Homopolymer contamination

To avoid homopolymer formation, it is necessary to ensure true molecular contact between the monomer and the polymer. Even if this is initially established, it needs to be maintained during the radiation treatment while the monomer is undergoing conversion. Several methods are used for minimizing the homopolymer formation. These include the addition of metal cations, such as Cu(II) and Fe(II). However, by this metal ion technique, both grafting and homopolymerization are suppressed to a great extent, thus permitting reasonable yield of graft with little homopolymer contamination by the proper selection of the optimum concentration of the inhibitor [83,90,91]. 

GTP was employed for the synthesis of block copolymers with the first block PDMAEMA and the second PDEAEMA, poly[2-(diisopropylamino)e-thyl methacrylate], PDIPAEMA or poly[2-(N-morpholino)ethyl methacrylate], PM EM A (Scheme 33) [87]. The reactions took place under an inert atmosphere in THF at room temperature with l-methoxy-l-trimethylsiloxy-2-methyl-1-propane, MTS, as the initiator and tetra-n-butyl ammonium bibenzoate, TBABB, as the catalyst. Little or no homopolymer contamination was evidenced by SEC analysis. Copolymers in high yields with controlled molecular weights and narrow molecular weight distributions were obtained in all cases. The micellar properties of these materials were studied in aqueous solutions. 

Employing similar procedures, PPO-fc-POEGMA block copolymers and POEGMA-fc-PPO-fc-POEGMA triblock copolymers were prepared from the corresponding PPO macroinitiators [129]. The polymerizations were performed in a isopropanol/water (70/30) mixture at 20 °C using CuCl and bpy. The methacrylate monomer was almost quantitatively polymerized, and the polydispersities were lower than 1.25 in most cases. Less than 5% PPO homopolymer contamination was detected by SEC analysis. 

Table 2 contains the characteristics of the amic ester-aryl ether copolymers including coblock type, composition, and intrinsic viscosity. Three series of copolymers were prepared in which the aryl ether phenylquinoxaline [44], aryl ether benzoxazole [47], or aryl ether ether ketone oligomers [57-59] were co-re-acted with various compositions of ODA and PMDA diethyl ester diacyl chloride samples (2a-k). The aryl ether compositions varied from approximately 20 to 50 wt% (denoted 2a-d) so as to vary the structure of the microphase-separated morphology of the copolymer. The composition of aryl ether coblock in the copolymers, as determined by NMR, was similar to that calculated from the charge of the aryl ether coblock (Table 2). The viscosity measurements, also shown in Table 2, were high and comparable to that of a high molecular weight poly(amic ethyl ester) homopolymer. In some cases, a chloroform solvent rinse was required to remove aryl ether homopolymer contamination. It should also be pointed out that both the powder and solution forms of the poly(amic ethyl ester) copolymers are stable and do not undergo transamidization reactions or viscosity loss with time, unlike their poly(amic acid) analogs.

Solutions of the copolymers (2a-k) were cast into thin films and heated to 350 °C to imidize the copolymer (Schemes 6-8), yielding polymers 3a-k. In each case, clear tough films were obtained indicating minimal homopolymer contamination. The thermal analyses for the copolymers are shown in Table 3 together with that of a polyimide homopolymer for comparison. It is important to note that the aryl ether composition increased after imidization due to the loss of ethanol in the imidization process. No detectable T was observed for the polyimide... 

To determine the amount of homopolymer contaminants, select samples were solvent extracted. Thus 5 g of the star block was placed in a cellulose thimble and... 

It is concluded that we can easily detect homopolymers contaminating graft copolymers using the TLC method, even small amounts of 0.5—1.0%, though in... 

The S-B block copolymer produced (XII) was characterized by a variety of techniques including NMR, gel permeation chromatography (GPC), thin-layer chromatography (TLC), and transmission electron microscopy (TEM). The results of these analyses clearly showed that the block copolymer was fairly pure with very little homopolymer contamination. 

Becker et al. [64] functionalized a peptide, based on the protein transduction domain of the HIV protein TAT-1, with an NMP initiator while on the resin. They then used this to polymerize f-butyl acrylate, followed by methyl acrylate, to create a peptide-functionahzed block copolymer. Traditional characterization of this triblock copolymer by gel permeation chromatography and MALDI-TOF mass spectroscopy was, however, comphcated partly due to solubility problems. Therefore, characterization of this block copolymer was mainly hmited to ll and F NMR and no conclusive evidence on molecular weight distribution and homopolymer contaminants was obtained. Difficulties in control over polymer properties are to be expected, since polymerization off a microgel particle leads to a high concentration of reactive chains and a diffusion-limited access of the deactivator species. The traditional level of control of nitroxide-mediated radical polymerization, or any other type of controlled radical polymerization, will therefore not be straightforward to achieve. 

As is evident from the GPC chromatograms in Figure 3, both the diphenyl ethylene capped polystyrene and the PS-PIBM diblock copolymers have narrow molecular weight distributions. More importantly, no detectable homopolymer contamination is present in the very pure diblock. This high structural integrity was achieved by taking the following precautions. 

The reaction of ceric ions with polymer-bound functionalities gives polymer-bound radicals. I hus, one of the major applications of ceric ion initiation chemistry has been in grafting onto starch, cellulose/ polyurethanes and other polymers. The advantage of this over conventional initialing systems is that, ideally, no low molecular weight radicals which might give homopolymer contaminant are fonned. 

The Hving cationic polymerization of p-chloro-a-methylstyrene (pClorMeSt) can also be accompHshed under conditions identical to those used for the synthesis of poly(a MeSt-h-IB) copolymer [86, 182]. Using the above method poly(pClaMeSt-l)-IB) diblock copolymer was also prepared via sequential monomer addition. On the basis of GPC UV traces of the starting PpClaMeSt and the resulting poly(pClaMeSt-fo-IB) diblock copolymer, the Beff was 100% and homopolymer contamination was not detected. 

The most likely pathways of NCA polymerization are the so-called amine and the activated monomeP (AM) mechanisms. The amine mechanism is a nucleophilic ring-opening chain growth process where the polymer could grow linearly with monomer conversion if side reactions were absent (eqn [2]). On the other hand, the AM mechanism is initiated by deprotonation of an NCA, which then becomes the nucleophile that initiates chain growth (eqn [3]). It is important to note that a polymerization can switch back and forth between the amine and AM mechanisms many times a propagation step for one mechanism is a side reaction for the other, and vice versa. It is because of these side reactions that block copolypeptides and hybrid block copolymers prepared from NCAs using amine initiators have structures different than predicted by monomer feed compositions and most likely have considerable homopolymer contamination. These side reactions also prevent control of chain-end functionality desirable for many applications. 

The majority of amine-initiated block copolypeptides were often subjected to only limited characterization (e.g., amino acid compositional analysis) and, as such, their structures, and the presence of homopolymer contaminants, were not conclusively determined. Some copolymers, which had been subjected to chromatography, showed polymodal molecular weight distributions containing substantial high- and low-molecular-weight fractions.The compositions of these copolymers were found to be different from the initial monomer feed compositions and varied widely for different molecular weight fractions. It appears that most, if not all, block copolypeptides prepared using amine initiators have structures different than predicted by monomer feed compositions and likely have considerable homopolymer contamination due to the side reactions described above. 

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