Mechanism for the polymerization

FIGURE 14 5 Mechanism for the polymerization of ethylene in the presence of a Ziegler-Natta catalyst... 

Several mechanisms for the polymerization of vinyl ether and epoxies have been suggested [20,22,23,25,27,28,33-35]. On irradiation with gamma rays or electrons, pure epoxies polymerize via a cationic mechanism [35]. However, this cationic polymerization is inhibited by just traces of moisture, as shown below for cyclohexene oxide in reaction 5. 

Lapin [33] also suggests the following cationic mechanism for the polymerization of vinyl ethers... 

The most probable mechanism for the polymerization bv metal center (XV) is 3... 

Scheme 10.4. Suggestions for an alternative mechanism for the polymerization of acetylene.

The combination of these findings with the aforementioned results of Beckhaus, Rosenthal, and Mach may also be interpreted in terms of an alternative mechanism for the polymerization of acetylene, which differs from that of Alt [12] (Scheme 10.3). In the absence of coupling, as in 11, or of twofold C—H activation as found in 12, the steps after substitution of MeiSiC=CSiMci by HC=CH and formation of Cp2Ti(i]2-HC2H) are (i) oxidative addition to give the hydrido-acetylide Cp2Ti(H)(C=CH),... 

Figure 3. Modified Cossee mechanism for the polymerization of olefins with early transition metals. Green, Rooney and Brookhart introduced the presence of the adjuvant a-agostic interaction in the transition state.

The reaction mechanism for the polymerization of a hydroxyalkanoic acid (Eqs. 2-243 through 2-246) is a chain polymerization, often called an activated monomer polymerization. The active site of lipase is its serine a-amino acid unit, which contains a hydroxyl group. The acyl carbon of the hydroxyalkanoic acid undergoes nucleophilic attack by the hydroxyl group of serine to form lipase-activated monomer (Eq. 2-243). Initiation consists of reaction... 

The pattern observed in Fig. 10 can be explained in terms of the following mechanism for the polymerization of ethane. 

The mechanism for the polymerization probably fits the general pattern of electrophilic attack by the initiator on the carbonyl oxygen atom for the initiation step, with propagation taking place by attack of the electron deficient C-4 of the last unit of the growing chain on the carbonyl group of a monomer molecule. Certain modifications of this general mechanism are apparently observed with specific catalysts, and the subject appears to need further study. 

In the previous communication(7a) we postulated the initiation mechanism involving the attack of BuLi on the vinyl double bond of the butyl lsopropenyl ketone formed. However, from the results mentioned above we will propose the following mechanism for the polymerization of MMA with BuLi in toluene, which is slightly different from the previous one. On mixing the monomer with BuLi the initiator reacts with both olefinic and carbonyl double bonds of the monomer. The attack on the olefinic double bond produces the MMA anions, which add the monomer to form the growing chains(E). 

A generally accepted mechanism for the polymerization of ethylene by typical Ziegler-Natta catalysts has been proposed by Cossee (81). More recently a mechanism for transition metal hydrocarbyl catalyzed polymerization of olefins has been discussed (41, 42, 43) which is essentially similar to that shown in Reaction 5. 

The mechanism for the polymerization of dienes to the ds 1,4 structure is also parallel. Sterns and Foreman (112) presented a cyclic 6-membered transition state to produce the cis structure. Orr (113) concluded that the configuration of the monomer itself has no role to play in determining the amount of cis or trans isomer produced from isoprene and butadiene. There is no requirement for the prealignment of the diene monomer by coordination between a diene molecule and the gegen ion. The diene must only assume the ds transition state during reaction. 

Exercise 16-20 Write a reasonable mechanism for the polymerization of methanal in water solution under the influence of a basic catalyst. Would you expect base catalysis to produce any 1,3,5-trioxacyclohexane Why ... 

Fig. 3 a and b. Proposed mechanisms for the polymerization of benzene by the Kovacik route... 

Scheme 26 Proposed mechanism for the polymerization of dienes by the catalyst system Nd(N(SiMe3)2)3/[HNMe2Ph]+ [B(C6F5)4] /AlR3 [320], copyright Soc Chem Ind. Reproduced with permission. Permission is granted by John Wiley Sons Ltd. on behalf of the SCI...

Most reaction models which describe the mechanism of diene polymerization by Nd catalysts have been adopted from models developed for the polymerization of ethylene and propylene by the use of Ti- and Ni-based catalysts systems. A monometallic insertion mechanism which accounts for many features of the polymerization of a-olefins has been put forward by Cossee and Arlman in 1964 [624-626]. Respective bimetallic mechanisms date back to Patat, Sinn, Natta and Mazzanti [627,628]. The most important and generally accepted mechanisms for the polymerization of dienes by Nd-based catalysts are discussed in the following. 

Use of triphenylmethyl and cycloheptatrienyl cations as initiators for cationic polymerization provides a convenient method for estimating the absolute reactivity of free ions and ion pairs as propagating intermediates. Mechanisms for the polymerization of vinyl alkyl ethers, N-vinylcarbazole, and tetrahydrofuran, initiated by these reagents, are discussed in detail. Free ions are shown to be much more reactive than ion pairs in most cases, but for hydride abstraction from THF, triphenylmethyl cation is less reactive than its ion pair with hexachlorantimonate ion. Propagation rate coefficients (kP/) for free ion polymerization of isobutyl vinyl ether and N-vinylcarbazole have been determined in CH2Cl2, and for the latter monomer the value of kp is 10s times greater than that for the corresponding free radical polymerization. 

All the findings collected to date do not enable us to formulate a detailed initiation mechanism for the polymerization of alkenes and dienes on ZN centres. The initiation and growth mechanisms are usually combined in a single scheme which will be discussed in detail in Chap. 5, Sect. 4. 

Two proposed mechanisms for the polymerization of a,a -bis(dialkyl-sulfonio)-p-xylene dihalides (463) are shown in Fig. 70 [302]. Both mechanisms begin with the abstraction of an a-proton to produce ylid 464. The 1,6-elimination of dialkyl sulfide produces the p-xylylene pseudo-diradical 465. One mechanism involves the formation of polymer from this species via the dimerization of 465 to give the dication diradical 466. This species was proposed to grow rapidly to high molecular weight polymer (467) by head-to-tail additions to both ends [301]. An alternative mechanism involving polymer formation from 465 via a putative anionic mechanism has been proposed [302]. There were two main factors in the proposal of... 

It seems that formation of dimers and trimers from soybean oil is the slow step of polymerization, an evidence for step polymerization mechanism. Once formed, these dimers and trimers polymerize very quickly to a high molecular weight and exhibits chain reaction kinetics. This phenomenon is caused by the soybean oil molecule itself, a large molecule with low activity, due to mid chain double bond location. However, the formed dimers and trimers have more unsaturated carbon-carbon double bonds per molecule, and they are easily polymerized to high molecular weight polymers. We propose further investigation on the possible combination of two polymerization mechanisms for the polymerization of soybean oil in SCCO2. Kinetic study of soybean oil polymerization currently is carried out in our laboratory. 

Draw a stepwise mechanism for the polymerization of isoprene to gutta-percha using (CH3)3CO—OC(CH3)3 as the initiator. 

In this context, the polymerization kinetic of MMA, under light irradiation, using a CT complex between a polymeric donor such as poly(A -vinylcarbazole) [poly(NVC)] and bromine as the acceptor has been investigated [69]. A radical mechanism for the polymerization is suggested. Moreover, the presence of bromine atoms as end groiqjs in the obtained poly(MMA) and the lack of any evidence of poly(NVC) chemically linked to poly(MMA) have allowed the authors to propose that the radical generation process may be described as in Scheme 11 ... 

In general, the mechanisms which have been discussed form the basis of most polymerizations of NCAs. Clearly, they often may lead to very complex kinetic behaviour, particularly since primary and secondary amines can initiate either by direct nucleophilic attack or by proton abstraction. The elucidation of the various chemical steps, their consequences and their relative importance in specific instances have been based on extensive chemical and kinetic data which we present below. Alternative mechanisms for the polymerization of NCAs have been proposed, but have in the main been rejected because they are not consistent with some of the experimental observations. The reader is referred to the extensive... 

Scheme 4. Proposed initiation mechanism for the polymerization of MMA by Ln" complexes.

Figure 2 Coordination-insertion mechanism for the polymerization of lactide by a metal complex with an alkoxide initiator...

---