Intermolecular chain transfer polymer

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 ... 

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

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... 

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... 

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

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... 

Scheme 3.7 LCB formation by (a) intermolecular chain transfer to polymer and (b) addition to a terminally unsaturated polymer chain.

Complexities also emerge when chain transfer to polymer mechanisms are examined. The rate of chain transfer to polymer is dependent on both the reactivity of the radical and the abstractability of the hydrogen atom on the repeat unit in the polymer chain. For the case of intermolecular chain transfer (LCB), this is represented by ... 

Non-linear polymers are frequently characterized by their solubility in a given solvent. The insoluble part, which corresponds to high molecular weight heavily branched polymer and polymer networks, is called gel. In the polymerization of monofunctional monomers that form gel by intermolecular chain transfer to polymer followed by termination by combination (e.g., butyl acrylate), the addition of CTA may reduce the gel content to nil, whereas the sol MWD remains essentially unaffected [89]. 

Termination by -Scission of Polymer Radicals. Internal polymer radicals, formed by either intra-or intermolecular chain transfer (see eqs. (16) and (18)), can undergo chain cleavage to form a terminated polymer chain and a new radical. 

As already discussed (see Section 7.2.4), inter- and intramolecular transfer reactions to polymer are of great importance with regard to the molecular weight distribution (Scheme 7.6). In complete thermodynamic equilibrium, the conversion should be given by the equilibrium monomer concentration and the molecular weight distribution should be the most probable one with D = 2, because of the intermolecular chain transfer, which is responsible for scrambling of the molecular... 

NMR and stability studies, that the comonomers are randomly distributed along the chains [237, 238]. This is attributed to the fact that intermolecular chain transfer (transacetalization), contrary to THF polymerization, proceeds on a time scale similar to propagation, and the same holds for the intramolecular chain transfer leading to cycUc polymer [239, 240]. Cationic polymerization of trioxane is usually a precipitation polymerization leading to crystalline polymers, and the size of the macrocydes is determined by the size of the crystalline lamdlae [240]. 

It was believed for a long time that head-to-head radical addition to monomers is a major route for formation of labile structures. Kinetic studies, in association with NMR measurements, reveal that formation of internal allylic and tertiary chlorine structures actually proceeds through an intramolecular or intermolecular chain-transfer reaction to polymer [Eqs. (31), (32) VC = vinyl chloride]. 

Even the polymer itself can react as a chain transfer agent. In the latter case, one has to distinguish between intramolecular and intermolecular chain transfer. 

Intermolecular chain transfer to polymer is well documented in the cationic polymerization of cyclic acetals. In the polymerization of TOX, as will be discussed in Section 4.10.3, chain transfer to polymer is essential for the preparation of thermally stable polyacetal. Intermolecular chain transfer to polymer is detrimental to the synthesis of monofunctional polymers such as macromonomers because segment exchange (scrambling) leads to disproportionation and formation of products having two, one, and none of the functional groups (Scheme 21). " Intermolecular chain transfer to polymer prohibits also the synthesis of block copolymers by sequential polymerization of two cyclic acetals. Addition of DXP to a solution of living polyDXL resulted in further polymerization but the copolymer formed had a nearly statistical distribution of units." ... 

Scheme 21 Disproportionation of end-groups by intermolecular chain transfer to polymer.

Scheme 24 Intermolecular chain transfer to polymer leading to formation of cyciic fraction in the cationic poiymerization of cyclic acetals.

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