer and block copolymer macromonomers were copolymerized with MMA by free-radical polymerization in benzene at 60 °C using AIBN as an initiator typical concentration were [MMA]=1.2 mol 1 1 and [macromonomer] =0.020 mol l"1. MMA was completely converted in 18 h and the macromonomers conversion reached more than 70% as determined by lH NMR. Incomplete conversion was explained by steric hindrance. Free-radical copolymerization resulted in high MW graft copolymers with PMMA backbone and relatively rigid, nonpolar poly(P-pinene) branches. 

The anionic homopolymerization of polystyrene macromonomers was carried out successfully. The methacrylic ester sites at the chain end do not require very strong nucleophiles to be initiated diphenylmethylpotassium was used, and the process was carried out at — 70 °C in THF solution24). The products are comparable with those obtained by free-radical polymerization. The molecular weight distribution should be narrower but this cannot be easily checked because these polymer species are highly branched and compact as already mentioned. 

Although non-ZiEGLER catalysts are involved, there is excellent evidence for free radical polymerization of yr-complexed monomer. Bier et al (348) have shown that ethylene forms complexes with silver salts in neutral aqueous solution at 10—40° C. and 5—50 atmospheres ethylene. Initiation by peroxides produced high molecular weight, branched... 

Chain branching occurs in cationic polymerization much as it does in free-radical polymerization. Propose a mechanism to show how branching occurs in the cationic polymerization of styrene. Suggest why isobutylene might be a better monomer for cationic polymerization than styrene. 

Chain branching is not as common with anionic polymerization as it is with free-radical polymerization and cationic polymerization. 

A second factor is the nature of the polymer backbone its flexibility, steric regularity, electric charge, and branching. This factor also reflects the compatibility and penetration ability of the polymer and the stability of the complex. A polynucleotide has a flexible, sterically regular and negatively charged polymer backbone, whereas that of a vinyl polymer obtained by free-radical polymerization is probably less flexible, sterically inhomogeneous and neutral. 

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