Back-biting process

During polymer chain growth, a back-biting process can lead to cyclic carbonate formation. In general, this process is more facile for aliphatic epoxides than for alicyclic epoxides and when the growing polymer chain dissociates from the metal center (Scheme 3). 

Pathway (2) leads to TMC formation by a back-biting process with ring closure. 

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... 

The peak at m/z 76 corresponding to carbon disulfide is present in the spectrum. Similarly, in the case of polythiotrimethylene, a back biting process occurs to form cyclic monomers on thermal decomposition at 260 °C ... 

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]. 

ROP of lactide was already demonstrated by Carothers in 1932, but the products had low molecular weight. New developments in the purification of lactide monomer and improvements in the synthesis techniques led recently to PLA with suitable properties being synthesized by this method. The lactide monomer was obtained from oligomeric PLA in an internal transesterification (back-biting) process performed at high temperatures and reduced pressure. This process, due to racemization, yields a mixture of the three possible dilactide forms D,D-lactide, L,L-lactide and D,L-lactide (meiu-lactide) (Scheme 6.3). The desired isomer can be obtained from this mixture by distillation and crystallization. 

This reaction also plays a role in the degradation of polysulftdes. A back-biting mechanism as shown in equation 6 results in formation of the cycHc disulfide (5). Steam distillation of polysulftdes results in continuous gradual collection of (5). There is an equiUbrium between the linear polysulftde polymer and the cycHc disulfide. Although the linear polymer is favored and only small amounts of the cycHc compound are normally present, conditions such as steam distillation, which remove (5), drive the equiUbrium process toward depolymerization. 

Heterochain polymers produced by ring-opening polymerization contain the hetero-atoms in the main chain as well as in the monomer and the polymer chain competes with the monomer for the reaction with the propagating species. This competition leads to polymer transfer and back-biting reactions during the polymerization. Heterochain polymers are also susceptible to depolymerization by the ionic active species which are easily formed during processing. 

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... 

These structures might of course be regarded as the first propagating species of the polymerisation of 23 initiated by methyl iodide. The ring closure of the open chain form is a process similar to a back-biting reaction, and as such is likely to be thermodynamically favoured by the presence of a ring substituent at the point of closure (157,158). Thus the polymerisation of 23 with I" counterion must involve a similar equilibrium in propagation and any data for rate constants will therefore be of a composite nature. 

Termination and Transfer Processes Macrocyclization. End-Biting and Back-Biting... 

In addition to short branches from back-biting reactions, long branches are seen to be formed both in LDPE and in PVC from intmmolecular chain transfer to polymer. These are much less frequent ( 1 per 2000 repeat units) but have a great effect on the viscosity of the melt and thus the processing. This is discussed in more detail in later chapters. 

Reaction a is the addition of monomer or any cyclic oligomer to the growing species, reaction b is back-biting and reaction c is end-to-end closure. When x = 3, the monomer (D3) is more strained and more nucleophilic than any other component in the system. Thus the dominant process is reaction a (where D(n = D3) which rapidly gives high polymer. At lower conversions (small n) reaction c also takes place. Reactions a with D(jy. where q > 1, and back-biting (b) are slow (lower nucleophilicity of the units) so that the early linear and cyclic polymers obey the 3n pattern. 

The GTP, performed in the presence of trimethylsilyldimethylketene acetal as initiator in conjunction with tetrabutyl ammonium acetate catalyst, showed the formahon of cyclic oligomers in addihon to the expected linear oligomers. In contrast, PMMA prepared by radical polymerizahon does not have this functional heterogeneity. MALDl-TOF spectra of low molar mass PMMA, obtained by the polymerizahon of MMA using zir-conocene as initiator, reveal that the process is not a living polymerizahon and that a back-biting cyclizahon process is involved. ... 

The branching on chains of low-density polyethylene results from a back-biting reaction in which the radical end group abstracts a hydrogen from the fourth carbon back (the fifth carbon in the chain). Abstraction of this hydrogen is particularly facile because the transition state associated with the process can adopt a conformation like that of a chair cyclohexane. In addition, the less stable 1° radical is converted to a more stable 2° radical. This side reaction is called a chain-transfer reaction, because the activity of the end group is transferred from one chain to another. Continued polymerization of monomer from this new radical center leads to a branch four carbons long ... 

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