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Chain-Growth Polymerization
Published in Timothy P. Lodge, Paul C. Hiemenz, Polymer Chemistry, 2020
Timothy P. Lodge, Paul C. Hiemenz
The three-step mechanism for free-radical polymerization represented by Reaction (3.A) through Reaction (3.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it further complicates the picture presented so far. On the other hand, this additional reaction can be turned into an asset in actual practice. One consequence of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer, without necessarily affecting the rate of polymerization. A certain minimum average molecular weight is often needed to achieve a desired physical property, but further increases in chain length simply make material processing more difficult.
Surfactants and emulsion polymerisation: an industrial perspective
Published in David R. Karsa, Surfactants in Polymers, Coatings, Inks and Adhesives, 2020
In addition to the monomers, a formulation will include a chain-transfer agent to control molecular weight, surfactants, an initiator and a chelating agent to mop up any metal ions arising from pipework, etc. Metal ions can affect the initiator decomposition, the stability of the mix and the ageing of the final product. The most common initiator is a persulfate salt as they readily decompose in the 60–95 °C range within which the bulk of polymerisations are carried out. For lower temperatures, peroxides or hydroperoxides activated by reducing agents are used. Thiols are the most widespread chain-transfer agents used. Some earlier formulations included carbon tetrachloride for this purpose on account of lower product odour, but its ozone-depleting properties have led to its demise. Other minor ingredients are likely to be electrolytes, buffers and pH-adjusting agents.
Polymerization at High Pressure
Published in Ian L. Spain, Jac Paauwe, High Pressure Technology, 2017
Chain-transfer reactions can play a major role in the control of the molecular weight of a polymer system. While there do not appear to be reported studies on the effect of pressure on the chain-transfer reaction for ionic polymerization, there are several studies on the effect of pressure on the transfer reaction in free-radical polymerization and values of the activation volume for chain transfer appear to be both positive and negative depending on the monomer and solvent. The measurement is usually reported as ΔVcm or ΔVcs where ΔVcm = ΔVfm, – ΔVp and ΔVcs = ΔVfs− ΔVp, where the subscripts fm and fs denote the transfer to monomer and solvent, respectively. Values for ΔVCS range from -15 to +7 cc/mole [8, 10, 41, 50, 79, 80] and for chain-transfer reactions where steric and polar effects are small, ΔVCS is reported to follow the empirical relation [10] () ΔVCS=1.6−4.8logC′fS
Preparation, characterization and properties of an organic siloxane-modified cassava starch-based wood adhesive
Published in The Journal of Adhesion, 2018
Jianping Sun, Lijun Li, Hao Cheng, Wenyi Huang
Figure 3 shows the effect of different polymerization temperatures on both the shear adhesive strength and the viscosity of starch-based adhesive. When the polymerization temperature was increased from 50 to 60°C, the shear adhesive strength in the dry state increased from 3.25 to 6.11 MPa and the shear adhesive strength in the wet state increased from 2.02 to 3.05 MPa. Because an increase in the reaction temperature leads to an increase in the mobility of acrylamide and BA molecules, this increases the probability of collision between acrylamide, BA molecules and starch macro-radicals. [33]. Therefore, the shear adhesive strength was increased. When the temperature increased from 60 to 70°C, the shear adhesive strength in the dry state reduced to 3.75 MPa rapidly and the shear adhesive strength in the wet state reduced to 1.66 MPa. It can be explained by the fact that the high polymerization temperature caused both the chain transfer and chain termination rate to increase, which resulted in a grafted copolymer with decreased molecular weight. The lower molecular weight of the grafted copolymer weakens the adhesive bonding strength. At the same time, when the lower molecular weight starch adhesive was used in a humid environment, it was easier, for the water molecules, to get into the polymer. Thus, the shear adhesive strength in both the dry and wet states reduced. It also can be seen that the viscosity of the starch-based adhesive changed little in the whole temperature range. Finally, the proper polymerization temperature was chosen to be 60°C for the starch-based wood adhesive.
Facile fabrication of patternable and large-area elastic liquid crystal polymer films
Published in Liquid Crystals, 2023
Lifan Lu, Xiaofang Chen, Wei Liu, Hongkun Li, Yi Li, Yonggang Yang
In this work, a one-step approach is presented to produce large-area CLCE films with uniform colours, which is reproducible and easily scalable. This approach is based on the property of chain transfer agent which can effectively reduce the molecular weights and cross-linking degrees of polymers. The resultant CLCE films can be applied as car films and in wearable devices. This method shown here will have a significant impact on the commercial application of CLCE films.