Molecular weight radical polymerization

A polymer with purposely introduced chain end groups of a specific type, e.g., ionic, hydroxyl, acidic, etc. Low-molecular-weight radical polymerization product obtained in a reaction in which extensive chain transfer to a solvent (or specifically introduced chain transfer agent) has occurred, so that the telomer contains fragments of these reactants as end groups. 

Keywords molecular weight radical polymerization RAFT agents facticity... 

The trapped radicals, most of which are presumably polymeric species, have been used to initiate graft copolymerization [127,128]. For this purpose, the irradiated polymer is brought into contact with a monomer that can diffuse into the polymer and thus reach the trapped radical sites. This reaction is assumed to lead almost exclusively to graft copolymer and to very little homopolymer since it can be conducted at low temperature, thus minimizing thermal initiation and chain transfer processes. Moreover, low-molecular weight radicals, which would initiate homopolymerization, are not expected to remain trapped at ordinary temperatures. Accordingly, irradiation at low temperatures increases the grafting yield [129]. 

However, upon terminating chains with thiols, sulphur centered low-molecular weight radicals are formed that are able to start a polymerization of the remaining monomer B. Therefore, formation of homopolymer consisting of B is inevitable if thiols are used. A suitable alternative to the classical transfer additives are degra-dative chain transfer agents, such as allylmalonic acid... 

However, the polymerization of monomers in the gaseous phase by first procedure (1) mentioned above leads to the formation of considerable amounts of homopolymers. It is caused by low molecular weight radicals H and OH , not bounded chemically with the solid support [70, 73], In this case the subsequent... 

By 1951 (29) it had been concluded that polyacetaldehyde is an amorphous polyacetal of very high molecular weight. The polymerization of acetaldehyde is initiated by free radicals assisted by the crystallization of the monomer (crystallization polymerization). 

Several researchers have carried out experimental and/or theoretical investigations on emulsion polymerizations initiated with oil-soluble initiators and reported that the kinetics of the emulsion polymerizations is basically similar to that initiated with water-soluble initiators [193-202]. Breitenbach et al. [193] carried out the emulsion polymerization of St initiated by BPO at 50 and 60 °C. The authors interpreted the experimental results by assuming a relatively rapid exchange of low molecular weight radicals between the micelle-polymer... 

The initiation efficiency depends on the relationship between the rate of macroradical formation and that of initiation. In Table 1 are compiled results on certain redox systems employed for the initiation of cellulose grafting either by direct oxidation of cellulose (or its derivative) or chain transfer from active low molecular weight radicals. Table 1 indicates that in systems where the matrix acts as a reducing agent (1st group), the initiation efficiency does not exceed 15%, i.e. only a minor portion of macroradicals formed at the first stage of oxidation initiates graft polymerization while the rest is oxidized to stable... 

In bulk- and solution-phase free-radical polymerization, there is a tradeoff between molecular weight and polymerization rate. This is especially true for controlled/living radical polymerization. In emulsion polymerization, however, high molecular weight polymers can be made at fast polymerization rates. Emulsion polymerization is a type of radical polymerization that is frequently used for making polymers of high molecular weight. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are emulsified with surfactants in a continuous phase of water. 

PTFE was first produced in large scale in 1950 (Kaufman, 1963). There is little published information containing details of the commercial polymerization process for PTFE, but the product is known to form either as granules or dispersed low molecular weight particles. Polymerization is by free radical reaction at temperatures of 80-90°C and gives a yield around 85 per cent (Brydson, 1999). 

The determination of k ka by direct analysis of a polymerization or the resultant polymer often requires data on aspects of the polymerization mechanism that are not readily available. For this reason, it is appropriate to consider the self-reactions of low-molecular-weight radicals that are stmcturally analogous to the propagating species. These model smdies provide valuable insights by demonstrating the types of reaction that are likely to occur during polymerization and the factors influencing koi/k. ... 

These terminal radicals may be used to initiate the polymerization of a second monomer, whereby block copolymers are formed. Notably, homopolymerization is not observed in this process, as no low-molecular-weight radicals are generated upon the reaction of Mn(CO)s with the macroinitiator. 

Many advantages are shown by emulsion and suspension polymerization. First, the continuous water phase can act as an excellent heat conductor and allow the heat to be removed from the system. This is an effective way to increase the polymerization rate of many reactions. Second, the viscosity remains close to that of water and is not dependent on molecular weight since the polymer molecules are contained within the particles. Third, emulsion polymerization is unique in the sense that an increase in molar mass can be achieved without reducing the rate of polymerization. Therefore, high molecular weight polymers can be obtained at fast polymerization rates. By contrast, there is a tradeoff between molecular weight and polymerization rate in bulk and solution free-radical polymerization. 

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