Intermolecular chain transfer

Intermolecular chain transfer reactions may occur between two propagating polymer chains and result in the termination of one of the chains. Alternatively, these reactions take place by an intramolecular reaction by the coiling of a long chain. Intramolecular chain transfer normally results in short branches ... 

This feature can be explained by intermolecular chain transfer, giving branched macromolecules with high molecular weight or by the occurrence of dehydrocoupling cross-linking reactions, such as described previously for polyborazylene (scheme 1). 

Intermolecular chain transfer, 20 220 Intermolecular forces, in solvent-solute interactions, 23 91... 

As stated earlier, LDPE consists of molecules which are branched. The branching occurs during the polymerisation process, either by intermolecular chain transfer reactions or by intermolecular chain transfer as under ... 

Radical chain polymerization of ethylene to polyethylene is carried out at high pressures of 120-300 MPa (17,000-43,000 psi) and at temperatures above the Tm of polyethylene (Fig. 3-18) [Doak, 1986]. Batch processes are not useful since the long residence time gives relatively poor control of product properties. Long-chain branching due to intermolecular chain transfer becomes excessive with deleterious effects on the physical properties. Continuous processes allow better control of the polymerization. 

Intermolecular chain transfer between a growing polymer chain (32) and a previously formed polymer molecule (33) results in long-chain branching ... 

Richards and Slater (75) used a labelled polystyrene to demonstrate the existence of intermolecular chain transfer in the thermal degradation process. Polystyrene-14C, prepared in the normal way, was mixed with an inactive polystyrene specially prepared with weak links so that it degraded at temperatures where the polystyrene-14C was stable when on its own. Appearance of styrene- C monomer in the volatile degradation products proved the existence of intermolecular chain transfer (Reaction 9). 

Cyclization results from intramolecular chain transfer to polymer, being also indicative of the extent of intermolecular chain transfer leading to formation of branched (nonreactive in the case of oxetanes) tertiary oxonium ion (cf., Section II.D.3). 

The intermolecular chain transfer to polymer (scrambling), however, is detrimental for preparation of functional polymers, leading to disproportionation of monofunctional macromolecules, as shown schematically below ... 

Formation of block copolymers in the sequential polymerization may be affected by chain transfer to polymer. As already discussed, in several systems the intramolecular chain transfer to polymer leads to formation of cyclic fraction. Cyclic macromolecules, being neutral, do not participate in further reaction and constitute the homopolymer fraction in resulting copolymer. Intermolecular chain transfer to polymer may lead to disproportionation, i.e., formation of fraction of macromolecules which do not carry active species ... 

Intermolecular chain transfer to polymer leads also to the exchange of segments between macromolecules (scrambling). This may effectively preclude the isolation of block copolymers. This phenomenon is especially pronounced in the polymerization of cyclic acetals. 

In the second case, the soluble polymer 68 was treated with 10-undecenoyl chloride 72 to form the resin-bound alkene 73 (Scheme 16) [11]. Radical addition of xanthate 74, mediated by lauroyl peroxide (50 mol %), gave the product xanthate 75 and tetralone 76 as a 9 1 mixture, in an overall yield of 70%. By-product 76 is formed by intramolecular addition of the intermediate alkyl radical to the aromatic ring competing with intermolecular chain transfer. It was not possible to achieve complete consumption of 73 in this case possibly due to the decrease in the rate of the intermolecular radical addition step as the alkene was consumed. These experiments show, however, that xan-... 

Intermolecular Chain Transfer (Termination) to Polymer Chains... 

These results provide additional confirmation for the mechanism of pyrolysis of simple polyolefins. The absence of monomer in the volatile products, the maxima in the rate curves, and the sharp decrease in the intrinsic viscosity for linear polymethylene (29) and polypropylene (2, 6, 13, 30) all point to an essentially random scission, due to pronounced intermolecular chain transfer, Equation 2. However, deviations appear when a, the fraction of bonds broken, or, what amounts to the same, the number average DP is examined as a function of time. For small a, the former relation should be one of simple proportionality and hnearity in 1/P. Instead, for both polypropylene (6) and polymethylene [see Figure 5, in (29)] curvature appears, indicating a reduction of the scission rate after an initial period of rapid degradation. For polypropylene this has been interpreted as a breaking of weak and normal bonds. Between 250° and 280° C., one weak link per 2.4 X 10 is found (6). At 295° C., the existence of more than two types of bonds would have to be postulated. 

The disproportionation reaction of the free radical chain can generate the monomer as a successive process. There are, however, some other issues regarding the propagation for free radical chain reactions. In addition to the "regular" propagation step, different reactions may occur in a so-called transfer step. In this step, the free radical chain reacts with another molecule and generates a different radical chain and a new polymeric molecule. There are two possible types of transfer reactions. The transfer step can be an intermolecular chain transfer or an intramolecular chain transfer. An example of an intermolecular chain transfer is... 

After the radical formation, intermolecular chain transfer reactions take place. A special case of chain transfer is the propagation reaction (see Section 2.1) of the type ... 

The intermolecular chain transfer (termination) reaction was demonstrated by polymerizing a given monomer in the presence of another preformed polymer. A copolymer is obtained in this case... 

Although there may be some minor contribution of intermolecular chain transfer, these systematic studies have provided a clearer perspective of the mechanism of the intramolecular chain transfer reaction of propagating acrylate radicals. Nevertheless further investigation was required to provide decisive proof of the mechanism. 

Two distinct but related strategies that rely on templates to control the number of monomers incorporated into an oligomeric product can be envisioned. One of these approaches, shown in Scheme 8-2, relies on templated radical macrocyclization reactions to control telomer size [14, 15]. This strategy requires attachment of all of the monomer units to the template backbone and uses macrocyclization, which faces competition from intermolecular chain transfer, to control the telomer length. The chain transfer agent T-I (i.e., telomerization terminator) is not attached to the template. 

The MCRs can also be formed by intermolecular chain transfer to polymer (leading to long-chain branches), but its contribution is small. 

The mechanism of polymerization can be described as an equilibration among these various components in addition to reaction with the monomer, the growing silanolate chain ends react with all siloxane bonds via intramolecular cyclization and intermolecular chain transfer as shown in Scheme 7.18. 

Intermolecular chain transfer to polymer results in long-chain branches and proceeds via abstraction of either a backbone tertiary hydrogen atom (in repeat units from vinyl monomers) or an atom from a substituent group. An example... 

Branched polymer is known to form in polymerization of vinyl acetate via abstraction of hydrogen atoms from pendent acetyl methyl groups, which is an example of the second type of intermolecular chain transfer to polymen... 

Thus, intermolecular chain transfer to polymer leads to premature termination of the growth of one propagating chain and the reactivation of a dead chain which then grows a long-chain branch. As a consequence, the molar mass distribution of the polymer broadens. The changes in skeletal structure and molar mass distribution inevitably have major effects upon bulk polymer properties. 

However, the low-temperature oxidation of solid polypropylene (70-110°C) proceeds with alternating intramolecular and intermolecular chain transfer. Intramolecular kinetic of extension chains is limited to small parts of the macromolecule with a favorable set of conformations. As a result, blocks of hydroperoxide can be short. In the solid polypropylene has found about 60% of paired units and about 20% of triads, the share of units with a higher number of hydroperoxide groups is small. It should be noted that in other carbon-chain polymers increases the probability of intramolecular reaction at the high rate of conformational motions. For example, in the polymers with a saturated C-C bond (such as... 

---