Branching ring-opening polymerization

Intramolecular rearrangement of the initially formed radical may occur occasionally (e.g. backbiting - Section 4.4.3) or even be the dominant pathway (e.g. cyelopolymerization - Section 4.4.1, ring-opening polymerization - Section 4.4.2). These pathways can give rise to branches, rings, or internal unsaturation in the polymer chain. 

Moreover, alcohol functionalities have been introduced into the polynor-bornene (PNB) backbone by copolymerization of norbornene with a few percent of 5-acetate norbornene and subsequent acetate reduction. After transformation of the pendant hydroxyl functions into diethyl aluminum alkoxides, sCL has been ring opening polymerized (Scheme 31). Owing to the controlled/ liv-ing character of both polymerization processes the isolated poly(NB- -CL) graft copolymers were characterized by well-defined composition, controlled molecular weight and branching density, and narrow MWD (PDI=1.2-1.4) [92]. 

In principle, one can obtain an unbranched polyethyleneimine by saponifying the amide groups that are located on the polymer (Example 5-4). It is important to note that the ring-opening polymerization of aziridine does not yield linear polyethyleneimines but rather highly branched polymer structmes. 

Scheme I Synthesis of hyperbranched poly(ethylene imine) (PEI) via acid catalyzed ring-opening polymerization of aziridine. Each color represents a different branching unit blue for linear units (L), black for dendritic units (D), red for terminal units (T). PEI has a degree of branching (DB) of 62-73%, the depicted structure represents only a small idealized fragment. Reproduced with permission from [91]...

T. Uryu, M. Yamanaka, M. Henmi, K. Hatanaka, and K. Matsuzaki, Ring-opening polymerization of 1,6-an hydro-2,4-di-O-bcnzy 1-3-0-twt-butyldimcthylsilyl-/i-i)-glucopyranose and synthesis of a-( —> 3)-branched dextrans, Carbohydr. Res., 157 (1986) 157-169. 

A similar technique was employed for the synthesis of miktoarm stars having PS, PEO, poly(e-caprolactone) (PCL) or PMMA branches [57]. A PS-h-PMMA diblock copolymer possessing a central DPE derivative, bearing a protected hydroxyl function was prepared. After deprotection and transformation of the hydroxyl group to an alkoxide the anionic ring opening polymerization of the third monomer (EO or e-CL) was initiated. Only limited characterization data were given in this communication. 

In the literature, linear polymers are referred to as poly(iminoethylene), and branched polymers as poly(ethylenimine), since the latter derive from the ring-opening polymerization of ethylenimine. In this review, both polymers will be referred to as PEI. Mention will be made, however, to their structure whenever opportune. 

Branched poly(ethyleneimine) is usually obtained by ring opening polymerization of ethyleneimine with cationic initiators. The various aspects of this reaction have been reviewed elsewhere 7). This is the origin of commercial PEI. For this kind of PEI, it has been definitely established that the molecular structure is highly branched, independently of the modalities of preparation. As a result, there are three types of functional groups, i.e. primary, secondary and tertiary amino groups. For instance,... 

Poly(N-alkyl-ethylene imine) can be prepared by cationic ring-opening polymerization of N-alkylaziridines however, branched polymers are obtained by this way 19>. Linear poly-MEI may be obtained by N-methylation of linear PEI by the Clarke-Eschiweiler variation of the Leukart reaction, in which an excess of formic acid is used together with formaldehyde. The reaction may be performed either on PEI, or... 

As discussed in Section II.C, in any system in cationic ring-opening polymerization, a reaction of active species with polymer repeating unit may lead to chain transfer to polymer or termination (if the resulting branched or cyclic onium ions are not active), whereas recombination with counterion leads to termination in the case of irreversible reaction. The later reaction may be avoided by the proper choice of counterion. As the onium ions are generally inherently stable there is no other termination reaction, provided that impurities that may act as terminating agents are eliminated. 

It must be considered that silyl triflates are not stable in THF for an extended time, because they can initiate the cationic ring-opening polymerization (ROP) of cyclic ethers. Therefore, the silyl triflate (in diethyl ether) is added in drops to the (aminosilyl)lithium compound dissolved in THF. The reaction proceeds very quickly, and ROP of THF is not observed under these conditions. The trisilanes la - Ic can be converted by reaction with triflic acid into the silyl bis (triflates) 2a - 2c. These compounds are precursors for further chain elongation. The reaction with (diethylamino)-diphenylsilyllithium leads to the pentasilanes 3a - 3c. The reaction of bis(diethylamino)phenylsilyl-lithium [9] with silyl triflates proceeds analogously. The formation of the disilane 4 is shown in Scheme 2 as an example. Conversion with triflic acid and chain elongation with LiSiPh2(NEt2) leads to the branched tetrasilane 6. 

Synthetic PAs are produced by polycondensation of bifunctional monomers or by cationic and anionic ring-opening polymerization of lactams. Polymers obtained with the first technique are linear, whereas chain branching may occur with anionic polymerization. Based on their chemical structure, synthetic polyamides may be classified into two categories [1] ... 

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