Synthesis and styrene copolymerization of novel trisubstituted ethylenes: 3. Alkoxy ring-substituted isopropyl 2-cyano-3-phenyl-2-propenoates

ABSTRACT Novel trisubstituted ethylenes, alkoxy ring-substituted isopropyl 2-cyano-3-phenyl-2-propenoates, RPhCH = C(CN)CO2CH(CH3)2 (where R is 2-methoxy, 3-methoxy, 4-methoxy, 2-ethoxy, 3-ethoxy, 4-ethoxy, 4-propoxy, 4-butoxy, 4-hexyloxy) were prepared and copolymerized with styrene. The ethylenes were synthesized by the piperidine catalyzed Knoevenagel condensation of ring-substituted benzaldehydes and isopropyl cyanoacetate, and characterized by CHN analysis, FTIR, 1H and 13C NMR. All the ethylenes were copolymerized with styrene in solution with radical initiation (ABCN) at 70°C. The compositions of the copolymers were calculated from nitrogen analysis and the structures were analyzed by FTIR, 1H and 13C NMR. Decomposition of the copolymers in nitrogen occurred in two steps, first in the 250-500ºC range with residue (2.6–3.9% wt.), which then decomposed in the 500-800ºC range.

It was shown that electrophilic tri-and tetrasubstituted ethylenes are particularly useful in delineating the transition from radical chemistry to ionic chemistry [7].
In regards to polymerization reactivity, previous studies showed that TSE containing substituents larger than fluorine have very low reactivity in radical homopolymerization due to polar and steric reasons. Although steric difficulties preclude homopolymerization of most TSE monomers, their copolymerization with a monosubstituted alkene makes it possible to overcome these steric problems [8]. Copolymerization of electrophylic TSE compounds having double bonds substituted with halo, cyano, and carbonyl groups and electron-rich monosubstituted ethylenes such as styrene, N-vinylcarbazole, and vinyl acetate [9][10][11] show a tendency toward the formation of alternating copolymersthus suggesting a way of functionalization of commercial polymers via introduction of isolated TSE monomer units.
With the objective to design unique structures, that could serve as a spring board for further development of novel compounds and materials with new properties and applications we have reported synthesis and styrene copolymerization of alkoxy ring-substituted methyl [12][13][14], ethyl [15,16], propyl [17], and butyl [18] esters of 2-cyano-3-phenyl-2-propenoates. Our purposes in exploration of novel isopropyl 2-cyano-3-phenyl-2-propenoates (ICPP) were twofold: (1) to utilize aldol condensation for synthesis of alkenes with a variety of potentially reactive functional groups; (2) to employ conventional radical copolymerization of novel functional comonomers with a commercial monomer styrene.

General procedures
Infrared spectra of the ICPP compounds and polymers (NaCl plates) were determined with an ABB FTLA 2000 FTIR spectrometer. The melting points of the ICPP compounds and the glass transition temperatures (T g ), of the copolymers were measured with TA (Thermal Analysis, Inc.) Model Q10 differential scanning calorimeter (DSC). The thermal scans were performed in a 25 to 150ºC range on second heat at heating rate of 10ºC/min. T g was taken as a midpoint of a straight line between the inflection of the peak's onset and endpoint. The thermal stability of the copolymers was measured by thermogravimetric analyzer (TGA) TA Model Q50 from ambient temperature to 800ºC at 20ºC/min in the flow of nitrogen (20 mL/min). The molecular weights of the polymers was determined relative to polystyrene standards in THF solutions with sample concentrations 0.8% (w/v) by gel permeation chromatography (GPC) using a Altech 426 HPLC pump at an elution rate of 1.0 mL/min; Phenogel 5μ Linear column at 25ºC and Viscotek 302 RI detector. 1 H and 13 C NMR spectra were obtained on 10-25% (w/v) ICPP or polymer solutions in CDCl 3 at ambient temperature using Avance 300 MHz spectrometer. Elemental analyses of ICPP compounds and the copolymers were performed by Midwest Microlab, LLC (IN).

Synthesis of styrene -ICPP copolymers
Styrene (≥99%), 1,1ʹ-azobiscyclohexanecarbonitrile (98%), (ABCN), and toluene (98%) supplied from Sigma-Aldrich Co., were used as received. Copolymers of the ST and the ICPP compounds, P(ST-co-ICPP) were prepared in 25-mL glass screw cap vials at ST/ICPP = 3 (mol) the monomer feed using 0.12 mol/L of ABCN at an overall monomer concentration 2.44 mol/L in 10 mL of toluene. The copolymerization was conducted at 70ºC. After 5 hours the mixture was cooled to room temperature, and precipitated dropwise in methanol. The composition of the copolymers was determined based on the nitrogen content. The compounds were characterized by FTIR, 1 H and 13 C NMR spectroscopies. Thermal behavior was studied by DSC and TGA.
The condensation reaction yielded crystalline products. No stereochemical analysis of the novel alkoxy ring-substituted ICPP was performed since no stereoisomers (E or/and Z) of known configuration were available.

Homopolymerization
An attempted homopolymerization of the ICPP compounds in the presence of ABCN did not produce any polymer as indicated by the lack of a precipitate in methanol. The inability of the monomers to polymerize is associated with steric difficulties encountered in homopolymerization of 1,1-and 1,2-disubstituted ethylenes [8]. Homopolymerization of ST under conditions identical to those in copolymerization experiments yielded 18.3% of polystyrene, when polymerized for 30 min.

Copolymerization
The novel synthesized ICPP compounds copolymerized readily with ST under free-radical conditions (Scheme 2) forming white flaky precipitates when their solutions were poured into methanol.
The conversion of the copolymers was kept below 20% to minimize compositional drift (Table 1). Nitrogen elemental analysis showed that between 18.0 and 28.0 mol% of ICPP is present in the copolymers prepared at ST/ ICPP = 3 (mol), which is indicative of relatively high reactivity of the ICPP monomers towards ST radical which is typical of alkoxy ring-substituted esters of 2-cyano-3-phenyl-2-propenoates [12][13][14][15][16][17][18]. Since ICPP monomers do not homopolymerize, the most likely structure of the copolymers would be isolated ICPP monomer units alternating with short ST sequences (Scheme 2).
The copolymers prepared in the present work are all soluble in ethyl acetate, THF, DMF and CHCl 3 and insoluble in methanol, ethyl ether, and petroleum ether.
Relative reactivities of ST and the ICPP monomers in the copolymerization can be estimated by application of the copolymerization equation for the terminal copolymerization model [8] Equation (2) assumes a minimal copolymer compositional drift during a copolymerization reaction, i.e., a low conversion. The fact that ICPP monomers do not self-propagate allows the use of Equation (2)  than styrene in the addition to a ST-ended polymer radical. The above reactivities of isopropyl cyanophenylpropenoates are relatively close to those of methyl cyanophenylpropenoates [12][13][14], (1/r 1 ) 3-methoxy

Copolymer structure
The structure of ST-ICPP copolymers was characterized by FTIR and NMR spectroscopy. A comparison of the spectra of the monomers, copolymers and polystyrene shows, that the reaction between the ICPP monomers and ST is a copolymerization. FTIR spectra of the copolymers show overlapping bands in 3350-2680 cm −1 region corresponding to C-H stretch vibrations. The bands for the ICPP monomer unit are 2240-2228 (w, CN), 1756-1710 (s, C = O), and 1280-1210 cm −1 (m, C-O). Phenyl rings of both monomers show ring stretching bands at 1516-1320 cm −1 as well as a doublet 778-610 cm −1 , associated with C-H out-of-plane deformations. These bands can be readily identified in styrene copolymers with TSE monomers containing cyano and carbonyl groups.
The 1 H NMR spectra of the ST-ICPP copolymers show a broad double peak in a 7.5-5.7 ppm region corresponding to phenyl ring protons. A resonance at 5.3-5.0 ppm is assigned to the methineoxy proton of isopropyl ester in both head-to-tail, H-T (structure A) and head-to-head, H-H (structure B) alternating ST-ICPP structures. A signal at 4.2-3.8 ppm is assigned is assigned to the ICPP methoxy protons. Broad overlapping resonances at 3.9-3.3 ppm are assigned to the methylene protons of H-T structures and methine protons of ST in H-H structures. Overlapping resonances at 3.5-2.5 ppm are assigned to mithine protons of ICPP in both A and B structures. Resonance in 2.4-210 is assigned to mithine proton in A structure and methylene protons of ST monomer unit in B structure close to the ICPP, which are more subjected to deshielding than the ones in polystyrene (structure C). A broad resonance peak in 2.0-0.9 ppm range is attributed to the methine and methylene protons of ST monomer sequences, whereas the absorption in 1.0-0.2 ppm is assigned to methyl groups of ICPP isopropyl ester.
The peak in 172-163 ppm range is assigned to the carbonyl of the ICPP isopropyl group. Quaternary carbons  Broadening of the NMR signals in the spectra of the copolymers (independent of magnetic field strength) is apparently associated with head-to-tail and head-to-head structures, which formed though the attack of a styreneended radical on both sides of ICPP monomer or/and participation of both free monomers and monomer donor-acceptor complexes [8] The FTIR and NMR data  showed that these are true copolymers, composed of both ICP and ST monomer units.

Thermal behavior
Thermal transitions of the ST-ICPP copolymers were analyzed by differential scanning calorimetry. All the copolymers were amorphous and show no crystalline DSC endotherm on repeated heating and cooling cycles. The glass transition temperatures T g of the copolymers were measured by DSC. The second heating results were obtained in all cases so that the samples become drier without 'thermal memory'. Figure 4 shows DSC trace of ST copolymer with isopropyl 2-cyano-3-(3-methoxyphenyl)-2-propenoate. Table 2 shows glass transition values for the ST-ICPP copolymers prepared in this work with no correlation to the size and position of the ICPP alkoxy ring substitution apparently due to non-uniform composition, monomer unit distribution, and/or molecular weight and MWD. A single T g value was observed for all the copolymers with values close or higher than polystyrene (104ºC) [23]. We compared glass transitions of styrene copolymers with different esters of cyanophenylpropenoates. Thus T g values of ST-ICPP copolymers are lower than those of the ST copolymers with methyl cyanophenylpropenoates [12][13][14] (ºC), 3-methoxy (220) >4-methoxy (210) >4-ethoxy (179) >2-methoxy (172) >4-butoxy (147), 4-propoxy (131) >3-ethoxy (126) >2-ethoxy (120) >4-hexyloxy (110), ethyl cyanophenylpropenoates [15,16] Information on thermal stability of the copolymers was obtained from thermogravimetric analysis ( Table 2). Figure 4 shows TGA of the P(ST-co-ICPP) copolymer. The copolymer has lower thermal stability than is reported for polystyrene (PS) [24], the onset of decomposition at 221ºC (PS 350ºC), 10% weight loss at 287ºC (PS 425ºC), 50%  weight loss at 362ºC (PS 428ºC). Lower thermal stability of the ST-ICPP copolymers apparently associated with presence of ICPP quaternary carbon in the chain similarly to poly-alpha-methylstyrene [25]. Decomposition of the copolymers in nitrogen occurred in two steps, first in the 250-500ºC range with residue (2.6-3.9% wt), which then decomposed in the 500-800ºC range. The decomposition products were not analyzed in this study, and the mechanism has yet to be investigated.

Conclusions
Novel trisubstituted ethylenes, alkoxy ring-substituted isopropyl 2-cyano-3-phenyl-2-propenoates were prepared and copolymerized with styrene. The compositions of the copolymers were calculated from nitrogen analysis and the structures were analyzed by IR, H 1 and 13 C NMR. The thermal gravimetric analysis indicated that the copolymers decompose in two steps, first in the 250-500°C range with residue (2.6-3.9%wt), which then decomposed in the 500-800ºC range.

Disclosure statement
No potential conflict of interest was reported by the authors.