Ring-expansion polymerizations

There are only a few studies dealing with cyclic aliphatic polyesters owing to their difficult synthesis [169]. Until recently, most works were based on ring-expansion polymerization. In the pioneering work of Kricheldorf and coworkers, cyclic... 

Fig. 39 Synthesis of macrocyclic aliphatic polyester by ring-expansion polymerization initiated by cyclic tin(IV) alkoxides...

Fig. 40 Synthesis of cyclic poly((3-lactone) by metal-free ring-expansion polymerization...

Kricheldorf HR (2004) Biodegradable polymers with variable architectures via ring-expansion polymerization. J Polym Sci A Polym Chem 42 4723-4742... 

Kricheldorf HR, Fechner B (2002) Polylactones. 59. Biodegradable networks via ring-expansion polymerization of lactones and lactides. Biomacromolecules 3 691-695... 

Jeong W, Hedrick JL, Waymouth RM (2007) Organic spirocyclic initiators for the ring-expansion polymerization of P-lactones. J Am Chem Soc 129 8414-8415... 

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... 

Kricheldorf et al. reported an anionic polymerization of y-D,L-butyro-lactone or D,L-lactide with cyclic dibutyltin initiators, such as 2,2-dibutyl-2-stanna-l,3-dioxepane, to give cyclic polymers [ 138-140]. Figure 41 shows the ring expansion polymerization of lactone for synthesizing a cyclic polymer as an example. They also synthesized the cyclic polymer with a living mechanism in the polymerization of e-caprolactone [141]. 

This type of ring expansion polymerization was extended for a synthesis of multiblock copoly(ether-ester)s of poly(THF) and e-caprolactone with cyclic dibutyltin initiators [142]. Such ring expansion polymerizations of lactones to synthesize macrocyclics with spirocyclic tin initiators are shown in Fig. 42 [143],... 

Fig. 42 Ring expansion polymerization of lactone with spiro-ring initiator...

Star-shaped polylactones with functional end groups via ring-expansion polymerization of f-caprolactone or y-o,i-butyrolactonc with a spiroinitia-tor was synthesized as shown in Fig. 44 [146]. The spirocyclic polylactones formed were reacted with various carboxylic acid chlorides and yielded fourarmed stars with the elimination of Bu2SnCl2. Star arms with chloroacetate, 4-bromobenzoate, 4-nitrobenzoate, cinnamate, stearate, or methacrylate end groups were obtained by varying the acid chlorides. 

Many works on the synthesis of cyclic polymers and block copolymers using kinetically controlled ring-expansion polymerizations of cyclic monomers, such as lactones and lactides with various types of cyclic tin initiators, were reviewed by Kricheldorf [147,148]. Kricheldorfs group continued the synthesis of cyclic polymers, and their recent works have focused on the following. Polycondensations of 4,4/-difluorodiphenylsulfone with tris(4-hydroxy phenyl)ethane were performed in DMSO to give multi-cyclic poly(ether sulfone)s derived from tris(4-hydroxyphenyl)ethane [149]. 

Fig. 54 Pyridine catalyzed ring-expansion polymerization of dithiolane-2,4-dione...

The large difference in reactivity ratio between L-LA and DXO makes it difficult to synthesize a tri-block copolymer, but the task can be carried out by using a cyclic tin alkoxide as initiator. The DXO macrocycle can initiate polymerization of l-LA and by ring expansion polymerization the two side blocks of l-LA are formed simultaneously. Scheme 7 shows the reaction pathway for the synthesis of tri-block copolymers from l-LA and DXO. 

Two possible explanations for observed distributions are (1) end-to-end closure of growing linear macromolecules which, for polymerization of D3, give DP = 3n and (2) ring expansion polymerization. 

A cyclic (ring) chain (Fig. 4b) has no chain termini compared with a linear chain (two termini). Ring polymers are synthesized by ring-closure reactions or ring-expansion polymerizations [94]. They manifest unique diffusion behavior and intrachain interactions. 

Scheme 12.14 Ring-expansion polymerization of thiiranes, using a cyclic thiocarbamate initiator [54],...

Scheme 12.17 Zwitterionic ring-expansion polymerization of lactide using a N-heteraqrolic carbene catalyst [59].

Macrocydes are present as a more or less important fraction in polymer systems exhibiting ring-chain equilibrium. Macrocydes can also be synthesized on purpose by using different synthetic strat es such as ring-expansion polymerization (REP) or end-to-end coupling of a linear polymer. The advantages and limitations of these different methods will be examined and illustrated... 

TEG, or PEG-2000. These spirocycles were used as initiators for the ring-expansion polymerization of Z-lactide in chlorobenzene at 120 °C. The resulting cyclic block co-polymers were reacted in situ with aliphatic dicarboxylic acid dichlorides (ADADs) or with hexamethylene diisocyanate (HMDI) to achieve chain extension. 

An interesting study has been performed where the ROP vs ring-expansion polymerization of e-caprolactone to form cychc polycaprolactones has been evaluated always employing heteroscorpionate complexes as initia-... 

A ring-expansion polymerization proceeds with the successive insertion of monomers into a cyclic initiator to form a ring polymer. The process does not require dilution, in contrast to an alternative and conventional end-to-end polymer cyclization process by linear polymer precursors. Until recently, however, the ring-expansion process has not been considered as... 

Scheme 1,2 New ring-expansion polymerizations (Bielawski et al, 2002 Bielawski et al, 2003 Culkin et al, 2007 Jeong et al, 2009 Xia et al, 2009).

Scheme 1.4 Ring-expansion polymerization and the subsequent covalent linking for the synthesis of ring polymers (Li et al, 2006).

Kudo, H., Sato, M., Wakai, R. et al (2008) A novel approach to cyclic polysulfides via the controlled ring-expansion polymerization of cyclic thiourethane with thiiranes. Macromolecules, 41,521-523. 

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