Chain transfer with initiators

Using the methods described, the values of Cm and Ci in the benzoyl peroxide polymerization of styrene have been found to be 0.00006 and 0.055 respectively [Mayo et al., 1951]. The amount of chain transfer to monomer that occurs is negligible in this polymerization. The chain-transfer constant for benzoyl peroxide is appreciable, and chain transfer with initiator becomes increasingly important as the initiator concentration increases. These effects are shown in Fig. 3-7, where the contributions of the various sources of chain ends are indicated. The topmost plot shows the total number of polymer molecules per 105 styrene monomer units. The difference between successive plots gives the number of polymer molecules terminated by normal coupling termination, transfer to benzoyl peroxide, and transfer to styrene. 

This quadratic in Rp is of the form required by the data for styrene-benzoyl peroxide shown in Fig. 14. The first term, corresponding to the intercept, represents the creation of chain ends through transfer with monomer. It occurs to an extent which is independent of the polymerization rate. The second term corresponds to 1/2 according to Eq. (27) it represents the pairs of ends created at the initiation step. Its coefficient is given by the initial slope of the line in Fig. 14. The third term, which accounts for the curvature at higher rates, represents the contribution of chain transfer with benzoyl peroxide. This becomes more prominent at higher rates because of the larger amounts of the initiator which are present. The marked rise in the curves for... 

Polymer production proceeds as described in structure 5.25. An initiator, such as sulfuric acid, produces an oxonium ion and a gegenion. The oxonium ion then adds to the oxirane, ethylene oxide, producing a macrooxonium ion with growth eventually terminated by chain transfer with water. 

Termination may also occur by chain transfer with the initiator (e.g., water or alcohol) or a deliberately added chain-transfer agent. Deliberate termination of growth is carried out to produce polymers with specific molecular weights or, more often, telechelic polymers with specific end groups. Hydroxyl and amine end groups are obtained by using water and ammonia as chain-transfer agents. Carboxyl-ended telechelics can be obtained by termination with ketene silyl acetal followed by hydrolysis with base [Kobayashi et al., 1989]. 

Poly vinylidene fluoride is polymerized under pressure at 25-150°C in an emulsion using a fluorinated surfactant to minimize chain transfer with the emulsifying agent. Ammonium persulfate is used as the initiator. The homopolymer is highly crystalline and melts at 170°C. It can be injection molded to produce articles with a tensile strength of 7000 psi (48 MPal. a modulus of elasticity in tension of 1.2 x 105 psi and a heat deflection of 3003F (149°C). 

Mayo, F. R. Chain transfer in the polymerization of styrene. VIII. Chain transfer with bromobenzenc and mechanism of thermal initiation. J. Am. Chem. Soc. 75, 6133 (1953). 

Polystyrene. The polymerization of styrene is most commonly done under free radical conditions. Peroxides are used to initiate the reaction at low temperatures. At 100°C styrene acts as its own initiator. Below 80°C the termination mechanism primarily involves combination of radicals. Above 80°C both disproportionation and chain transfer with the Diels-Alder dimer are important. 

Russian workers have proposed that the increased activity of allyl acetate and allyl alcohol in free radical or gamma ray initiated polymerization in the presence of zinc chloride may be connected with the decreased degradative chain transfer with complexed monomer or the activation of the stabilized allyl radical in the complexed monomer—i.e., the conversion of degradative chain transfer to effective transfer (55, 87). However, these explanations have been partially rejected as inadequate. 

In other applications, phthalides 328-330 were added to the polymerization of MMA and styrene in an effort to add thermostability to the favorable properties of the vinyl polymers. The examples with Cl directly attached to the phthalide ring were found to act principally as chain-transfer agents. Nonhalogenated compounds participated in chain-transfer and initiation processes <2001MI37>. Aryl phthalides 331-333 have been employed as stabilizers for the processing of polymers to limit chain cleavage and oxidation <2001WO132762>. 

Mayo [16] proposed an alternative mechanism that is currently widely supported. Figure 7.7 shows a schematic of the Mayo mechanism. A Diels-Alder reaction between two styrene molecules produces an intermediate dimer (DH), also referred to as Mayo dimer . DH is highly reactive and has never been isolated. To complete the auto-initiation, DH reacts with a third styrene molecule via molecular assisted homolysis [17] to form a phenyltetraline radical (D ) and a phenethyl radical (SH ). A second reaction involving DH is to undergo chain transfer with a growing radical chain to produce a dead polymer chain (PS-H) and a new growing radical. The chain transfer constant (A ct) of DH has been estimated at 10, which is the highest Kcl ever reported for a molecule that contains no heteroatoms [18,19]. 

The precise experimental conditions for the measurements of chain lifetimes of polyethylene with the TiCl4/Al(i-Bu2 )H catalyst are not explicitly stated, but there is clear evidence for a steady increase in lifetime with polymerization time. For an average lifetime of 4 min after 40 min polymerization time, the instantaneous values were 4 min after 18 min polymerization and 10 min after 40 min polymerization. As the concentration of active centres remains almost steady after a sharp initial fall, the increase cannot be accounted for wholly by changes in the monomer/active sites ratio. The explanation may lie in a reduced rate of chain transfer with increase in conversion, as has been found for propene with a-TiClj/AlEt2 Cl [121]. In accord with this view average chain lifetimes of polypropene have been calculated to increase with conversion [123]. 

The case of chain transfer to initiator may be exploited in special forms of controlled free-radical polymerization, e.g. with sulfur-containing initiators that are discussed later within the topic of living Ifee-radical polymerizations. 

Polymerization of cyclic ethers by macromolecular dioxolenium salt yielded block copolymers. No termination or chain transfer was observed with THF, but some termination with BCMO and considerable chain transfer with OBH occurred. Cyclic formals such as TEX or DOL do not form block copolymers, showing that the initiation mechanism is not via bonding. 

BARNER L., QUINN J.F., BARNER-KOWOLLIK C.H., VANA P., DAVIS T.P., Reversible addition-fragmentation chain transfer polymerization initiated with y-radiation at ambient temperature an overview. European Polymer Journal, (2003), 39, 449-459. 

Chain transfer with water involves formation of the corresponding oxonium ion with the subsequent release of a proton which initiates a new chain ... 

Since chemisorbed ethylene disappears by initiation reaction with adsorbed hydrogen, pro-pagation reaction with surface-bound polymer chain, and polymer chain transfer with monomer (Fig. 9.8), one may write (Friedlander and Oita, 1957) ... 

The reaction between acetylene and ethene, catalyzed by WCle in benzene at 20°C, gives polybutadiene (85% 1,2-, 15% 1,4-) of high MW. The initial formation of butadiene probably occurs by alternate addition of acetylene to [W]=CH2 to form [W]=CHCH=CH2, and chain transfer with ethene (Zaliznaya 1990). 

Graft epoxy-acrylic copolymer prepared with a free radical initiator is an example of the "grafting from" process. In the case where benzoyl peroxide was used as the free radical initiator, it is determined that about 77% of the free radical initiator instead of causing initiation of monomers, chain transfers with the epoxy resin backbone, followed by the "grafting from" of22onomers onto the epoxy resin. Benzoyl peroxide is known to decompose mostly (90 ) to the benzoyloxy radical and the (10 ) phenyl radical. Mechanisms of grafting can be demonstrated in the following two schemes. (Schemes A and B). 

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