Intramolecular back-biting

Probabilities of configurations conducive to the intramolecular back-biting abstraction of a hydrogen atom are evaluated for growing unperturbed PVAc chains. A realistic RIS model is used for the chain statistics, Probabilities are found to be smaller than those seen in an earlier treatment of the polyethylene chain. The smaller probabilities of PVAc contribute to the virtual absence of short branches. The present study therefore provides support for the validity of the Roedel mechanism for the formation of short branches in the free radical initiated polymerization of ethylene. 

Polymerisations of 8,9, and 10 were rapid but stopped at limited conversions owing to a termination process involving an intramolecular back-biting reaction, the tertiary amino groups on the polymer backbone being more... 

The A-B di-block copolymer of -CL and oxepan-2,7-dione has been synthesized using aluminum isopropoxide as initiator [114] (Scheme 16). In order to prepare the ABA tri-block copolymer, a difunctional initiator [Et2AlO(CH2)4OAlEt2] was used to polymerize B followed by the addition of monomer A. However, the rate of polymerization was lower than in the Al(0 Pr)3-initiated system. Increasing the temperature to 70 °C increased the rate but a broadening of MWD was observed due to intramolecular back-biting reactions and intermolecular transesterification reactions. The addition of 1 equiv. of pyridine with respect to Al increased the polymerization rate and reduced the MWD from 1.95 to 1.25 [95]. 

As with all polycondensation reactions, the formation of cychc ohgomers by ADMET is possible and has been demonstrated in a variety of cases [31-33]. This occurs by intramolecular back-biting metathesis of an active metal carbene with an internal olefin of the polymer (Scheme 6.5) to hberate cyclooctadiene, for example, from ADMET polybutadiene, although larger cychcs have also been observed. A related undesired cyclization is the intramolecular cyclization of the monomer by RCM. 

Polystyrene represents a case in which monomer is only one of several spedes formed by thermal degradation at 350°C as monomer, dimer, trimer, and tetramer are formed in the relative proportions of 40 10 8 1. The thermal breakdown process is beheved to begin at unsaturated linkages which constitute the weak points along the chain. A cleavage at these sites initiates a fi-ee-radical mechanism leading to liberation of monomer and to an intramolecular back-biting process. The process Kberates dimer, trimer, and so on, by a transfer mechanism such as the shown in Equation 1.60... 

Chain transfer to polymer, either by intramolecular (back-biting) or intermolecular reaction, characterizes many cationic ROPs. This is due to the nucleophilicity of heteroatoms along the polymer backbone which compete with that of the monomer (Scheme 45). For instance, 1,4-dioxane is predominantly formed as the cyclic dimer in the cationic ROP of EO. Cyclic trimers or tetramers are also generated by cationic ROP of PO or epichlorohydrin. Under optimal conditions, the cationic ROP of THF can proceed, however, in a controlled/living manner, because the rates of back-biting and intermolecular transfer are slow relative to the rate of propagation. 

Intermolecular (interchange) and intramolecular (back-biting) chain transfer to polymer can occur but usually are less significant than in cationic ring-opening polymerization. 

Butane. The VPO of butane (148—152) is, in most respects, quite similar to the VPO of propane. However, at this carbon chain length an important reaction known as back-biting first becomes significant. There is evidence that a P-dicarbonyl intermediate is generated, probably by intramolecular hydrogen abstraction (eq. 32). A postulated subsequent difunctional peroxide may very well be the precursor of the acetone formed. 

The production of these branches is ascribed to intramolecular transfer, i.e., back-bite. CH2 —CH, CH. — CH,... 

It is well known from the ROP of lactones and lactides that the catalyst or initiator causes transesterification reactions at elevated temperatures [39], or at long reaction times (Scheme 5) [40]. Intermolecular transesterification reactions modify the sequences of copolylactones and prevent the formation of block copolymers. Intramolecular transesterification reactions, i.e., back-biting, cause... 

Intramolecular chain transfer to polymer (back-biting) should lead to the formation of cyclic oligomers ... 

The a and j3 methylene CHj groups (endo- and exocyclic ones) change their diem-ical environment. In the new environnffint their chemical shifts in the H- and C-NMR spectra differ from the previous ones. The corre ronding ectra cannot, however, discriminate between the intermolecular and intramolecular (backbiting) chain transfer, because in the non-strained rit formed by back-biting. 

For certain monomers such as vinyl acetate and ethylene, branching is much more significant. The free-radical (high-pressure) polymerization of low-density polyethylene (LDPE) includes a back-biting internal chain-transfer reaction that results in the formation of a short branch. It is this branching that results in an upper limit for the crystallinity of LDPE of about 60%-70% and a melt temperature of 110 C the backbiting reaction preferentially occurs with the formation of an intramolecular six-membered ring that results in preferential formation of a C4 short-chain branch as shown in Scheme 1.41. 

Low molecular weight dicarboxylic acids, keto acids and hydroxy acids have been shown to form as photooxidation products of polyethylene and polypropylene. These are almost certainly formed by intramolecular reactions of alkylperoxyl and peracyl radicals shown typically in Scheme 3.7. Back-biting along the aliphatic chain gives rise to unstable hydroperoxides and the elimination of small molecular fragments. It will be seen in Chapter 5 that these low molar mass oxidation products, which are already present in the environment from natural sources, are the first point of microbial attack in the surface of environmentally degraded polymers, leading to oxidation initiated bioerosion (Chapter 5). 

Chain transfer to polymer also is possible and results in the formation of branched polymer molecules. This can occur intramolecularly, in which case it is known as back-biting and mainly produces short-chain branches, as for example in the polymerization of ethylene ... 

Transesterification reactions could occur during anionic ROP of six-membered carbonates (Scheme 4.5). The intramolecular nucleophilic attack on carbonyl carbon atom (back-biting) leads to cyclic oligomers. The control of the polymerisation is rather poor, and bimodal distribution of molar masses is often observed (Matsuo et al., 1998b Pahovnik and Hadjichristidis, 2015). 

Figure 8.3 Postulated reactions of polylactide degradation, (a) Intramolecular transesterfication (back-biting) (b) Intramolecular transesterfication (c) Intermolecular transesterfication (d) Hydrolysis (e) Pyrolytic elimination...

In ROMP these by-products arise from back-biting processes, because already formed polymer chains as well as monomers may approach the propagating carbene species with their double bonds. If these secondary metathesis reactions proceed intramolecularly cyclic products are formed as shown in structure (31). The cyclic structure of such oligomers has been proven by mass spectroscopy [261,262]. 

The situation is more complicated in the case of intramolecular transfer, which occurs through the formation of a six-membered ring. In the case of acrylate (1)/ methacrylate (2), it can be assumed that the methacrylate radical is not reactive enough to back-bite and that the acrylate radical can only abstract hydrogen if the antepenultimate unit on the chain is also an acrylate unit. Thus back-biting can occur only for two monomer sequences (Mi Mi Mi and Mi M2 Mi) at the radical end, as shown in Scheme 4.16 [70]. The overall back-biting rate must be corrected for the sequence probabilities [Eqs. (40)-(43)] at the chain end, according to Eq. (59). 

Fragmentation after intramolecular transfer results in the formation of a long-chain radical and trimer species or a dimer radical and an unsaturated dead chain (Scheme 4.13). Consideration of all possible pathways and structures becomes complex, but the resulting model requires no additional parameters from the homopolymerization back-biting/scission case and provides a good representation of high temperature acrylate-methacrylate copolymerization [70]. 

---