The use of chain-transfer agents

Once the rate constants are known, the relative amoimts of 

The possibility of using alkenylcyclopentenes as transfer agents has been examined, with the idea that the cyclopentene ring will first be opened in the normal way, and that this will be followed by an internal metathesis reaction involving the alkenyl group, so as to split out a cyclohexenyl unit forming the end-group of the polymer chain. A combination of unfavourable circumstances makes this difficult to achieve in practice (Schrock 1989). 

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One normatty assumes that systems such as styrene and methyl methacrylate, where transfer to monomer is not prominent, follow Case 2 kinetics when latex particles are small and termination in polymer particles is instantaneous. It has recently been shown that at low initiation rates radical desorption can be significant relative to radical absorption, and as a consequence w values appreciably smaller than 0.5 were found (Gilbert et al., 1980). At higher initiation rates n = 0-5 was approached. The use of chain-transfer agents would of course increase the desorption rate and lower n. 

The copolymerization is performed in solution with radical-forming initiators at temperatures of about 70 °C. The silicone macromer contains polymerizable vinyl groups on both chain ends. Such silicone macromers are produced by Wacker GmbH and are available with different chain lengths. The copolymerization with vinyl acetate is very facile and take place statistically [2] after polymerization, no double bonds can be found. To avoid crosslinking, the molecular weight has to be adjusted by the use of chain transfer agents or by the solvent and its concentration. 

Following the observations by Oshika (1968) and Dall Asta (1969) that DCPD could be polymerized by metathesis catalysts, it was shown by Devlin (1970) that it could be polymerized by M0OCI3 to a gel at 35°C and then cured to a hard brown resin at 140°C. Initially attention was concentrated on finding conditions for producing substantially gel-free polymers and copolymers of DCPD through careful control of the order of mixing, the use of chain-transfer agents, e.g. but-1-... 

Richard at Rh6ne-Poulenc [67] and Zosel at BASF [68] have carried out extensive studies on the dynamic mechanical properties of latex films. They examined latex made from monomers (like butyl acrylate and butadiene) that crosslink spontaneously during emulsion polymerization. Here crosslink density was reduced through the use of chain transfer agents. Alternatively, with monomers like BMA which form non-crosslinked latex, crosslinking was induced through the addition of bifimctiona] monomers such as methallyl methacrylate (MAMA). Richard has recently published a review of his work in this area [75]. 

Of course, the manufacture of polymers need not be carried out in solution emulsion polymerization has a long and honorable tradition in the field of macromolecular chemistry. This polymerization technique is performed using water-insoluble monomers, which are caused to form an emulsion with the aqueous phase by the addition of a surfactant. Polymerization may be effected by the use of a variety of radical generating initiating species, and the molecular weight of the polymer is controlled by the use of chain transfer agents and the concentration of initiator employed. The final polymer dispersion is often described as a latex, named after natural rubber, which is also an emulsion polymer ... 

One key feature of the bis(NHC) organometallic polymers is their reversibility, or dynamicity. The dynamic nature of the copolymerization prompted us to investigate the use of chain-transfer agents to modulate both polymer molecular weight and end-group structure (Scheme 1.15). Copolymerization with Pd(OAc)2 in... 

However, the use of chain transfer agents containing at least one P-H bond, allows to produce polymers which combine excellent activity as oilfield scale inhibitors with good biodegradability, often in excess of that required by the OECD 306B standard (11). 

The use of chain transfer agents in emulsion polymerization was briefly discussed in Sect. 2.2.1. As stated, the most commonly used chain transfer agents are the mer-captans (thioalcohols) RSH, although a wide range of other compounds also exert a modifying effect during polymerization, for example carbon tetrachloride, certain disulfides, rosin acid salts, 4-vinylcyclohexene (butadiene dimer) amongst many others, which may also include impurities in other raw materials. 

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