The material of the walls:
All materials are valid for a Sukkah [if it does not move with a common wind as explained in Halacha B]. Even if the material does not lend shade to the Sukkah [such as glass or transparent plastic], and hence it gives off more sun then shade, it is valid. [Thus, the walls may be made even initially from material that is Halachically unfit for use as Sechach.]
Material with bad odor, or material that dries and shrinks: Nevertheless, one is not to use materials that give off a foul odor or which dry and shrink within the 7 days to below the minimum requirement for the wall.
Using sheets as a wall: It is not proper to use sheets [whether of fabric or plastic] as material for a wall even if it is tied down well. However, it is permitted to use sheets if one places poles within three Tefach of each other and hence uses Lavud to make an additional backup wall. [According to all, Bedieved if one used sheets for a wall, then if they are tied down well that they do not move with the wind, the Sukkah is Kosher even if Lavud was not used.]
Practically are Sukkos with plastic walls initially valid?
If one places string or poles to perform Lavud to make three walls in addition to the plastic fabric walls, then even initially it is valid according to all. However, if Lavud is not being used, then one should not use such a Sukkah being it is possible that the plastic material walls will become loose and move with the wind. However, from the letter of the law, if the plastic does not move with the wind it is valid. Furthermore, some Poskim rule that it is even initially permitted to use plastic sheets if they are tightly attached as it is not possible to move with the wind.
How far must the sheet/plastic walls move with the wind to be considered problematic?
If the wind can move it even slightly back and forth then it is invalid, even if the wind is unable to make it fall completely to the ground. [Some interpret this to mean as follows: If the common wind can blow the sheet walls a distance of three Tefachim in a way that it invalidates the wall, then it is invalid. For example, if it can blow it three Tefachim, upwards from the ground, or three Tefachim away from the Sechach, or that it creates a stomach of three Tefachim deep in a way that it does not contain ten Tefachim of wall within three Tefachim of width, then it is invalid even when the wind does not blow.]
 Michaber 630:1
 Michaber 630:10
 Meaning that although the main aspect of a Sukkah is its ability to provide shade, of which lack there of invalidates the Sukkah, this is only in regards to the Sechach. However, the walls are not required that they provide shade. Hence even if in the morning or evening when the sun shines at an angle towards the walls of the Sukkah there is more sun light than shade, nevertheless the Sukkah is Kosher.
 Michaber ibid
 Biur Halacha 630:1 “Kol Hadevarim” in name of Elya Raba and Birkeiy Yosef; Sefer Haminhagim p. 65
Other opinions: Some Poskim rule that the wall of the Sukkah follows the same laws as the Sechach, and hence any material that is Biblically invalid for Sechach is likewise invalid to be used as material for the walls of the Sukkah. [Darkei Moshe 630 in name of Hagahos Ashri in name of Or Zarua, brought in Bach 630 and Biur Halacha 630:1] Some Poskim conclude that it is proper to be stringent like this opinion. [Bach ibid, brought in Biur Halacha ibid; Peri Megadim in 630; Biur Halacha ibid according to Gr”a who is Machmir to invalidate an invalid Mamad, and the Chazon Ish extends this to invalidate even a Mamad Demamad]
 Rama ibid
 Michaber 630:10; Piskeiy Teshuvos 630:9
 The reason: Being that it is difficult to ascertain that the sheets will not move with the wind, as even if one ties them well, occasionally the ropes tear unoticebly and he ends up having a wall that cannot stand in face of a common wind. [Michaber ibid]
 The reason: As in such a case even if the sheets move with the wind the wall remains Kosher due to the poles which are Lavud.
 Aruch Hashulchan 630:32
 Piskeiy Teshuvos 630:9
 Salmas Chaim 253; Kinyan Torah 4:70; Mishneh Halachos 5:77; Moadim Uzmanim 1:84
 M”B 630:48
 Chazon Ish 77:6; Piskeiy Teshuvos 631:9