Ring-opening polycondensation

The polycondensation of linear monomers is characterized by elimination of one small molecule (e.g., H2O, HCl, or CH3OH) in each growing step. The liberation of the byproduct compensates for the loss of entropy resulting from the loss of three-dimensional translational motion of the monomers. Hence, the reaction entropy is close to zero (the exact value depends on the extent of rotational motion in monomer and repeating unit). A typical RO-PC is based on two different growing steps. The first one is an addition reaction and elimination of a byproduct only occius in the second step which is a condensation reaction as illustrated for the polycondensation of an a,co-alkanediol with maleic anhydride (see Formula 9.1). Therefore, RO-PCs have a negative reaction entropy which needs to be overcompensated by a negative reaction enthalpy (i.e., by an exothermic 

Kricheldorf, Polycondensation, DOI 10.1007/978-3-642-39429-4 9, Springer-Verlag Berlin Heidelberg 2014 

A handful of RO-PCs is known which is based on two cyclic monomers, a cyclic a2 monomer and a cycUc b2 monomer so to say. The classification of cyclic monomers given above is valid again, but the gain in rotational freedom may now be higher by a factor of two. This means, that the reaction entropy is close to zero, when two monomers of class II react with each other. 

RO-PCs involving cyclic anhydrides have a tradition of almost 100 years, and thus polycondensations of cyclic anhydrides will be discussed first. 

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Polyamide 6 is produced by ring opening polycondensation of e- caprolactame. If no other reactants are used, the polymer chains contain one carboxylic acid and one amine end group. 

Kricheldorf HR, Eechner B (2001) Polylactones. 51. Resorbable networks by combined ring-expansion polymerization and ring-opening polycondensation of -caprolactone or DL-lactide. Macromolecules 34 3517-3521... 

Development of polybutadiene, polychloroprene and especially copolymers of butadiene andstyrene, as best replacements for natural rubber for tire-applications. Sodium used as catalyst Ring-opening polycondensation of caprolactam discovered by Schlack Formulation of the well-known Mark-Houwink equation for the viscometric determination of the molecular weight (mass)... 

Kricheldorfs group also proposed another approach to multiblock copolymers of TMC and LLA synthesis. Sequential copolymerizations of TMC and LLA were performed with 2,2-dibutyl-2-stanna-l,3-oxepane as a bifunctional cyclic initiator. The cyclic triblock copolymers were transformed in situ into multiblock copolymers by ring-opening polycondensation with sebacoyl chloride (Scheme 94). 

The enchainment of OA condensation polymers with asymmetric carbon atoms in the main chain prepared by polycondensation between diacids and diamines is not regular. At least one of the two reagents must be optically active and, therefore, its two reacting groups are not strictly equivalent, and head-to-head or head-to-tail arrangements are possible. This is not the case in ring opening polycondensation of lactams which are used to obtain OA polyamides derived from substituted-co-amino-alkanoic acids. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

The preparation of PGA given below (Scheme 2.51) is a polycondensation of glycolic acid followed by a solid-state postpolycondensation.374 (Reproduced from ref. 374. Copyright 2000 Elsevier Science Ltd, by permission of the copyright owner.) This method is different from the commonly used ring-opening polymerization of lactide (see Section 2.3.6). 

The main polymerization method is by hydrolytic polymerization or a combination of ring opening as in (3.11) and hydrolytic polymerization as in (3.12).5,7 9 11 28 The reaction of a carboxylic group with an amino group can be noncatalyzed and acid catalyzed. This is illustrated in the reaction scheme shown in Fig. 3.13. The kinetics of the hydrolytic polyamidation-type reaction has die form shown in (3.13). In aqueous solutions, die polycondensation can be described by second-order kinetics.29 Equation (3.13) can also be expressed as (3.14) in which B is die temperature-independent equilibrium constant and AHa the endialpy change of die reaction5 6 812 28 29 ... 

The mechanism of ring-opening polymerization has received much attention in recent years. The studies of it has made control of the polymerization reactions possible resulting in desirable products. However, many problems still remain unsolved in this field. In fact, the situation is far less satisfactory than the fields of vinyl polymerization and polycondensation. Extensive studies of copolymerization should be useful for the establishment of the chemistry of ring-opening polymerization. 

Interest in anionic polymerizations arises in part from the reactivity of the living carbanionic sites4 7) Access can be provided to polymers with a functional chain end. Such species are difficult to obtain by other methods. Polycondensations yield ro-functional polymers but they provide neither accurate molecular weight control nor low polydispersity. Recently Kennedy51) developed the inifer technique which is based upon selective transfer to fit vinylic polymers obtained cationically with functions at chain end. Also some cationic ring-opening polymerizations52) without spontaneous termination can yield re-functional polymers upon induced deactivation. Anionic polymerization remains however the most versatile and widely used method to synthesize tailor made re-functional macromolecules. 

At the same time, ring-opening-polymerization (ROP) processes, which dominated the phosphazene field for decades [38], tend now to be substituted by polycondensation reactions. These seem to be more feasible and reproducible, easier to carry out, and able to guarantee predictable MWs and MW distributions for these materials [10]. 

Random hyperbranched polymers are generally produced by the one-pot polymerization of ABX monomers or macromonomers involving polycondensation, ring opening or polyaddition reactions hence the products usually consist of broad statistical molecular weight distributions. 

Over the past decade, literally dozens of new AB2-type monomers have been reported leading to an enormously diverse array of hyperbranched structures. Some general types include poly(phenylenes) obtained by Suzuki-coupling [54, 55], poly(phenylacetylenes prepared by Heck-reaction [58], polycarbosilanes, polycarbosiloxanes [59], and polysiloxysilanes by hydrosilylation [60], poly(ether ketones) by nucleophilic aromatic substitution [61] and polyesters [62] or polyethers by polycondensations [63] or by ring opening [64]. 

Until now the discussion has centered on the addition polymers obtained fiom unsaturated monomers by reaction of the C=C or C=0 double bond. However, polymers obtained by other methods (ring-opening polymerization, polycondensation, etc.) offer interesting stereochemical phenomena also. As a rule, in these classes of macromolecular compounds the monomer units are clearly defined, the direction of the chain is often distinguishable and the stereo-isomeric elements present in the chain already preexist in the monomer. There are, however, numerous exceptions and further clarification is called for. 

The microstructural approach can be extended to the polymers of 1,2-disub-stituted ethylenes, to diene polymers, and to those obtained by ring-opening polymerization and by polycondensation. Differences with regard to vinyl and vinylidene polymers are observed in the type and number of the sequences, their nomenclature and the statistic model for the quantitative interpretation of their distribution. 

The polymerization of enantiomerically pure monomers presents no relevant stereochemical problems when the asymmetric carbon atom is not involved in the reaction and no new centers of stereoisomerism are formed. This is the case, for example, in polycondensation of chiral diacids with diamines (274) and in ring-opening polymerization of substituted lactams (275) and A -carboxyanhy-drides of a-amino acids (276). Interest here lies mainly in the properties of the polymer. Accidental racemization may sometimes occur but is not necessarily related to the mechanism of polymerization. 

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